22 Apr
2023
22 Apr
'23
8:07 a.m.
As some of you may be aware I am trying to find a fault in a Rainbow H7842
PSU. I am using Tony Duell's schematic from here
http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf.
I have been testing the Control Module by using a bench PSU to supply 15VDC
to the input of the 7812 regulator (p2, PSU Sheet 1).
My diagnosis shows that the control module is shutting down the PWM (p6,
Control Module Sheet 2) because it is detecting an overcurrent in the -12V
side (E3d on Control Module sheet 1, although I have determined that it is
actually E3c).
This seems to be because I measure a steady 0.6V on pin 6 of the transformer
(p4, PSU Sheet 3). I just can't imagine where it might be coming from as the
chopper won't be running. I had previously removed the transformer and there
are no shorts between the pin 5-pin 6 winding and any of the other pins on
the transformer. I checked all the DC outputs of the PSU when powering the
7812 from the bench, both on a working PSU and the non-working one. They are
all at zero except the -12V output on the non-working PSU, which is +0.6V.
But the voltage can't come out of nowhere.
I am stumped and would appreciate any suggestions.
Thanks
Rob
22 Apr
22 Apr
8:28 p.m.
This seems to be because I measure a steady 0.6V on pin 6 of the transformer
(p4, PSU Sheet 3). I just can't imagine where it might be coming from as the
chopper won't be running. I had previously removed the transformer and there
are no shorts between the pin 5-pin 6 winding and any of the other pins on
the transformer. I checked all the DC outputs of the PSU when powering the
7812 from the bench, both on a working PSU and the non-working one. They are
all at zero except the -12V output on the non-working PSU, which is +0.6V.
But the voltage can't come out of nowhere.
Is the -12V output connected to it's load while you are testing? If so,
perhaps there is a path through the load to the the +12V or +15V supplys
available elsewhere which would forward bias the two diodes connected to
pin 6 of the transformer giving you a small positive voltage at that point.
(I would have thought it would be a bit higher though, depending on the type
of diode. Neither of the two diodes is shorted, is it?)
10:53 p.m.
This seems to be because I measure a steady 0.6V on pin 6 of the transformer
(p4, PSU Sheet 3). I just can't imagine where it might be coming from as the
chopper won't be running. I had previously removed the transformer and there
are no shorts between the pin 5-pin 6 winding and any of the other pins on
the transformer. I checked all the DC outputs of the PSU when powering the
7812 from the bench, both on a working PSU and the non-working one. They are
all at zero except the -12V output on the non-working PSU, which is +0.6V.
But the voltage can't come out of nowhere.
I've looked at it some more. On page 5 (control module sheet 1), at the
non-inverting input of E1b, there is a 75k resistor and 16k9 I think?
resistor between -12V and V2 which is derived from Vstart. Perhaps this
would account for the +0.6V?
(Ignore what I said earlier about the possibility of one of the two diodes
connected to pin 6 of the transformer being shorted. The 51 Ohm (or is it a
5 Ohm?) resistor across one of them would look like a short compared to 75k.)
Regards,
Peter Coghlan.
23 Apr
23 Apr
2:59 a.m.
On 2023-Apr-22, at 3:53 PM, Peter Coghlan via cctalk wrote:
On 2023-Apr-22, at 1:07 AM, Rob Jarratt via cctalk wrote:
Following on Peter's observation above, note that under normal (as opposed to
test-bench) operation, a limited -12V is supplied at startup by the same little mains
transformer that supplies VStart (schematic: PSU sheet 1).
This startup -12V would appear to be or may be required to bias things correctly for
startup to proceed.
It follows that the -12V current-sense shutdown being observed is not (necessarily) part
of the fault being looked for, but just a consequence of the absence of the -12V startup.
Also, minor note: not indicated on the schematic is the power supply pin connections for
the comparator ICs (E1,2,3) (or I haven't spotted them). Presumably the +supply is
VStart, but you might check and record whether the -supply is GND or is it -12V (and check
all 3 ICs).
This seems to
be because I measure a steady 0.6V on pin 6 of the transformer
(p4, PSU Sheet 3). I just can't imagine where it might be coming from as the
chopper won't be running. I had previously removed the transformer and there
are no shorts between the pin 5-pin 6 winding and any of the other pins on
the transformer. I checked all the DC outputs of the PSU when powering the
7812 from the bench, both on a working PSU and the non-working one. They are
all at zero except the -12V output on the non-working PSU, which is +0.6V.
But the voltage can't come out of nowhere.
I've looked at it some more. On page 5 (control module sheet 1), at the
non-inverting input of E1b, there is a 75k resistor and 16k9 I think?
resistor between -12V and V2 which is derived from Vstart. Perhaps this
would account for the +0.6V?
(Ignore what I said earlier about the possibility of one of the two diodes
connected to pin 6 of the transformer being shorted. The 51 Ohm (or is it a
5 Ohm?) resistor across one of them would look like a short compared to 75k.)
1:58 p.m.
Thanks for the various replies. Here are my responses and further
observations.
I was asked if I had a load on the PSU while bench testing it. The answer to
that is no.
I did check the two diodes around the -12V current sensing resistor (PSU
Sheet 3), but as was pointed out the current sensing resistor is very low
value (51 ohms). The two diodes measure the same, in circuit, on the working
and non-working PSUs. I suppose I may need to lift them to check them
properly.
I am not sure about the comment suggesting that the bench test conditions
could result an overcurrent being sensed on the -12V output. This is because
I use the same bench test conditions on both the working and non-working
PSUs, and on the working one an overcurrent is not sensed.
I hadn't noticed the connection to -12V on the non-inverting input of E1b.
However, I don't think this can account for the 0.6V because in both the
working and non-working PSUs the non-inverting input to E1b is 4V. However,
when I was checking this, I noticed that I must have made a mistake, because
the -12V output actually measures +0.4V on the working PSU (not zero as I
first thought), but the non-working PSU measures +0.6V. Also, I noticed that
the -12V output on the working PSU rises more slowly to +0.4V than on the
non-working PSU where it rises more quickly to +0.6V.
I still don't understand where this +ve voltage on the -12V output can come
from though? Whether on the working PSU or the non-working PSU.
Thanks
Rob
3:04 p.m.
Hi Rob,
Thanks for the various replies. Here are my responses and further
observations.
I was asked if I had a load on the PSU while bench testing it. The answer to
that is no.
I initially thought that the +0.6V on the -12V line might have been arriving
there via a load which was connected to both the -12V line and a positive
supply derived from the +15V you are applying from the bench power supply.
It seems that this cannot be the case as there is no load connected and all
the positive outputs from the PSU are at zero anyway. An the internal to
the PSU path from Vstart to the -12V line is a much more likely explaination
for this +0.6V.
I did check the two diodes around the -12V current sensing resistor (PSU
Sheet 3), but as was pointed out the current sensing resistor is very low
value (51 ohms). The two diodes measure the same, in circuit, on the working
and non-working PSUs. I suppose I may need to lift them to check them
properly.
I don't think there is any need to check these further. I was barking up
the wrong tree whe I suggested suspecting them.
I am not sure about the comment suggesting that the bench test conditions
could result an overcurrent being sensed on the -12V output. This is because
I use the same bench test conditions on both the working and non-working
PSUs, and on the working one an overcurrent is not sensed.
It looks like the conditions in the control and overcurrent sensing parts of
the working and the non-working PSUs are different so some further
investigation may be required as to why this is the case.
I hadn't noticed the connection to -12V on the non-inverting input of E1b.
However, I don't think this can account for the 0.6V because in both the
working and non-working PSUs the non-inverting input to E1b is 4V.
See further down.
However,
when I was checking this, I noticed that I must have made a mistake, because
the -12V output actually measures +0.4V on the working PSU (not zero as I
first thought), but the non-working PSU measures +0.6V. Also, I noticed that
the -12V output on the working PSU rises more slowly to +0.4V than on the
non-working PSU where it rises more quickly to +0.6V.
I still don't understand where this +ve voltage on the -12V output can come
from though? Whether on the working PSU or the non-working PSU.
The 4V at the non-inverting input of E1b is linked to the -12V line via the
75k resistor and from there via a smoothing choke and the chopper transformer
secondary to the -12V line rectifier diode connected to pin 6 and on to ground
via the parallel diode/resistor combination. This should result in a current
of about 45 microamps flowing to ground through those components mentioned.
This in turn should cause the rectifier to be forward biased and drop
approximately 0.6V across it which is characteristic of silicon diodes.
Does that make sense?
(It's rather confusing in that the -12V line rectifier is on the ground side
of the chopper transformer secondary instead of on the supply side where it
would more usually be encountered. It's electrically all the same wherever
it is put as the components involved are in series anyway.)
It ought to be possible to measure the same 0.6V across the diode to confirm
this is where it is being dropped (and to measure the remaining 3.4V of the
4V at E1b across the 75k resistor). As to why it is only 0.4V on the working
power supply, I haven't thought that far ahead yet :-)
I think the +0.6V on the -12V line is explainable and to be expected under the
test conditions described. It looks like there could be something wrong in
the control circuitry which is preventing the power supply from starting up.
This might also account for the difference between the 0.4V and 0.6V.
Brent's suggestions for checking the condition around the comparators and how
they are supplied with power are good ones. I haven't made any further
suggestions because I don't have any right now :-)
Regards,
Peter Coghlan.
9:11 p.m.
On 2023-Apr-23, at 8:04 AM, Peter Coghlan via cctalk wrote:
...
This should result in a current
of about 45 microamps flowing to ground through those components mentioned.
...
It ought to be possible to measure the same 0.6V
across the diode to confirm
this is where it is being dropped (and to measure the remaining 3.4V of the
4V at E1b across the 75k resistor). As to why it is only 0.4V on the working
power supply, I haven't thought that far ahead yet :-)
The 0.4V across the working-supply diode in contrast to 0.6V is because the current is way
down in the microAmps. This is well on the low side of the curvy knee of typical Si diode
switching, so lower voltage drop and high per-component in-circuit variability are not a
surprise.
I think the +0.6V on the -12V line is explainable and
to be expected under the
test conditions described. It looks like there could be something wrong in
the control circuitry which is preventing the power supply from starting up.
This might also account for the difference between the 0.4V and 0.6V.
Brent's suggestions for checking the condition around the comparators and how
they are supplied with power are good ones. I haven't made any further
suggestions because I don't have any right now :-)
In addition to the IC power pins:
The voltage value of Vz (V of zener by 390 ohm R) is not presented in the schematic.
Calculating from the measurement of 4V suggests it will be ~ 5V.
It tends to be helpful if the voltages of zener diodes are presented in a schematic.
Also note:
The ISense+/Isense- labeling in the schematic is the reverse of the polarities which will
actually be present/expected on those lines (for all three outputs). That is, the lines
labeled ISense- will be more + than the lines labeled ISense+ under normal operation as
well as at times of over-current shutdown.
29 Apr
29 Apr
12:30 p.m.
It looks like the conditions in the control and
overcurrent sensing parts
of the
working and the non-working PSUs are different so some
further
investigation
may be required as to why this is the case.
Yes, that is certainly the problem. Presumably more current is flowing
through the current sense resistor than should be. I thought that perhaps
the higher positive voltage might be enough to switch on the rectifier diode
while the lower positive voltage on the good PSU won't, but it seems to
switch on in both cases because I measured a voltage across the current
sense resistor in both cases. I measured 0.08V on the bad PSU and 0.01 on
the good PSU. So this would explain why an over current is being detected.
What I don't now get is why the -12V output is higher on the bad PSU because
if there is something shorted or a capacitor with high current leakage on
the -12V output, that would surely result in a lower voltage on the -12V
output, not a higher one?
>
> I hadn't noticed the connection to -12V on the non-inverting input of
E1b.
However, I
don't think this can account for the 0.6V because in both
the working and non-working PSUs the non-inverting input to E1b is 4V.
See further down.
>
> However,
> when I was checking this, I noticed that I must have made a mistake,
> because the -12V output actually measures +0.4V on the working PSU
> (not zero as I first thought), but the non-working PSU measures +0.6V.
> Also, I noticed that the -12V output on the working PSU rises more
> slowly to +0.4V than on the non-working PSU where it rises more quickly +0.6V.
the 75k
I still don't understand where this +ve voltage on the -12V output can
come from though? Whether on the working PSU or the non-working PSU.
The 4V at the non-inverting input of E1b is linked to the -12V line via resistor and from there via a smoothing choke and the
chopper transformer
secondary to the -12V line rectifier diode connected to pin 6 and on to
ground via the parallel diode/resistor combination. This should result in
a
current of about 45 microamps flowing to ground
through those components
mentioned.
This in turn should cause the rectifier to be forward biased and drop
approximately 0.6V across it which is characteristic of silicon diodes.
Does that make sense?
It took me a while to understand this, but yes I get that now.
(It's rather confusing in that the -12V line rectifier is on the ground
side of
the
chopper transformer secondary instead of on the supply
side where it would
more usually be encountered. It's electrically all the same wherever it
is
put
as the components involved are in series anyway.)
It ought to be possible to measure the same 0.6V across the diode to
confirm
this is where it is being dropped (and to measure the
remaining 3.4V of
the
4V at E1b across the 75k resistor). As to why it is
only 0.4V on the
working
power supply, I haven't thought that far ahead yet
:-)
On the bad PSU I measure a 0.48V drop across the rectifier diode (the one
directly connected to the transformer, not the one in parallel with the
current sense resistor), on the good PSU I measure a 0.4V drop.
I think the +0.6V on the -12V line is explainable and to be expected under
the
test conditions described. It looks like there could
be something wrong
in the
control circuitry which is preventing the power supply
from starting up.
This might also account for the difference between the 0.4V and 0.6V.
Brent's suggestions for checking the condition around the comparators and
how they are supplied with power are good ones. I haven't made any
further
suggestions because I don't have any right now :-)
I had forgotten about Brent's suggestion. Still need to check this.
Regards,
Peter Coghlan.
11:22 p.m.
Yes, that is certainly the problem. Presumably more current is flowing
through the current sense resistor than should be.
Do you mean when the power supply is connected to the mains here?
If we are still talking about the test case when you are applying an external
supply to Vstart only, there should be almost zero current flowing through
the 51 Ohm current sense resistor because the chopper is not operating
because it has no HV supply.
We can calculate the current flowing through the current sense resistor
using Ohm's law given it's resistance and the voltage across it so we
don't need to presume.
I thought that perhaps
the higher positive voltage might be enough to switch on the rectifier diode
while the lower positive voltage on the good PSU won't,
My feeling is that (in the test case with external Vstart only) the -12V
rectifier diode is being caused to conduct by the forward bias voltage applied
to it from Vstart/V2 (should that be Vz I wonder?) via the 16k9 resistor, the
75k resistor and the wound components. The current available to flow through
the diode is limited to some microamps by the 75k resistor. As Brent said,
this is very low on the diode's characteristic curve and there are likely to
be variations between the voltages dropped across different diodes.
If diode was not present in the circuit, approx 4V (which IIRC you measured
at E1b non-inverting input) should appear across the points where the diode
would have been. It is more the characteristic of the diode which is
determining how much voltage is appearing across itself than anything else.
The other components involved have little say about what voltage appears
across the diode. They just allow a small current to flow through the diode
to forward bias it and the diode gets to decide how much voltage then appears
across itself, depending on the magnitude of the current flowing through it
when forward biased and it's characteristic curve.
but it seems to
switch on in both cases because I measured a voltage across the current
sense resistor in both cases. I measured 0.08V on the bad PSU and 0.01 on
the good PSU. So this would explain why an over current is being detected.
0.08V across a 51 Ohm resistor suggests that approx 1.6mA is flowing through
it from Ohm's law. 0.01V across the same resistor suggests that approx 200 uA
flowing through it. I assume this is with the power supply connected to the
mains, not using the external supply to energise Vstart only as I can't see
where 1.6mA would come from in the latter case.
1.6mA being drawn from the -12V supply is not suggestive of an overload
unless it's capacity is very limited indeed. Perhaps this is the normal
expected loading? 200uA seems very low indeed but perhaps this is to be
expected? If the -12V line is not being produced, the load cannot draw
any current from it. I don't think that either of these conditions should
trigger an overload response from the PSU.
(Are the 0.08V and and 0.01V voltages appearing in the same direction or
is one of them positive and the other one negative?)
(Does the resistor measure approximately 50 Ohms in both cases with the
power switched off)?
What I don't now get is why the -12V output is higher on the bad PSU because
if there is something shorted or a capacitor with high current leakage on
the -12V output, that would surely result in a lower voltage on the -12V
output, not a higher one?
Sorry, this is not very clear to me and I already get confused enough when
dealing with negative supply rails. The -12V output being higher could
refer to it's voltage being more positive with respect to 0V or it's voltage
being more negative with respect to 0V. In any case, I'm not sure I recall
what the voltages measured on the -12V lines of the working and non-working
power supplies were with them connected to the mains. I think I recall they
were +0.6V and +0.4V respectively when power was applied to Vstart only.
I think it is likely that most of what we are seeing here is to be expected
and is explainable either by the test conditions created by applying power
to Vstart only or by conditions created by the presence of the fault and
not necessarily because we are close to the location of the fault.
I think the next step might be to check what is happening with the
comparators. Are their positive power supply pins connected to Vstart or
V2 (Vz?) or somewhere else? Are their negative supply pins connected to 0V
or maybe to the -12V line or maybe somewhere else? Are they getting suitable
volages on both supply pins that will enable them to operate under startup
conditions? If either the positive or negative supply to one or more of
them is missing or wildly incorrect at startup, they cannot be expected to
do their jobs properly.
It might be worth doing some more testing without a mains supply connected
but with an external supply for Vstart and also an external negative supply
going to the -12V line via a 2k7 resistor to mimic both the positive and
negative startup supplies on sheet 1. Comparing (no pun intended) what
appears on the positive and negative power supply pins of the comparators
between the working and non-working power supplies under these conditions
might reveal something.
If the two behave similarly, it may be time to focus on whether the
comparators in the faulty PSU are behaving correctly. However, I would
be wary of attempting to make tests around any of the comparators with
the mains supply connected to the PSU in case of inadvertently disabling
a protection circuit which could be detecting and protecting against a
real overload. This would probably lead to something going bang :-(
I hadn't noticed the connection to -12V on the non-inverting input of E1b.
However, I don't think this can account for the 0.6V because in both
the working and non-working PSUs the non-inverting input to E1b is 4V.
See further down.
However,
when I was checking this, I noticed that I must have made a mistake,
because the -12V output actually measures +0.4V on the working PSU
(not zero as I first thought), but the non-working PSU measures +0.6V.
Also, I noticed that the -12V output on the working PSU rises more
slowly to +0.4V than on the non-working PSU where it rises more quickly
to +0.6V.
I still don't understand where this +ve voltage on the -12V output can
come from though? Whether on the working PSU or the non-working PSU.
The 4V at the non-inverting input of E1b is linked to the -12V line via the 75k
resistor and from there via a smoothing choke and the chopper transformer
secondary to the -12V line rectifier diode connected to pin 6 and on to
ground via the parallel diode/resistor combination. This should result in
a current of about 45 microamps flowing to ground through those components
mentioned.
This in turn should cause the rectifier to be forward biased and drop
approximately 0.6V across it which is characteristic of silicon diodes.
Does that make sense?
(It's rather confusing in that the -12V line rectifier is on the ground side of the
chopper transformer secondary instead of on the supply side where it would
more usually be encountered. It's electrically all the same wherever it
is put as the components involved are in series anyway.)
It ought to be possible to measure the same 0.6V across the diode to confirm
this
is where it is being dropped (and to measure the remaining 3.4V of the
4V at E1b across the 75k resistor). As to why it is only 0.4V on the working
power supply, I haven't thought that far ahead yet :-)
On the bad PSU I measure a 0.48V drop across the rectifier diode (the one
directly connected to the transformer, not the one in parallel with the
current sense resistor), on the good PSU I measure a 0.4V drop.
I think the +0.6V on the -12V line is explainable
and to be expected under the
test conditions described. It looks like there could be something wrong in the
control circuitry which is preventing the power supply from starting up.
This might also account for the difference between the 0.4V and 0.6V.
Brent's suggestions for checking the condition around the comparators and
how they are supplied with power are good ones. I haven't made any
further
suggestions because I don't have any right now :-)
I had forgotten about Brent's suggestion. Still need to check this.
Regards,
Peter Coghlan.
30 Apr
30 Apr
6:26 p.m.
I think I have found a possible cause, but not too sure what to do about it
if I have.
I had to replace the 7812 regulator that supplies Vstart. The new one has an
output that seems to vary between 12.2 and 12.4V. The 7812 on the good PSU
outputs a steady 12.1V. I think that is enough to cause the E3d comparator
to turn the comparator off and allow its output to float high.
I suspect I may have used one with a wide tolerance rather than a narrow
one, or just have a bad one. It should be a genuine part as it came from CPC
Farnell. Not quite sure why the output varies on the bad one though.
Regards
Rob
6:37 p.m.
Spec sheet for the 7812 shows 2 versions - 4% and 2% tolerances. So the voltages your
measuring are within the 4% part. Maybe you do need the 2% one.
Sent from my iPhone
On Apr 30, 2023, at 11:26, Rob Jarratt via cctalk
<cctalk(a)classiccmp.org> wrote:
I think I have found a possible cause, but not too sure what to do about it
if I have.
I had to replace the 7812 regulator that supplies Vstart. The new one has an
output that seems to vary between 12.2 and 12.4V. The 7812 on the good PSU
outputs a steady 12.1V. I think that is enough to cause the E3d comparator
to turn the comparator off and allow its output to float high.
I suspect I may have used one with a wide tolerance rather than a narrow
one, or just have a bad one. It should be a genuine part as it came from CPC
Farnell. Not quite sure why the output varies on the bad one though.
Regards
Rob
6:42 p.m.
The CT version is 4%, the ACT is 2%.
Sent from my iPhone
On Apr 30, 2023, at 11:37, Wayne S
<wayne.sudol(a)hotmail.com> wrote:
Spec sheet for the 7812 shows 2 versions - 4% and 2% tolerances. So the voltages your
measuring are within the 4% part. Maybe you do need the 2% one.
Sent from my iPhone
> On Apr 30, 2023, at 11:26, Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
wrote:
>
> I think I have found a possible cause, but not too sure what to do about it
> if I have.
>
> I had to replace the 7812 regulator that supplies Vstart. The new one has an
> output that seems to vary between 12.2 and 12.4V. The 7812 on the good PSU
> outputs a steady 12.1V. I think that is enough to cause the E3d comparator
> to turn the comparator off and allow its output to float high.
>
> I suspect I may have used one with a wide tolerance rather than a narrow
> one, or just have a bad one. It should be a genuine part as it came from CPC
> Farnell. Not quite sure why the output varies on the bad one though.
>
> Regards
>
> Rob
>
>
>
8:34 p.m.
Looks like the one I used is an L7812CV, which has a looser tolerance. I am going to have
to order some tighter tolerance parts.
-----Original Message-----
From: Wayne S <wayne.sudol(a)hotmail.com>
Sent: Sunday, April 30, 2023 7:42 PM
To: Wayne S <wayne.sudol(a)hotmail.com>
Cc: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>rg>; Rob Jarratt <robert.jarratt(a)ntlworld.com>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
The CT version is 4%, the ACT is 2%.
Sent from my iPhone
On Apr 30, 2023, at 11:37, Wayne S
<wayne.sudol(a)hotmail.com> wrote:
Spec sheet for the 7812 shows 2 versions - 4% and 2% tolerances. So the
voltages
your measuring are within the 4% part. Maybe you do need the 2%
one.
Sent from my iPhone
> On Apr 30, 2023, at 11:26, Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
wrote:
>
> I think I have found a possible cause, but not too sure what to do
> about it if I have.
>
> I had to replace the 7812 regulator that supplies Vstart. The new one
> has an output that seems to vary between 12.2 and 12.4V. The 7812 on
> the good PSU outputs a steady 12.1V. I think that is enough to cause
> the E3d comparator to turn the comparator off and allow its output to
float
high.
>
> I suspect I may have used one with a wide tolerance rather than a
> narrow one, or just have a bad one. It should be a genuine part as it
> came from CPC Farnell. Not quite sure why the output varies on the bad
one
though.
>>
>> Regards
>>
>> Rob
>>
>>
>>
1 May
1 May
6:16 a.m.
I need to order higher tolerance ones. However, I was wondering, would a sensible strategy
be to go through the ones that I have and find one that outputs a voltage closer to the
spec? Or would that just be storing up trouble for the future?
Regards
Rob
-----Original Message-----
From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Sent: Sunday, April 30, 2023 9:35 PM
To: 'Wayne S' <wayne.sudol(a)hotmail.com>
Cc: rob(a)jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
Looks like the one I used is an L7812CV, which has a looser tolerance. I am
going to have to order some tighter tolerance parts.
> -----Original Message-----
> From: Wayne S <wayne.sudol(a)hotmail.com>
> Sent: Sunday, April 30, 2023 7:42 PM
> To: Wayne S <wayne.sudol(a)hotmail.com>
> Cc: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic
> Posts <cctalk(a)classiccmp.org>rg>; Rob Jarratt
> <robert.jarratt(a)ntlworld.com>
> Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
>
> The CT version is 4%, the ACT is 2%.
>
> Sent from my iPhone
>
> > On Apr 30, 2023, at 11:37, Wayne S <wayne.sudol(a)hotmail.com> wrote:
> >
> > Spec sheet for the 7812 shows 2 versions - 4% and 2% tolerances. So
> > the
> voltages your measuring are within the 4% part. Maybe you do need the
> 2% one.
> >
> > Sent from my iPhone
> >
> >> On Apr 30, 2023, at 11:26, Rob Jarratt via cctalk
> >> <cctalk(a)classiccmp.org>
> wrote:
> >>
> >> I think I have found a possible cause, but not too sure what to do
> >> about it if I have.
> >>
> >> I had to replace the 7812 regulator that supplies Vstart. The new
> >> one has an output that seems to vary between 12.2 and 12.4V. The
> >> 7812 on the good PSU outputs a steady 12.1V. I think that is enough
> >> to cause the E3d comparator to turn the comparator off and allow
> >> its output to
> float high.
> >>
> >> I suspect I may have used one with a wide tolerance rather than a
> >> narrow one, or just have a bad one. It should be a genuine part as
> >> it came from CPC Farnell. Not quite sure why the output varies on
> >> the bad
> one though.
> >>
> >> Regards
> >>
> >> Rob
> >>
> >>
> >>
9:25 a.m.
It seems a bit odd that a power supply from someone like DEC of that era would
be designed to depend so critically on the absolute value of a rail used
for startup purposes.
The output of the replacement 7812 varying between 12.2V and 12.4V suggest that
the 7812 itself could be faulty. Or maybe it could be oscillating due to a
failure in the smoothing/reservoir/decoupling capacitors on either side of it,
especially the 10uF and 2.2uF capacitors connected to it's output? It might be
useful to see what Vstart looks like with an oscilloscope.
If the value of Vstart is really critical enough to change the state of E3d,
I would suggest checking the resistors connected to the inputs and output
of E3d, especially the 360k resistor. Assuming it is getting a reasonable
power supply on pins 3 and 12 (whose value should not be critical), the
output of E3d should depend solely on the relationship between the voltages
at it's two inputs. If the output does not correspond with what the inputs
say it should be, then the comparator itself is probably faulty.
Regards,
Peter Coghlan.
I need to order higher tolerance ones. However, I was wondering, would a
sensible strategy be to go through the ones that I have and find one that
outputs a voltage closer to the spec? Or would that just be storing up
trouble for the future?
Regards
Rob
> -----Original Message-----
> From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
> Sent: Sunday, April 30, 2023 9:35 PM
> To: 'Wayne S' <wayne.sudol(a)hotmail.com>
> Cc: rob(a)jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts'
> <cctalk(a)classiccmp.org>
> Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
>
> Looks like the one I used is an L7812CV, which has a looser tolerance. I am
> going to have to order some tighter tolerance parts.
>
> > -----Original Message-----
> > From: Wayne S <wayne.sudol(a)hotmail.com>
> > Sent: Sunday, April 30, 2023 7:42 PM
> > To: Wayne S <wayne.sudol(a)hotmail.com>
> > Cc: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic
> > Posts <cctalk(a)classiccmp.org>rg>; Rob Jarratt
> > <robert.jarratt(a)ntlworld.com>
> > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> >
> > The CT version is 4%, the ACT is 2%.
> >
> > Sent from my iPhone
> >
> > > On Apr 30, 2023, at 11:37, Wayne S <wayne.sudol(a)hotmail.com> wrote:
> > >
> > > Spec sheet for the 7812 shows 2 versions - 4% and 2% tolerances. So
> > > the
> > voltages your measuring are within the 4% part. Maybe you do need the
> > 2% one.
> > >
> > > Sent from my iPhone
> > >
> > >> On Apr 30, 2023, at 11:26, Rob Jarratt via cctalk
> > >> <cctalk(a)classiccmp.org>
> > wrote:
> > >>
> > >> I think I have found a possible cause, but not too sure what to do
> > >> about it if I have.
> > >>
> > >> I had to replace the 7812 regulator that supplies Vstart. The new
> > >> one has an output that seems to vary between 12.2 and 12.4V. The
> > >> 7812 on the good PSU outputs a steady 12.1V. I think that is enough
> > >> to cause the E3d comparator to turn the comparator off and allow
> > >> its output to
> > float high.
> > >>
> > >> I suspect I may have used one with a wide tolerance rather than a
> > >> narrow one, or just have a bad one. It should be a genuine part as
> > >> it came from CPC Farnell. Not quite sure why the output varies on
> > >> the bad
> > one though.
> > >>
> > >> Regards
> > >>
> > >> Rob
> > >>
> > >>
> > >>
10:23 p.m.
On 2023-May-01, at 2:25 AM, Peter Coghlan via cctalk wrote:
It seems a bit odd that a power supply from someone
like DEC of that era would
be designed to depend so critically on the absolute value of a rail used
for startup purposes.
Further to Peter's point above, the 1988 NatSemi databook - which is to say, from the
era of this power supply - specs the 7812 output min-max to be 11.4 to 12.6V (+/-5%). Your
measured Vstart=12.4V is well within this.
Looking at the schematic, nothing stands out where the distinction from 12 or 12.1 would
matter.
You still haven't reported the IC power pin connections. If the neg-supply pins are
supplied by -12 rather than GND, it could explain the odd voltage seen on the E3d +input.
There are 3 explicit components in the design which provide -12V at startup. They
didn't throw those components in there just to fill up board space and look pretty.
Why would you expect the control circuit to be testable for valid startup state when you
haven't provided the startup environment?
11:29 p.m.
On May 1, 2023, at 6:23 PM, Brent Hilpert via cctalk
<cctalk(a)classiccmp.org> wrote:
On 2023-May-01, at 2:25 AM, Peter Coghlan via cctalk wrote:
The usual DEC spec for power supplies was +/- 5%. In reality there occasionally were
issues: FS told me on one occasion when we had DMC-11s not working right that those
actually need +5%, -0%, and that FS people knew this and would adjust power supplies as
needed.
paul
It seems a bit odd that a power supply from
someone like DEC of that era would
be designed to depend so critically on the absolute value of a rail used
for startup purposes.
Further to Peter's point above, the 1988 NatSemi databook - which is to say, from the
era of this power supply - specs the 7812 output min-max to be 11.4 to 12.6V (+/-5%). Your
measured Vstart=12.4V is well within this.
Looking at the schematic, nothing stands out where the distinction from 12 or 12.1 would
matter.
2 May
2 May
6:39 a.m.
-----Original Message-----
From: Brent Hilpert via cctalk <cctalk(a)classiccmp.org>
Sent: Monday, May 1, 2023 11:23 PM
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Brent Hilpert <bhilpert(a)shaw.ca>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
On 2023-May-01, at 2:25 AM, Peter Coghlan via cctalk wrote:
say,
It seems a bit odd that a power supply from
someone like DEC of that
era would be designed to depend so critically on the absolute value of
a rail used for startup purposes.
Further to Peter's point above, the 1988 NatSemi databook - which is to from the era of this power supply - specs the 7812
output min-max to be
11.4 to 12.6V (+/-5%). Your measured Vstart=12.4V is well within this.
Looking at the schematic, nothing stands out where the distinction from 12
or
12.1 would matter.
You still haven't reported the IC power pin connections. If the neg-supply
pins are supplied by -12 rather than GND, it could explain the odd voltage
seen on the E3d +input.
Sorry, I thought I had done this. The two LM339 comparator power pins are
connected to Vstart and GND.
The reason for the odd output of E3d seems to be that the positive input is
actually higher than the negative one, but only by a small amount, and that
small amount would seem to correspond to the slightly higher value of
Vstart, because the negative input is the output of a voltage divider from
Vstart to ISense -12V+, which is GND.
There are 3 explicit components in the design which provide -12V at
startup.
They didn't throw those components in there just
to fill up board space
and
look pretty. Why would you expect the control circuit
to be testable for
valid
startup state when you haven't provided the
startup environment?
Not quite sure what you mean here. I had advice from a friend to bench test
the control module by providing 14V to the input of the 7812. On the good
PSU I can see the PWM operate, on the bad one the PWM is shutdown.
Regards
Rob
6:54 a.m.
Not quite sure what you mean here. I had advice from a friend to bench test
the control module by providing 14V to the input of the 7812. On the good
PSU I can see the PWM operate, on the bad one the PWM is shutdown.
And what happens if you feed in exactly 12 V on the output of the 7812?
Would the PWM work then? By using a lab supply you could check if the
circuit is sensitive to variation in the 12V supply.
/Mattis
Regards
Rob
7:19 a.m.
That’s a good idea, I will try that
From: Mattis Lind <mattislind(a)gmail.com>
Sent: Tuesday, May 2, 2023 7:55 AM
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
Not quite sure what you mean here. I had advice from a friend to bench test
the control module by providing 14V to the input of the 7812. On the good
PSU I can see the PWM operate, on the bad one the PWM is shutdown.
And what happens if you feed in exactly 12 V on the output of the 7812? Would the PWM work
then? By using a lab supply you could check if the circuit is sensitive to variation in
the 12V supply.
/Mattis
Regards
Rob
7 May
7 May
8:54 a.m.
The comments about the tolerance of the 7812 were right, it doesn’t appear to be an issue
with the replacement 7812 regulator because when I tried using the bench PSU to feed
exactly 12V to the circuit from the output of the 7812 the comparator still gave the wrong
result. It was still wrong if I applied only 11V
I then looked at the value of Vz on the good and bad PSUs, when applying 12V to the 7812
output. That was 5.4V in both the good and bad PSUs. Where I saw a difference was on the
-12V output, it was +0.4V on the good PSU and 0.56V on the bad one (the voltage varied so
this was an average). I checked the voltage drop across the current sense resistor. It is
0.01V on the good PSU and 0.08V on the bad PSU, which would explain the higher positive
voltage on the -12V output and the comparator being turned on.
I am wondering if there could be a problem on the -12V output circuit (PSU Sheet 3). I am
struggling to understand the purpose of the two transistors and the Zener diode there, but
maybe one of them should be switched on and isn’t. I am also unsure now as to which diode
is doing the rectification (to -12V). Someone said it was the one attached to pin 6 of the
transformer, but is that right? Isn’t it the Zener diode half way across the page?
Regards
Rob
From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Sent: 02 May 2023 08:19
To: 'Mattis Lind' <mattislind(a)gmail.com>om>; rob(a)jarratt.me.uk; 'General
Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org>
Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
That’s a good idea, I will try that
From: Mattis Lind < <mailto:mattislind@gmail.com> mattislind(a)gmail.com>
Sent: Tuesday, May 2, 2023 7:55 AM
To: <mailto:rob@jarratt.me.uk> rob(a)jarratt.me.uk; General Discussion: On-Topic and
Off-Topic Posts < <mailto:cctalk@classiccmp.org> cctalk(a)classiccmp.org>
Cc: Rob Jarratt < <mailto:robert.jarratt@ntlworld.com>
robert.jarratt(a)ntlworld.com>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
Not quite sure what you mean here. I had advice from a friend to bench test
the control module by providing 14V to the input of the 7812. On the good
PSU I can see the PWM operate, on the bad one the PWM is shutdown.
And what happens if you feed in exactly 12 V on the output of the 7812? Would the PWM work
then? By using a lab supply you could check if the circuit is sensitive to variation in
the 12V supply.
/Mattis
Regards
Rob
9:33 a.m.
The comments about the tolerance of the 7812 were right, it doesn’t
appear to be an issue with the replacement 7812 regulator because when
I tried using the bench PSU to feed exactly 12V to the circuit from the
output of the 7812 the comparator still gave the wrong result. It was
still wrong if I applied only 11V
What do you mean by "gave the wrong result"? If the power supply to the
comparator is "reasonable" and the comparator's output does not reflect
what is happening at it's inputs, the comparator is faulty.
If the comparator is giving the "wrong result" because it's inputs
are telling it to, it is behaving correctly.
With no mains supply connected and a positive startup voltage applied
to Vstart and a negative startup voltage applied to the -12V line via
a 2k7 resistor, you could try shorting the inputs of the comparator
together and see whether this changes the comparator's output. I want
to emphasise doing this without power going to the mains rectifier
feeding the chopper so that if there is real overload, the magic smoke
will not be released.
If the output if the comparator is then "correct" and this output being
"wrong" was the source of the PSU not working, then the PWM should start
up like it does in the good power supply. This would confirm that there
is a problem in the components providing and/or mmonitoring the -12V line.
If the PWM does not start up, this suggests the problem is elsewhere.
I then looked at the value of Vz on the good and bad PSUs, when applying
12V to the 7812 output. That was 5.4V in both the good and bad PSUs. Where
I saw a difference was on the -12V output, it was +0.4V on the good PSU
and 0.56V on the bad one (the voltage varied so this was an average). I
checked the voltage drop across the current sense resistor. It is 0.01V
on the good PSU and 0.08V on the bad PSU, which would explain the higher
positive voltage on the -12V output and the comparator being turned on.
I am wondering if there could be a problem on the -12V output circuit
(PSU Sheet 3). I am struggling to understand the purpose of the two
transistors and the Zener diode there, but maybe one of them should be
switched on and isn’t. I am also unsure now as to which diode is doing
the rectification (to -12V). Someone said it was the one attached to
pin 6 of the transformer, but is that right? Isn’t it the Zener diode
half way across the page?
The TIP121 darlington transistor is a shunt regulator for the -12V line.
The voltage at the base of the MPSA55 transistor varies with the -12V line
because it is connected to the -12V line via the potential divider formed by
the 1k24 and 1k10 resistors. If the voltage on the -12V line increases
in the negative direction, the voltage at the base of the MPSA55 increases
in proportion to it via the potential divider. It's emitter voltage is
fixed by the zener diode so the MPSA55 is turned on more and it pulls more
current through the base of the TIP121 which results in the TIP121
conducting more and pulling the -12V line down closer to it's correct
voltage. The opposite happens if the -12V goes lower than it should be,
the TIP121 is turned on less and this allows the -12V line to increase
negatively to it's correct value.
None of this circuitry should be doing very much until the chopper
transformer is producing the source for the -12V line.
I suppose if the TIP121 is sborted or the zener diode is shorted, it
could be causing problems, however, from the test results etc we have
been given so far, I am not completely convinced there is a problem
with the -12V line.
If a negative startup voltage is applied to the -12V line as well as
a positive voltage to Vstart to better simultate startup conditions,
it may reveal more about what is happening with the -12V line.
It might then be possible to compare the voltages across the zener
diodes in the working power supply and the non-working power supply
for example.
Regards,
Peter.
Regards
Rob
From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Sent: 02 May 2023 08:19
To: 'Mattis Lind' <mattislind(a)gmail.com>om>; rob(a)jarratt.me.uk; 'General
Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org>
Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
That’s a good idea, I will try that
From: Mattis Lind < <mailto:mattislind@gmail.com> mattislind(a)gmail.com>
Sent: Tuesday, May 2, 2023 7:55 AM
To: <mailto:rob@jarratt.me.uk> rob(a)jarratt.me.uk; General Discussion: On-Topic and
Off-Topic Posts < <mailto:cctalk@classiccmp.org> cctalk(a)classiccmp.org>
Cc: Rob Jarratt < <mailto:robert.jarratt@ntlworld.com>
robert.jarratt(a)ntlworld.com>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
Not quite sure what you mean here. I had advice from a friend to bench test
the control module by providing 14V to the input of the 7812. On the good
PSU I can see the PWM operate, on the bad one the PWM is shutdown.
And what happens if you feed in exactly 12 V on the output of the 7812? Would the PWM
work then? By using a lab supply you could check if the circuit is sensitive to variation
in the 12V supply.
/Mattis
Regards
Rob
7:03 p.m.
Quick comment w/o looking at the schematic yet, is that zeners don’t do rectification, but
do do regulation. If you suspect bad a bad rectifier you can easily check with a scope.
Sent from my iPhone
On May 7, 2023, at 03:33, Peter Coghlan via cctalk
<cctalk(a)classiccmp.org> wrote:
The comments about the tolerance of the 7812 were right, it doesn’t
appear to be an issue with the replacement 7812 regulator because when
I tried using the bench PSU to feed exactly 12V to the circuit from the
output of the 7812 the comparator still gave the wrong result. It was
still wrong if I applied only 11V
What do you mean by "gave the wrong result"? If the power supply to the
comparator is "reasonable" and the comparator's output does not reflect
what is happening at it's inputs, the comparator is faulty.
If the comparator is giving the "wrong result" because it's inputs
are telling it to, it is behaving correctly.
With no mains supply connected and a positive startup voltage applied
to Vstart and a negative startup voltage applied to the -12V line via
a 2k7 resistor, you could try shorting the inputs of the comparator
together and see whether this changes the comparator's output. I want
to emphasise doing this without power going to the mains rectifier
feeding the chopper so that if there is real overload, the magic smoke
will not be released.
If the output if the comparator is then "correct" and this output being
"wrong" was the source of the PSU not working, then the PWM should start
up like it does in the good power supply. This would confirm that there
is a problem in the components providing and/or mmonitoring the -12V line.
If the PWM does not start up, this suggests the problem is elsewhere.
I then looked at the value of Vz on the good and bad PSUs, when applying
12V to the 7812 output. That was 5.4V in both the good and bad PSUs. Where
I saw a difference was on the -12V output, it was +0.4V on the good PSU
and 0.56V on the bad one (the voltage varied so this was an average). I
checked the voltage drop across the current sense resistor. It is 0.01V
on the good PSU and 0.08V on the bad PSU, which would explain the higher
positive voltage on the -12V output and the comparator being turned on.
I am wondering if there could be a problem on the -12V output circuit
(PSU Sheet 3). I am struggling to understand the purpose of the two
transistors and the Zener diode there, but maybe one of them should be
switched on and isn’t. I am also unsure now as to which diode is doing
the rectification (to -12V). Someone said it was the one attached to
pin 6 of the transformer, but is that right? Isn’t it the Zener diode
half way across the page?
The TIP121 darlington transistor is a shunt regulator for the -12V line.
The voltage at the base of the MPSA55 transistor varies with the -12V line
because it is connected to the -12V line via the potential divider formed by
the 1k24 and 1k10 resistors. If the voltage on the -12V line increases
in the negative direction, the voltage at the base of the MPSA55 increases
in proportion to it via the potential divider. It's emitter voltage is
fixed by the zener diode so the MPSA55 is turned on more and it pulls more
current through the base of the TIP121 which results in the TIP121
conducting more and pulling the -12V line down closer to it's correct
voltage. The opposite happens if the -12V goes lower than it should be,
the TIP121 is turned on less and this allows the -12V line to increase
negatively to it's correct value.
None of this circuitry should be doing very much until the chopper
transformer is producing the source for the -12V line.
I suppose if the TIP121 is sborted or the zener diode is shorted, it
could be causing problems, however, from the test results etc we have
been given so far, I am not completely convinced there is a problem
with the -12V line.
If a negative startup voltage is applied to the -12V line as well as
a positive voltage to Vstart to better simultate startup conditions,
it may reveal more about what is happening with the -12V line.
It might then be possible to compare the voltages across the zener
diodes in the working power supply and the non-working power supply
for example.
Regards,
Peter.
>
> Regards
>
>
>
> Rob
>
>
>
> From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
> Sent: 02 May 2023 08:19
> To: 'Mattis Lind' <mattislind(a)gmail.com>om>; rob(a)jarratt.me.uk;
'General Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org>
> Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
>
>
>
> That’s a good idea, I will try that
>
>
>
> From: Mattis Lind < <mailto:mattislind@gmail.com> mattislind(a)gmail.com>
> Sent: Tuesday, May 2, 2023 7:55 AM
> To: <mailto:rob@jarratt.me.uk> rob(a)jarratt.me.uk; General Discussion: On-Topic
and Off-Topic Posts < <mailto:cctalk@classiccmp.org> cctalk(a)classiccmp.org>
> Cc: Rob Jarratt < <mailto:robert.jarratt@ntlworld.com>
robert.jarratt(a)ntlworld.com>
> Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
>
>
>
>
>
>
>
>
>
>
> Not quite sure what you mean here. I had advice from a friend to bench test
> the control module by providing 14V to the input of the 7812. On the good
> PSU I can see the PWM operate, on the bad one the PWM is shutdown.
>
>
>
> And what happens if you feed in exactly 12 V on the output of the 7812? Would the PWM
work then? By using a lab supply you could check if the circuit is sensitive to variation
in the 12V supply.
>
>
>
>
>
> /Mattis
>
>
>
>
> Regards
>
> Rob
>
>
8 May
8 May
12:38 p.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 07 May 2023 10:34
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
Sorry for the loose language. I mean that the comparator is working correctly but the
output is not what should be expected, because its inputs are wrong.
The comments about the tolerance of the 7812 were right, it doesn’t
appear to be an issue with the replacement 7812 regulator because when
I tried using the bench PSU to feed exactly 12V to the circuit from
the output of the 7812 the comparator still gave the wrong result. It
was still wrong if I applied only 11V
What do you mean by "gave the wrong result"? If the power supply to the
comparator is "reasonable" and the comparator's output does not reflect
what is happening at it's inputs, the comparator is faulty.
If the comparator is giving the "wrong result" because it's inputs are
telling it
to, it is behaving correctly.
With no mains supply connected and a positive startup voltage applied to
Vstart and a negative startup voltage applied to the -12V line via a 2k7
resistor, you could try shorting the inputs of the comparator together and see
whether this changes the comparator's output. I want to emphasise doing
this without power going to the mains rectifier feeding the chopper so that if
there is real overload, the magic smoke will not be released.
If the output if the comparator is then "correct" and this output being
"wrong" was the source of the PSU not working, then the PWM should start
up like it does in the good power supply. This would confirm that there is a
problem in the components providing and/or mmonitoring the -12V line.
If the PWM does not start up, this suggests the problem is elsewhere.
I can tell you that if I remove the diode on the output of the -12V comparator then the
PWM operates normally.
I then looked at the value of Vz on the good and bad PSUs, when
applying 12V to the 7812 output. That was 5.4V in both the good and
bad PSUs. Where I saw a difference was on the -12V output, it was
+0.4V on the good PSU and 0.56V on the bad one (the voltage varied so
this was an average). I checked the voltage drop across the current
sense resistor. It is 0.01V on the good PSU and 0.08V on the bad PSU,
which would explain the higher positive voltage on the -12V output and the
comparator being turned on.
I am wondering if there could be a problem on the -12V output circuit
(PSU Sheet 3). I am struggling to understand the purpose of the two
transistors and the Zener diode there, but maybe one of them should be
switched on and isn’t. I am also unsure now as to which diode is doing
the rectification (to -12V). Someone said it was the one attached to
pin 6 of the transformer, but is that right? Isn’t it the Zener diode
half way across the page?
The TIP121 darlington transistor is a shunt regulator for the -12V line.
The voltage at the base of the MPSA55 transistor varies with the -12V line
because it is connected to the -12V line via the potential divider formed by
the 1k24 and 1k10 resistors. If the voltage on the -12V line increases in the
negative direction, the voltage at the base of the MPSA55 increases in
proportion to it via the potential divider. It's emitter voltage is fixed by the
zener diode so the MPSA55 is turned on more and it pulls more current
through the base of the TIP121 which results in the TIP121 conducting more
and pulling the -12V line down closer to it's correct voltage. The opposite
happens if the -12V goes lower than it should be, the TIP121 is turned on less
and this allows the -12V line to increase negatively to it's correct value.
None of this circuitry should be doing very much until the chopper
transformer is producing the source for the -12V line.
I suppose if the TIP121 is sborted or the zener diode is shorted, it could be
causing problems, however, from the test results etc we have been given so
far, I am not completely convinced there is a problem with the -12V line.
If a negative startup voltage is applied to the -12V line as well as a positive
voltage to Vstart to better simultate startup conditions, it may reveal more
about what is happening with the -12V line.
It might then be possible to compare the voltages across the zener diodes in
the working power supply and the non-working power supply for example.
Regards,
Peter.
Regards
Rob
From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Sent: 02 May 2023 08:19
To: 'Mattis Lind' <mattislind(a)gmail.com>om>; rob(a)jarratt.me.uk; 'General
Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org>
Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
That’s a good idea, I will try that
From: Mattis Lind < <mailto:mattislind@gmail.com>
mattislind(a)gmail.com>
Sent: Tuesday, May 2, 2023 7:55 AM
To: <mailto:rob@jarratt.me.uk> rob(a)jarratt.me.uk; General Discussion:
On-Topic and Off-Topic Posts < <mailto:cctalk@classiccmp.org>
cctalk(a)classiccmp.org>
Cc: Rob Jarratt < <mailto:robert.jarratt@ntlworld.com>
robert.jarratt(a)ntlworld.com>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
Not quite sure what you mean here. I had advice from a friend to bench
test the control module by providing 14V to the input of the 7812. On
the good PSU I can see the PWM operate, on the bad one the PWM is
shutdown.
And what happens if you feed in exactly 12 V on the output of the 7812?
Would the
PWM work then? By using a lab supply you could check if the
circuit is sensitive to variation in the 12V supply.
>
>
>
>
>
> /Mattis
>
>
>
>
> Regards
>
> Rob
>
>
7:27 p.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 07 May 2023 10:34
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
Sorry for the loose language. I mean that the comparator is working correctly
but the output is not what should be expected, because its inputs are wrong.
The comments about the tolerance of the 7812 were right, it doesn’t
appear to be an issue with the replacement 7812 regulator because when
I tried using the bench PSU to feed exactly 12V to the circuit from
the output of the 7812 the comparator still gave the wrong result. It
was still wrong if I applied only 11V
What do you mean by "gave the wrong result"? If the power supply to the
comparator is "reasonable" and the comparator's output does not reflect
what is happening at it's inputs, the comparator is faulty.
If the comparator is giving the "wrong result" because it's inputs are
telling it
to, it is behaving correctly.
With no mains supply connected and a positive startup voltage applied to
Vstart and a negative startup voltage applied to the -12V line via a 2k7
resistor, you could try shorting the inputs of the comparator together and see
whether this changes the comparator's output. I want to emphasise doing
this without power going to the mains rectifier feeding the chopper so that if
there is real overload, the magic smoke will not be released.
If the output if the comparator is then "correct" and this output being
"wrong" was the source of the PSU not working, then the PWM should start
up like it does in the good power supply. This would confirm that there is a
problem in the components providing and/or mmonitoring the -12V line.
If the PWM does not start up, this suggests the problem is elsewhere.
I can tell you that if I remove the diode on the output of the -12V
comparator then the PWM operates normally.
I then looked at the value of Vz on the good and bad PSUs, when
applying 12V to the 7812 output. That was 5.4V in both the good and
bad PSUs. Where I saw a difference was on the -12V output, it was
+0.4V on the good PSU and 0.56V on the bad one (the voltage varied so
this was an average). I checked the voltage drop across the current
sense resistor. It is 0.01V on the good PSU and 0.08V on the bad PSU,
which would explain the higher positive voltage on the -12V output and the
comparator being turned on.
I am wondering if there could be a problem on the -12V output circuit
(PSU Sheet 3). I am struggling to understand the purpose of the two
transistors and the Zener diode there, but maybe one of them should be
switched on and isn’t. I am also unsure now as to which diode is doing
the rectification (to -12V). Someone said it was the one attached to
pin 6 of the transformer, but is that right? Isn’t it the Zener diode
half way across the page?
The TIP121 darlington transistor is a shunt regulator for the -12V line.
The voltage at the base of the MPSA55 transistor varies with the -12V line
because it is connected to the -12V line via the potential divider formed by
the 1k24 and 1k10 resistors. If the voltage on the -12V line increases in the
negative direction, the voltage at the base of the MPSA55 increases in
proportion to it via the potential divider. It's emitter voltage is fixed by the
zener diode so the MPSA55 is turned on more and it pulls more current
through the base of the TIP121 which results in the TIP121 conducting more
and pulling the -12V line down closer to it's correct voltage. The opposite
happens if the -12V goes lower than it should be, the TIP121 is turned on less
and this allows the -12V line to increase negatively to it's correct value.
None of this circuitry should be doing very much until the chopper
transformer is producing the source for the -12V line.
I suppose if the TIP121 is sborted or the zener diode is shorted, it could be
causing problems, however, from the test results etc we have been given so
far, I am not completely convinced there is a problem with the -12V line.
If a negative startup voltage is applied to the -12V line as well as a positive
voltage to Vstart to better simultate startup conditions, it may reveal more
about what is happening with the -12V line.
It might then be possible to compare the voltages across the zener diodes in
the working power supply and the non-working power supply for example.
>
> Regards,
> Peter.
>
> >
> > Regards
> >
> >
> >
> > Rob
> >
> >
> >
> > From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
> > Sent: 02 May 2023 08:19
> > To: 'Mattis Lind' <mattislind(a)gmail.com>om>; rob(a)jarratt.me.uk;
'General
> > Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org>
> > Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
> >
> >
> >
> > That’s a good idea, I will try that
> >
> >
> >
> > From: Mattis Lind < <mailto:mattislind@gmail.com>
> > mattislind(a)gmail.com>
> > Sent: Tuesday, May 2, 2023 7:55 AM
> > To: <mailto:rob@jarratt.me.uk> rob(a)jarratt.me.uk; General Discussion:
> > On-Topic and Off-Topic Posts < <mailto:cctalk@classiccmp.org>
> > cctalk(a)classiccmp.org>
> > Cc: Rob Jarratt < <mailto:robert.jarratt@ntlworld.com>
> > robert.jarratt(a)ntlworld.com>
> > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > Not quite sure what you mean here. I had advice from a friend to bench
> > test the control module by providing 14V to the input of the 7812. On
> > the good PSU I can see the PWM operate, on the bad one the PWM is
> shutdown.
> >
> >
> >
> > And what happens if you feed in exactly 12 V on the output of the 7812?
> Would the PWM work then? By using a lab supply you could check if the
> circuit is sensitive to variation in the 12V supply.
> >
> >
> >
> >
> >
> > /Mattis
> >
> >
> >
> >
> > Regards
> >
> > Rob
> >
> >
9 May
9 May
6:24 a.m.
I don't think we can draw any conclusions from voltage measurements
around the TIP121 without the -12V line being approximately correct or at
the very least being negative.
It's collector voltage being 0V on the good PSU suggests there is no current
flowing through the parallel 20R resistors and therefore no emitter-collector
current through the TIP121 so it's not being switched on meaning that it's
driver doesn't feel the need to pull the -12V line down nearer 0V because it's
already low enough. (Maybe having a load is enough to keep it at the correct
level and the shunt regulator only starts doing stuff if the load is less than
expected or removed altogether? Or maybe the -12V line is not present when
the test was done?)
The collector voltage of the TIP121 on the bad PSU being slightly positive is
probably due to the voltage on it's emitter (ie the -12V line) being positive
instead of negative which makes it's bias conditions all wrong and any
measurements around it mostly meaningless.
I am going to read your answer more carefully later. But I wanted to check one thing. I
measured the base-emitter voltage as negative in both cases, and yet the TIP121 appears to
be conducting on the bad PSU. Surely that means that the TIP121 is not working correctly?
How about applying -12V or slightly less from a bench supply current limited
to about 150mA to the -12V line and seeing what happens? If the PSU
and/or -12V load tries to draw more than that there would seem to be a real
overload and further investigation is needed to find out what is drawing the
current.
Measure the voltage across the zener diode in the -12V regulator and
compare with the good PSU. Also compare the voltage across the parallel
20R resistors.
A larger voltage here would indicate more current being drawn through the
TIP121.
Under these conditions, there shouldn't be any voltage across the 51R
current sensing resistor for the -12V line. If there is, it suggests there could
be reverse leakage through the -12V rectifier diode.
However, Brent's calculations show that the current trip value for the -12V
line is as low as 1.3mA and I can't see any reason to disagree with his
calculations or his conclusion that this seems very low (except that another
tiny smidgen of current is available from the negative startup supply but this
won't really have any bearing on things). If this is really the case, then placing
something like a 5k6 resistor across the -12V line on the good PSU should
cause enough current to flow for the trip to operate. Finding this level of
leakage in the failed PSU is not going to be easy.
On the other hand, if this test doesn't trip it, then please look very closely at
the resistors and connections to the inputs of E3d and verify that they are as
described on the circuit diagram.
It seems very strange indeed to have a trip value as low as 1.3mA combined
with a shunt regulator whose method of regulation is to pull the voltage
down by drawing current from the supply line. Perhaps the shunt regulator
might be able to pull enough current to cause the trip to operate if the -12V
line was too high (in the negative sense) or if the shunt regulator was under
the mistaken impression that the -12V line was too high?
(This is a bit unlikely but the 115V/230V switch is set correctly, isn't it?)
On the other other hand, if the manual says that the -12V line is supposed to
be able to supply 150mA, then it doesn't make sense for the current trip to
operate at 1.3mA and we must be going wrong somewhere.
Regards,
Peter.
>
> Regards,
> Peter.
>
> >
> > Regards
> >
> >
> >
> > Rob
> >
> >
> >
> > From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
> > Sent: 02 May 2023 08:19
> > To: 'Mattis Lind' <mattislind(a)gmail.com>om>; rob(a)jarratt.me.uk;
> > 'General
> > Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org>
> > Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
> >
> >
> >
> > That’s a good idea, I will try that
> >
> >
> >
> > From: Mattis Lind < <mailto:mattislind@gmail.com>
> > mattislind(a)gmail.com>
> > Sent: Tuesday, May 2, 2023 7:55 AM
> > To: <mailto:rob@jarratt.me.uk> rob(a)jarratt.me.uk; General
Discussion:
> > On-Topic and Off-Topic Posts <
<mailto:cctalk@classiccmp.org>
> > cctalk(a)classiccmp.org>
> > Cc: Rob Jarratt < <mailto:robert.jarratt@ntlworld.com>
> > robert.jarratt(a)ntlworld.com>
> > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > Not quite sure what you mean here. I had advice from a friend to
> > bench test the control module by providing 14V to the input of the
> > 7812. On the good PSU I can see the PWM operate, on the bad one
> > the PWM is
> shutdown.
> >
> >
> >
> > And what happens if you feed in exactly 12 V on the output of the
7812?
> > Would the PWM work then? By using a lab supply you could check if
> > the circuit is sensitive to variation in the 12V supply.
> > >
> > >
> > >
> > >
> > >
> > > /Mattis
> > >
> > >
> > >
> > >
> > > Regards
> > >
> > > Rob
> > >
> > >
7 a.m.
On 2023-May-08, at 11:24 PM, Rob Jarratt via cctalk wrote:
Yes, this was acknowledged. I was explaining how it might have been the source of the
mystery 0.08V, so if/as it's not then here's another possibility:
It's conceivable there is an oscillation being generated around this -12V output
circuitry, perhaps by a bad semiconductor junction or because some of the junctions are
operating in reverse to normal under the test setup in conjunction with the caps and
inductors, the mystery voltage being generated in consequence. Have you poked around the
-12V area with a scope, as opposed to multi-meter V measurements?
I am going to read your answer more carefully later. But I wanted to check one thing. I
measured the base-emitter voltage as negative in both cases, and yet the TIP121 appears to
be conducting on the bad PSU. Surely that means that the TIP121 is not working correctly?
The TIP121 would/could be conducting because under your test setup with no VStart-12
it's biased in the reverse-from-normal direction and could be conducting in reverse
through the BC junction. The 0.016V across the 10Ω R from C-to-GND is nonetheless higher
than would be expected from the trickle current available.
Which brings me to this, in reply to earlier message:
On 2023-May-08, at 5:03 AM, Rob Jarratt wrote:
From: Brent
Hilpert <bhilpert(a)shaw.ca>
From earlier measurements and the 45uA calc of current through the 51Ω
sense resistor, the V across the 51Ω ISense-12 resistor should be only 0.002V.
So a question is where is this 0.08V coming from? An unfulfilled -12V supply
for the E3 power pin might have been an explanation, as extra current might
be drawn out of the E3d.+in input due to the +in being pulled below
(negative to) the
E3 -power pin. But you say that pin is connected to GND, so the source of the
0.08V should be sought, some more comprehensive measurements around
the E3d inputs / ISenseR might help.
I thought I had answered where the Vcc and GND are connected on the comparator. Vcc is
connected to Vstart and GND to GND. ...
4:57 p.m.
I am going to read your answer more carefully later. But I wanted to check
one thing. I measured the base-emitter voltage as negative in both cases,
and yet the TIP121 appears to be conducting on the bad PSU. Surely that
means that the TIP121 is not working correctly?
I'd be very reluctant to draw any conclusions from measurements made when
the device is not biased correctly. As Brent says, there could be current
flowing through the presumably forward biased base-collector junction.
Also, this device is a not just a straighforward transistor. It's data sheet
says it is a package containing two transistors and some other components,
including a normally reverse biased diode between the collector and emitter
terminals. A positive voltage of sufficient magnitude on it's emitter could
be forward biasing this diode resulting in current flow through it and
therefore also through the 20R resistors causing a voltage to be developed
across them.
I think a better way to determine if the TIP121 is causing excess current
draw on the -12V line is to make the voltage at it's emitter approximately
correct and see if it then draws enough current through itself to cause
the excess current trip to operate. This current can then be measured
by observing the voltage across the 20R resistors (or the smoke coming
from them if they are not of sufficient power rating...). It should also
be possible to observe the conditions around the zener diode and the MPSA55
and see if these components are behaving reasonably.
Regards,
Peter.
9:52 p.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 09 May 2023 17:58
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
I will do all the suggested checks, but I won't be able to do this for a number of
days. However, I wanted to understand something in the meantime. The conditions I am
applying are (I think!) what would happen during startup, and during startup the control
board has to make the PWM run, otherwise the main switching transistor won't operate
and the transformer won't operate to produce the -12V in the first place. So surely in
the startup condition I am applying (which is to supply Vstart from a bench PSU) is valid?
Where is the flaw in my reasoning here?
I am going to read your answer more carefully later. But I wanted to
check one thing. I measured the base-emitter voltage as negative in
both cases, and yet the TIP121 appears to be conducting on the bad
PSU. Surely that means that the TIP121 is not working correctly?
I'd be very reluctant to draw any conclusions from measurements made
when the device is not biased correctly. As Brent says, there could be current
flowing through the presumably forward biased base-collector junction.
Also, this device is a not just a straighforward transistor. It's data sheet says
it
is a package containing two transistors and some other components,
including a normally reverse biased diode between the collector and emitter
terminals. A positive voltage of sufficient magnitude on it's emitter could be
forward biasing this diode resulting in current flow through it and therefore
also through the 20R resistors causing a voltage to be developed across
them.
I think a better way to determine if the TIP121 is causing excess current draw
on the -12V line is to make the voltage at it's emitter approximately correct
and see if it then draws enough current through itself to cause the excess
current trip to operate. This current can then be measured by observing the
voltage across the 20R resistors (or the smoke coming from them if they are
not of sufficient power rating...). It should also be possible to observe the
conditions around the zener diode and the MPSA55 and see if these
components are behaving reasonably.
Regards,
Peter.
10 May
10 May
11:03 a.m.
I will do all the suggested checks, but I won't be able to do this for a
number of days. However, I wanted to understand something in the meantime.
The conditions I am applying are (I think!) what would happen during
startup, and during startup the control board has to make the PWM run,
otherwise the main switching transistor won't operate and the transformer
won't operate to produce the -12V in the first place. So surely in the
startup condition I am applying (which is to supply Vstart from a bench PSU)
is valid? Where is the flaw in my reasoning here?
The PSU also gives a kick start to the -12V line by applying some negative
voltage probably around -15V to it via a 2k7 resistor (on sheet 1). If
you are not providing this, perhaps this is why the -12V line is able to
swing slightly positive and upset conditions in the -12V current sense
circuit enough to prevent the PWM from starting?
Even if you are providing this resistor limited -12V startup supply, could
it be insufficient due to a faulty component leaking current from the
-12V line to ground meaning that the -12V line does not become sufficiently
negative to fulfil the startup conditions?
Could the PWM be managing to start up and run for a cycle or two and then
stop due to a fault in the PWM circuit somehow attempting to draw too much
current from the -12V line? I don't see any connections from the -12V line
to the PWM so this is probably not the case.
Is this the same PSU whose chopper transistor exploded a while back? Could
there be any carbon deposits remaining on the board or conductive remnants
wedged under components etc causing leakage from the -12V line to ground?
Regards,
Peter.
19 May
19 May
9:24 p.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 10 May 2023 12:04
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
So, I did the following test. I used my bench PSU to apply +12V to the regulator output
and -12V to the actual -12V output. Under these conditions the PWM operated correctly. I
repeated the test on the good PSU and the result was the same.
The good PSU drew 13mA on the -12V line and 92mA from the regulator side. The bad PSU was
16mA and 86mA respectively. When I test only applying +12V to the regulator output, the
good PSU draws about 90mA and the bad one 140mA.
It is worth repeating that the PWM operates correctly on the good PSU when I only apply
+12V to the regulator, but not on the bad one. I guess there must be some kind of short
somewhere, but really not sure where it might be. It has to be something that is pulling
the -12V line slightly higher. The only place where this seems like it could happen is
around E1b I think. Perhaps there is a path through the positive input to E1b to Vcc on
the LM393 (Control Module Sheet 1). Does that seem logical?
I will do all the suggested checks, but I won't be able to do this for
a number of days. However, I wanted to understand something in the
meantime.
The conditions I am applying are (I think!) what
would happen during
startup, and during startup the control board has to make the PWM run,
otherwise the main switching transistor won't operate and the
transformer won't operate to produce the -12V in the first place. So
surely in the startup condition I am applying (which is to supply
Vstart from a bench PSU) is valid? Where is the flaw in my reasoning here?
The PSU also gives a kick start to the -12V line by applying some negative
voltage probably around -15V to it via a 2k7 resistor (on sheet 1). If you are
not providing this, perhaps this is why the -12V line is able to swing slightly
positive and upset conditions in the -12V current sense circuit enough to
prevent the PWM from starting?
Even if you are providing this resistor limited -12V startup supply, could it be
insufficient due to a faulty component leaking current from the -12V line to
ground meaning that the -12V line does not become sufficiently negative to
fulfil the startup conditions?
Could the PWM be managing to start up and run for a cycle or two and then
stop due to a fault in the PWM circuit somehow attempting to draw too much
current from the -12V line? I don't see any connections from the -12V line to
the PWM so this is probably not the case.
Is this the same PSU whose chopper transistor exploded
a while back? Could
there be any carbon deposits remaining on the board or conductive
remnants wedged under components etc causing leakage from the -12V line
to ground?
The component nearest to the exploded transistor is the 10uF capacitor on the output of
the 12V regulator. There are some carbon deposits on it. I did a cursory check for
resistance and ESR and it seemed OK.
Regards
Rob
20 May
20 May
8:19 a.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 10 May 2023 12:04
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
So, I did the following test. I used my bench PSU to apply +12V to the
regulator output and -12V to the actual -12V output. Under these conditions
the PWM operated correctly. I repeated the test on the good PSU and the
result was the same.
The good PSU drew 13mA on the -12V line and 92mA from the regulator side.
The bad PSU was 16mA and 86mA respectively. When I test only applying +12V
to the regulator output, the good PSU draws about 90mA and the bad one 140mA.
It is worth repeating that the PWM operates correctly on the good PSU when
I only apply +12V to the regulator, but not on the bad one. I guess there
must be some kind of short somewhere, but really not sure where it might be.
It has to be something that is pulling the -12V line slightly higher. The
only place where this seems like it could happen is around E1b I think.
Perhaps there is a path through the positive input to E1b to Vcc on the
LM393 (Control Module Sheet 1). Does that seem logical?
I will do all the suggested checks, but I won't be able to do this for
a number of days. However, I wanted to understand something in the
meantime.
The conditions I am applying are (I think!) what
would happen during
startup, and during startup the control board has to make the PWM run,
otherwise the main switching transistor won't operate and the
transformer won't operate to produce the -12V in the first place. So
surely in the startup condition I am applying (which is to supply
Vstart from a bench PSU) is valid? Where is the flaw in my reasoning here?
The PSU also gives a kick start to the -12V line by applying some negative
voltage probably around -15V to it via a 2k7 resistor (on sheet 1). If you are
not providing this, perhaps this is why the -12V line is able to swing slightly
positive and upset conditions in the -12V current sense circuit enough to
prevent the PWM from starting?
Even if you are providing this resistor limited -12V startup supply, could it be
insufficient due to a faulty component leaking current from the -12V line to
ground meaning that the -12V line does not become sufficiently negative to
fulfil the startup conditions?
Could the PWM be managing to start up and run for a cycle or two and then
stop due to a fault in the PWM circuit somehow attempting to draw too much
current from the -12V line? I don't see any connections from the -12V line to
the PWM so this is probably not the case.
Is this the same PSU whose chopper transistor exploded a while back? Could
there be any carbon deposits remaining on the board or conductive
remnants wedged under components etc causing leakage from the -12V line
to ground?
The component nearest to the exploded transistor is the 10uF capacitor on
the output of the 12V regulator. There are some carbon deposits on it. I did
a cursory check for resistance and ESR and it seemed OK.
Regards
Rob
25 May
25 May
8:43 p.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 20 May 2023 09:20
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
Ok, it looks like there is not a severe leak from the -12V line to ground then.
I am puzzled by the extra current draw on Vstart by the bad PSU but I'm not
sure that tracking this down would lead us to the real problem.
On the other hand, did you mention at one point that Vstart was varying?
If this is the case, the reason for this would probably need to be found and
fixed independent of whether it leads to finding the main problem as this is
supposed to be a stable supply.
I don't think there is likely to be any serious leakage via E1b because the link
to the -12V line is via a 75K resistor which would limit any leakage current to
roughly 160uA. Of course this applies if the resistor really is 75K and doesn't
have carbon deposits bridging the tracks and connections around it to
somewhere else.
I would suggest looking carefully at the resistors around E3d to make sure
they have the correct values, especially the 360K resistor and making sure
there is no debris etc around these components that could be bridging any
connections associated with them to somewhere else, also that no
connections have been severed. Problems here could be leading to E3d
falsely triggering when there is no real overload.
It might be useful to check the voltages and resistor values in the -12V
regulator and compare with same in the good power supply, especially the
voltage across the zener diode.
This capacitor is probably there to ensure the 7812 doesn't oscillate. Looking
at Vstart with an oscilloscope should confirm that this is not an issue. If it
doesn't have excessive leakage current and has approximately the correct
capacitance, it is probably ok. However, if there is gunk trapped underneath
it around the leads, this might account for the extra current draw on Vstart.
The explosion could have had other bad effects. Maybe E3 got damaged by a
surge in its power supply when the transistor blew up? Maybe the -12V
rectifier was affected? It is probably not as robust as the rectifiers for the
other lines and the chopper transistor shorting would have likely caused a
big current pulse in the chopper transformer primary, leading in turn to
surges at it's secondaries. Also the diode in parallel with the 51R sense
resistor might be suspect.
I'm not sure how to test these components comprehensively without trying
replacements for them.
If the 7812 was damaged at the time of the explosion, other components
powered from Vstart could have experienced surges as well. Maybe stuff on
the input side of the 7812 too?
This evening I went to check Vstart for any oscillation. However, all of a sudden, the
current draw is down to 85mA and PWM has started working. I am at a loss to explain it. I
wondered if there might be a dry joint, but I have tried a few light taps and shakes and
it continues to work. Perhaps your idea of some debris causing a short might explain it,
otherwise I just don't know.
I am thinking I may put it back together and test with a light bulb in series.
Regards
Rob
>
> Is this the same PSU whose chopper transistor exploded a while back?
> Could there be any carbon deposits remaining on the board or
> conductive remnants wedged under components etc causing leakage
from
the -12V
line to ground?
The component nearest to the exploded transistor is the 10uF capacitor
on the output of the 12V regulator. There are some carbon deposits on
it. I did a cursory check for resistance and ESR and it seemed OK.
10:43 p.m.
This evening I went to check Vstart for any oscillation. However, all of a
sudden, the current draw is down to 85mA and PWM has started working. I am
at a loss to explain it. I wondered if there might be a dry joint, but I
have tried a few light taps and shakes and it continues to work. Perhaps
your idea of some debris causing a short might explain it, otherwise I just
don't know.
This one is a real doozie :-(
I am thinking I may put it back together and test with a light bulb in series.
Go for it. I can't think of anything else to suggest trying at the moment.
Regards
Peter.
26 May
26 May
4:24 p.m.
On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote:
This evening I went to check Vstart for any oscillation. However, all of a sudden, the
current draw is down to 85mA and PWM has started working. I am at a loss to explain it. I
wondered if there might be a dry joint, but I have tried a few light taps and shakes and
it continues to work. Perhaps your idea of some debris causing a short might explain it,
otherwise I just don't know.
Operation with only VStart+12 places the circuitry into an unspecified operating region -
a region outside of the design intentions. In part, several semiconductor junctions and
portions of circuitry are polarised opposite to their normal/designed-for state. It is not
surprising that you are seeing odd/unpredictable behaviour under this operating
environment, nor is it surprising that it's different than the 'good' supply
under the same operating environment.
So why was it in shutdown earlier the other day but not now? :
Who knows - it's operating in an unspecified region. Perhaps the room temperature is 2
degrees higher. That's a serious point, not phase-of-the-moon satire.
When you supplied the proper startup environment with both Vstart+12 and Vstart-12 both
the bad and good unit behaved as expected for the design.
Why is the VStart+12 current draw higher when it was in shutdown versus when the PWM
controller IC is pulsing? :
Because in shutdown the 'Chopper Driver' transistor (PSU Sheet 2) is held hard ON
(conducting) (see datasheet).
Holding this transistor ON subtracts it's off-state current (~ 17mA) but adds it's
on-state base current (~ 37mA) and it's on-state collector current (~ 73mA), for a net
up-to ~ 93mA increase (may be less dependant on duty cycle of PWM), to the Vstart+12
current.
There remain two unexplained things here:
- Where was that unusual current-sense voltage that sends it into shutdown coming from?
(I provided one potential explanation earlier, but it remains unknown at this time).
- The 51-ohm current-sense resistor in the -12V supply vs the mode of operation of
the -12 supply remains unexplained/non-sensical. At the max current you mentioned
(150mA),
the V drop across that R would be >7V (!), which makes no sense. If I had it in hand,
I'd be
double-checking the drawing of that current-sense circuit around the 51-ohm R as a
start.
But this is not to say that either of these has anything to do with the fault you were/are
dealing with, they're just things that aren't understood at this point. Either or
both could be pursued out of curiosity or for the sake of completeness.
I am thinking I may put it back together and test with
a light bulb in series.
25 Jun
25 Jun
12:36 p.m.
Hello,
It has taken me ages to get back to this, but I think I have hit an
insurmountable problem.
I reassembled the PSU after the PWM started working again and tested it with
a dummy load and using two 100W light bulbs in series on the input side to
limit the current. However, it did not work. I traced this to the 7812
regulator that supplies Vstart (PSU Sheet 1) not getting enough voltage to
run, about 3.7VDC. In the same conditions the working one gets about 10 or
11VDC. I also noticed that on the good PSU the 100W bulbs pulsed and were
fairly dim, but on the bad PSU they were brighter and glowed steadily.
This made me suspect that the startup transformer may have been damaged. So
I checked the output of the startup transformer. On the bad PSU this was
about 2.4VAC, while on the working one it was about 12VAC. I lifted all 4 of
the diodes connected to the startup transformer and they all tested OK.
Sadly, I think this means the transformer must have been damaged. I have the
facility to do a ring test on the transformer, but don't want to remove it
unless there is a need to. Is there any other failure mode that I am not
considering? I didn't check the inputs to the transformer, so something
could be short on the input side and pulling the input to the startup
transformer down. It is worth reminding anyone reading this, that the 7812
on PSU Sheet 1 did fail with a short to ground.
I have included a couple of minor responses to Brent's last email to me
below.
Regards
Rob
-----Original Message-----
From: Brent Hilpert <bhilpert(a)shaw.ca>
Sent: 26 May 2023 17:24
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Rainbow H7842 PSU Fault
On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote:
>
> This evening I went to check Vstart for any oscillation. However, all of
a
sudden, the current draw is down to 85mA and PWM has
started working. I
am at a loss to explain it. I wondered if there might be a dry joint, but
I have
tried a few light taps and shakes and it continues to
work. Perhaps your
idea
of some debris causing a short might explain it,
otherwise I just don't
know.
Operation with only VStart+12 places the circuitry into an unspecified
operating region - a region outside of the design intentions. In part,
several
semiconductor junctions and portions of circuitry are
polarised opposite
to
their normal/designed-for state. It is not surprising
that you are seeing
odd/unpredictable behaviour under this operating environment, nor is it
surprising that it's different than the 'good' supply under the same
operating
environment.
So why was it in shutdown earlier the other day but not now? :
Who knows - it's operating in an unspecified region. Perhaps the room
temperature is 2 degrees higher. That's a serious point, not phase-of-the-
moon satire.
Well, the weather here has indeed got a bit warmer.
When you supplied the proper startup environment with both Vstart+12 and
Vstart-12 both the bad and good unit behaved as expected for the design.
Why is the VStart+12 current draw higher when it was in shutdown versus
when the PWM controller IC is pulsing? :
Because in shutdown the 'Chopper Driver' transistor (PSU Sheet 2) is held
hard ON (conducting) (see datasheet).
Holding this transistor ON subtracts it's off-state current (~ 17mA) but
adds
it's on-state base current (~ 37mA) and it's
on-state collector current (~
73mA), for a net up-to ~ 93mA increase (may be less dependant on duty
cycle of PWM), to the Vstart+12 current.
Thanks for explaining that, it makes more sense now. I need to learn to look
at other parts of the schematic beyond the immediate thing I am interested
in because I keep missing things like this.
There remain two unexplained things here:
- Where was that unusual current-sense voltage that sends it into
shutdown coming from?
(I provided one potential explanation earlier, but it remains
unknown
at this time).
- The 51-ohm current-sense resistor in the -12V supply vs the mode
of operation of
the -12 supply remains unexplained/non-sensical. At the max current
you mentioned (150mA),
the V drop across that R would be >7V (!), which makes no sense. If
I
had it in hand, I'd be
double-checking the drawing of that current-sense circuit around the
51-ohm R as a start.
Agreed that this does not make a lot of sense. I have checked the actual
value of the resistor (in circuit) and it matches the schematic. I have also
checked the schematic against the actual circuit and it really does seem
like the schematic is correct.
But this is not to say that either of these has anything to do with the
fault you
were/are dealing with, they're just things that
aren't understood at this
point.
Either or both could be pursued out of curiosity or
for the sake of
completeness.
> I am thinking I may put it back together and test with a light bulb in
series.
1:20 p.m.
On Sun, Jun 25, 2023 at 1:36 PM Rob Jarratt via cctalk
<cctalk(a)classiccmp.org> wrote:
Hello,
It has taken me ages to get back to this, but I think I have hit an
insurmountable problem.
I reassembled the PSU after the PWM started working again and tested it with
a dummy load and using two 100W light bulbs in series on the input side to
limit the current. However, it did not work. I traced this to the 7812
regulator that supplies Vstart (PSU Sheet 1) not getting enough voltage to
run, about 3.7VDC. In the same conditions the working one gets about 10 or
11VDC. I also noticed that on the good PSU the 100W bulbs pulsed and were
fairly dim, but on the bad PSU they were brighter and glowed steadily.
This made me suspect that the startup transformer may have been damaged. So
I checked the output of the startup transformer. On the bad PSU this was
about 2.4VAC, while on the working one it was about 12VAC. I lifted all 4 of
the diodes connected to the startup transformer and they all tested OK.
Sadly, I think this means the transformer must have been damaged. I have the
facility to do a ring test on the transformer, but don't want to remove it
unless there is a need to. Is there any other failure mode that I am not
considering? I didn't check the inputs to the transformer, so something
could be short on the input side and pulling the input to the startup
transformer down. It is worth reminding anyone reading this, that the 7812
on PSU Sheet 1 did fail with a short to ground.
I wouldn't be too hasty to condemn the transformer.
This is a mains transformer, not part of a switch mode power supply
circuit. It takes in mains via the voltage selector switch, the centre
tapped secondary is full-wave rectified twice to produce the +12V and
-12V rails needed to get the rest of the supply going.
Now, you mention the lamp limiter bulbs (series light bulbs, whatever
you call them) are bright. That means they are dropping significant
voltage. So the input voltage to the startup transformer will be less
than mains. Which will lead to a low output from its secondary
winding.
Try disconnectng the mains feed to the main chopper circuit. Perhaps
remove the mains bridge rectifier, or both of the inrush limiter
thermistors. Anything to prevent power getting to the main chopper
circuit. Power up again with the lamp limiter in series with the mains
(of course). My guess is that then the bulbs will be dark and the
startup transformer will give the right voltages. This would point to
a problem with the chopper circuit.
-tony
I have included a couple of minor responses to Brent's last email to me
below.
Regards
Rob
-----Original Message-----
From: Brent Hilpert <bhilpert(a)shaw.ca>
Sent: 26 May 2023 17:24
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Rainbow H7842 PSU Fault
On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote:
> This evening I went to check Vstart
for any oscillation. However, all of
a
sudden, the current draw is down to 85mA and PWM
has started working. I
am at a loss to explain it. I wondered if there might be a dry joint, but
I have
tried a few light taps and shakes and it
continues to work. Perhaps your
idea
of some debris causing a short might explain it,
otherwise I just don't
know.
Operation with only VStart+12 places the circuitry into an unspecified
operating region - a region outside of the design intentions. In part,
several
semiconductor junctions and portions of circuitry
are polarised opposite
to
their normal/designed-for state. It is not
surprising that you are seeing
odd/unpredictable behaviour under this operating environment, nor is it
surprising that it's different than the 'good' supply under the same
operating
environment.
So why was it in shutdown earlier the other day but not now? :
Who knows - it's operating in an unspecified region. Perhaps the room
temperature is 2 degrees higher. That's a serious point, not phase-of-the-
moon satire.
Well, the weather here has indeed got a bit warmer.
When you supplied the proper startup environment with both Vstart+12 and
Vstart-12 both the bad and good unit behaved as expected for the design.
Why is the VStart+12 current draw higher when it was in shutdown versus
when the PWM controller IC is pulsing? :
Because in shutdown the 'Chopper Driver' transistor (PSU Sheet 2) is held
hard ON (conducting) (see datasheet).
Holding this transistor ON subtracts it's off-state current (~ 17mA) but
adds
it's on-state base current (~ 37mA) and
it's on-state collector current (~
73mA), for a net up-to ~ 93mA increase (may be less dependant on duty
cycle of PWM), to the Vstart+12 current.
Thanks for explaining that, it makes more sense now. I need to learn to look
at other parts of the schematic beyond the immediate thing I am interested
in because I keep missing things like this.
There remain two unexplained things here:
- Where was that unusual current-sense voltage that sends it into
shutdown coming from?
(I provided one potential explanation earlier, but it remains
unknown
at this time).
- The 51-ohm current-sense resistor in the -12V supply vs the mode
of operation of
the -12 supply remains unexplained/non-sensical. At the max current
you mentioned (150mA),
the V drop across that R would be >7V (!), which makes no sense. If
I
had it in hand, I'd be
double-checking the drawing of that current-sense circuit around the
51-ohm R as a start.
Agreed that this does not make a lot of sense. I have checked the actual
value of the resistor (in circuit) and it matches the schematic. I have also
checked the schematic against the actual circuit and it really does seem
like the schematic is correct.
But this is not to say that either of these has anything to do with the
fault you
were/are dealing with, they're just things
that aren't understood at this
point.
Either or both could be pursued out of curiosity
or for the sake of
completeness.
> I am thinking I may put it back together and test with a light bulb in
series.
4:33 p.m.
Thanks Tony. I removed the mains bridge rectifier and found it measured short across two
of its terminals, so I am ordering a replacement. I tested the PSU, without the mains
bridge rectifier and without the diodes on the output of the startup transformer. I got
24VAC on the output of the , the lamp limiter bulbs did not light and the fan in the power
switch unit turned. So it looks like the startup transformer is actually OK.
Regards
Rob
.> -----Original Message-----
From: Tony Duell <ard.p850ug1(a)gmail.com>
Sent: 25 June 2023 14:21
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Brent Hilpert <bhilpert(a)shaw.ca>ca>; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
On Sun, Jun 25, 2023 at 1:36 PM Rob Jarratt via cctalk
<cctalk(a)classiccmp.org> wrote:
Hello,
It has taken me ages to get back to this, but I think I have hit an
insurmountable problem.
I reassembled the PSU after the PWM started working again and tested
it with a dummy load and using two 100W light bulbs in series on the
input side to limit the current. However, it did not work. I traced
this to the 7812 regulator that supplies Vstart (PSU Sheet 1) not
getting enough voltage to run, about 3.7VDC. In the same conditions
the working one gets about 10 or 11VDC. I also noticed that on the
good PSU the 100W bulbs pulsed and were fairly dim, but on the bad PSU
they were
brighter and glowed steadily.
This made me suspect that the startup transformer may have been
damaged. So I checked the output of the startup transformer. On the
bad PSU this was about 2.4VAC, while on the working one it was about
12VAC. I lifted all 4 of the diodes connected to the startup transformer and
they
all tested OK.
Sadly, I think this means the transformer must have been damaged. I
have the facility to do a ring test on the transformer, but don't want
to remove it unless there is a need to. Is there any other failure
mode that I am not considering? I didn't check the inputs to the
transformer, so something could be short on the input side and pulling
the input to the startup transformer down. It is worth reminding
anyone reading this, that the 7812 on PSU Sheet 1 did fail with a short to
ground.
I wouldn't be too hasty to condemn the transformer.
This is a mains transformer, not part of a switch mode power supply circuit. It
takes in mains via the voltage selector switch, the centre tapped secondary is
full-wave rectified twice to produce the +12V and -12V rails needed to get the
rest of the supply going.
Now, you mention the lamp limiter bulbs (series light bulbs, whatever you call
them) are bright. That means they are dropping significant voltage. So the
input voltage to the startup transformer will be less than mains. Which will
lead to a low output from its secondary winding.
Try disconnectng the mains feed to the main chopper circuit. Perhaps remove
the mains bridge rectifier, or both of the inrush limiter thermistors. Anything
to prevent power getting to the main chopper circuit. Power up again with
the lamp limiter in series with the mains (of course). My guess is that then
the bulbs will be dark and the startup transformer will give the right voltages.
This would point to a problem with the chopper circuit.
-tony
I have included a couple of minor responses to Brent's last email to
me below.
Regards
Rob
design.
> >
> > Why is the VStart+12 current draw higher when it was in shutdown
> > versus when the PWM controller IC is pulsing? :
> > Because in shutdown the 'Chopper Driver' transistor (PSU Sheet 2) is
> > held hard ON (conducting) (see datasheet).
> > Holding this transistor ON subtracts it's off-state current (~ 17mA)
> > but
> adds
> > it's on-state base current (~ 37mA) and it's on-state collector
> > current (~ 73mA), for a net up-to ~ 93mA increase (may be less
> > dependant on duty cycle of PWM), to the Vstart+12 current.
> >
>
> Thanks for explaining that, it makes more sense now. I need to learn
> to look at other parts of the schematic beyond the immediate thing I
> am interested in because I keep missing things like this.
>
> > There remain two unexplained things here:
> > - Where was that unusual current-sense voltage that sends it
> > into shutdown coming from?
> > (I provided one potential explanation earlier, but it remains
> unknown
> > at this time).
> >
> > - The 51-ohm current-sense resistor in the -12V supply vs the
> > mode of operation of
> > the -12 supply remains unexplained/non-sensical. At the max
> > current you mentioned (150mA),
> > the V drop across that R would be >7V (!), which makes no
> > sense. If
> I
> > had it in hand, I'd be
> > double-checking the drawing of that current-sense circuit
> > around the 51-ohm R as a start.
>
> Agreed that this does not make a lot of sense. I have checked the
> actual value of the resistor (in circuit) and it matches the
> schematic. I have also checked the schematic against the actual
> circuit and it really does seem like the schematic is correct.
>
> >
> > But this is not to say that either of these has anything to do with
> > the
> fault you
> > were/are dealing with, they're just things that aren't understood at
> > this
> point.
> > Either or both could be pursued out of curiosity or for the sake of
> > completeness.
> >
> >
> > > I am thinking I may put it back together and test with a light
> > > bulb in
> series.
> >
>
> -----Original Message-----
From: Brent Hilpert <bhilpert(a)shaw.ca>
Sent: 26 May 2023 17:24
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic
Posts <cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Rainbow H7842 PSU Fault
On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote:
>
> This evening I went to check Vstart for any oscillation. However,
> all of
a
sudden, the current draw is down to 85mA and PWM
has started
working. I am at a loss to explain it. I wondered if there might be
a dry joint, but
I have
tried a few light taps and shakes and it
continues to work. Perhaps
your
idea
of some debris causing a short might explain it,
otherwise I just
don't
know.
Operation with only VStart+12 places the circuitry into an
unspecified operating region - a region outside of the design
intentions. In part,
several
semiconductor junctions and portions of circuitry
are polarised
opposite
to
their normal/designed-for state. It is not
surprising that you are
seeing odd/unpredictable behaviour under this operating environment,
nor is it surprising that it's different than the 'good' supply
under the same
operating
environment.
So why was it in shutdown earlier the other day but not now? :
Who knows - it's operating in an unspecified region. Perhaps the
room temperature is 2 degrees higher. That's a serious point, not
phase-of-the- moon satire.
Well, the weather here has indeed got a bit warmer.
>
> When you supplied the proper startup environment with both Vstart+12
> and
> Vstart-12 both the bad and good unit behaved as expected for the
4:51 p.m.
On Sun, Jun 25, 2023 at 5:33 PM Rob Jarratt <robert.jarratt(a)ntlworld.com> wrote:
Thanks Tony. I removed the mains bridge rectifier and found it measured short across two
of its terminals, so I am ordering a replacement. I tested the PSU, without the mains
bridge rectifier and without the diodes on the output of the startup transformer. I got
24VAC on the output of the , the lamp limiter bulbs did not light and the fan in the power
switch unit turned. So it looks like the startup transformer is actually OK.
It's possible that the rectifier just failed. Or it might have been
damaged because it had to pass too much current due to a problem in
the main chopper circuit. I would certainly check the chopper
transistor for shorts now.
-tony
5:05 p.m.
Yes the chopper did blow up and I have since replaced it. So hopefully this is the last
part that needs to be replaced!
Regards
Rob
-----Original Message-----
From: Tony Duell <ard.p850ug1(a)gmail.com>
Sent: 25 June 2023 17:52
To: rob(a)jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>rg>; Brent Hilpert <bhilpert(a)shaw.ca>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
On Sun, Jun 25, 2023 at 5:33 PM Rob Jarratt <robert.jarratt(a)ntlworld.com>
wrote:
It's possible that the rectifier just failed. Or it might have been damaged
because it had to pass too much current due to a problem in the main
chopper circuit. I would certainly check the chopper transistor for shorts
now.
-tony
Thanks Tony. I removed the mains bridge rectifier and found it measured
short
across two of its terminals, so I am ordering a replacement. I tested the
PSU, without the mains bridge rectifier and without the diodes on the output
of the startup transformer. I got 24VAC on the output of the , the lamp limiter
bulbs did not light and the fan in the power switch unit turned. So it looks like
the startup transformer is actually OK.
4:52 p.m.
Before I confuse matters too much I should point out that I was not using the Rainbow
power switch unit but one from a DECmate. I had erroneously thought there was a fault with
the Rainbow one. I think the fan supply may be different on the DECmate switch, so ignore
the bit about the fan turning.
Regards
Rob
-----Original Message-----
From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Sent: 25 June 2023 17:34
To: 'Tony Duell' <ard.p850ug1(a)gmail.com>om>; 'rob(a)jarratt.me.uk'
<rob(a)jarratt.me.uk>uk>; 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: 'Brent Hilpert' <bhilpert(a)shaw.ca>
Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
Thanks Tony. I removed the mains bridge rectifier and found it measured
short across two of its terminals, so I am ordering a replacement. I tested the
PSU, without the mains bridge rectifier and without the diodes on the output
of the startup transformer. I got 24VAC on the output of the , the lamp limiter
bulbs did not light and the fan in the power switch unit turned. So it looks like
the startup transformer is actually OK.
Regards
Rob
.> -----Original Message-----
From: Tony Duell <ard.p850ug1(a)gmail.com>
Sent: 25 June 2023 14:21
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic
Posts <cctalk(a)classiccmp.org>
Cc: Brent Hilpert <bhilpert(a)shaw.ca>ca>; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
On Sun, Jun 25, 2023 at 1:36 PM Rob Jarratt via cctalk
<cctalk(a)classiccmp.org> wrote:
transformer will give the right voltages.
> This would point to a problem with the chopper circuit.
>
> -tony
>
>
>
>
> >
> > I have included a couple of minor responses to Brent's last email to
> > me below.
> >
> > Regards
> >
> > Rob
> >
> > > -----Original Message-----
> > > From: Brent Hilpert <bhilpert(a)shaw.ca>
> > > Sent: 26 May 2023 17:24
> > > To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic
> > > Posts <cctalk(a)classiccmp.org>
> > > Subject: Re: [cctalk] Rainbow H7842 PSU Fault
> > >
> > > On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote:
> > > >
> > > > This evening I went to check Vstart for any oscillation.
> > > > However, all of
> > a
> > > sudden, the current draw is down to 85mA and PWM has started
> > > working. I am at a loss to explain it. I wondered if there might
> > > be a dry joint, but
> > I have
> > > tried a few light taps and shakes and it continues to work.
> > > Perhaps your
> > idea
> > > of some debris causing a short might explain it, otherwise I just
> > > don't
> > know.
> > >
> > > Operation with only VStart+12 places the circuitry into an
> > > unspecified operating region - a region outside of the design
> > > intentions. In part,
> > several
> > > semiconductor junctions and portions of circuitry are polarised
> > > opposite
> > to
> > > their normal/designed-for state. It is not surprising that you are
> > > seeing odd/unpredictable behaviour under this operating
> > > environment, nor is it surprising that it's different than the
> > > 'good' supply under the same
> > operating
> > > environment.
> > >
> > > So why was it in shutdown earlier the other day but not now? :
> > > Who knows - it's operating in an unspecified region. Perhaps the
> > > room temperature is 2 degrees higher. That's a serious point, not
> > > phase-of-the- moon satire.
> >
> > Well, the weather here has indeed got a bit warmer.
> >
> > >
> > > When you supplied the proper startup environment with both
> > > Vstart+12 and
> > > Vstart-12 both the bad and good unit behaved as expected for the
> design.
> > >
> > > Why is the VStart+12 current draw higher when it was in shutdown
> > > versus when the PWM controller IC is pulsing? :
> > > Because in shutdown the 'Chopper Driver' transistor (PSU Sheet 2)
> > > is held hard ON (conducting) (see datasheet).
> > > Holding this transistor ON subtracts it's off-state current (~
> > > 17mA) but
> > adds
> > > it's on-state base current (~ 37mA) and it's on-state collector
> > > current (~ 73mA), for a net up-to ~ 93mA increase (may be less
> > > dependant on duty cycle of PWM), to the Vstart+12 current.
> > >
> >
> > Thanks for explaining that, it makes more sense now. I need to learn
> > to look at other parts of the schematic beyond the immediate thing I
> > am interested in because I keep missing things like this.
> >
> > > There remain two unexplained things here:
> > > - Where was that unusual current-sense voltage that sends it
> > > into shutdown coming from?
> > > (I provided one potential explanation earlier, but it
> > > remains
> > unknown
> > > at this time).
> > >
> > > - The 51-ohm current-sense resistor in the -12V supply vs
> > > the mode of operation of
> > > the -12 supply remains unexplained/non-sensical. At the max
> > > current you mentioned (150mA),
> > > the V drop across that R would be >7V (!), which makes no
> > > sense. If
> > I
> > > had it in hand, I'd be
> > > double-checking the drawing of that current-sense circuit
> > > around the 51-ohm R as a start.
> >
> > Agreed that this does not make a lot of sense. I have checked the
> > actual value of the resistor (in circuit) and it matches the
> > schematic. I have also checked the schematic against the actual
> > circuit and it really does seem like the schematic is correct.
> >
> > >
> > > But this is not to say that either of these has anything to do
> > > with the
> > fault you
> > > were/are dealing with, they're just things that aren't understood
> > > at this
> > point.
> > > Either or both could be pursued out of curiosity or for the sake
> > > of completeness.
> > >
> > >
> > > > I am thinking I may put it back together and test with a light
> > > > bulb in
> > series.
> > >
> >
> >
Hello,
It has taken me ages to get back to this, but I think I have hit an
insurmountable problem.
I reassembled the PSU after the PWM started working again and tested
it with a dummy load and using two 100W light bulbs in series on the
input side to limit the current. However, it did not work. I traced
this to the 7812 regulator that supplies Vstart (PSU Sheet 1) not
getting enough voltage to run, about 3.7VDC. In the same conditions
the working one gets about 10 or 11VDC. I also noticed that on the
good PSU the 100W bulbs pulsed and were fairly dim, but on the bad
PSU
they were brighter and glowed steadily.
This made me suspect that the startup transformer may have been
damaged. So I checked the output of the startup transformer. On the
bad PSU this was about 2.4VAC, while on the working one it was about
12VAC. I lifted all 4 of the diodes connected to the startup
transformer and
they all tested OK.
Sadly, I think this means the transformer must have been damaged. I
have the facility to do a ring test on the transformer, but don't
want to remove it unless there is a need to. Is there any other
failure mode that I am not considering? I didn't check the inputs to
the transformer, so something could be short on the input side and
pulling the input to the startup transformer down. It is worth
reminding anyone reading this, that the 7812 on PSU Sheet 1 did fail
with a short to
ground.
I wouldn't be too hasty to condemn the transformer.
This is a mains transformer, not part of a switch mode power supply
circuit. It takes in mains via the voltage selector switch, the centre
tapped secondary is full-wave rectified twice to produce the +12V and
-12V rails needed to get the rest of the supply going.
Now, you mention the lamp limiter bulbs (series light bulbs, whatever
you call
them) are bright. That means they are dropping significant voltage. So
the input voltage to the startup transformer will be less than mains.
Which will lead to a low output from its secondary winding.
Try disconnectng the mains feed to the main chopper circuit. Perhaps
remove the mains bridge rectifier, or both of the inrush limiter
thermistors. Anything to prevent power getting to the main chopper
circuit. Power up again with the lamp limiter in series with the mains
(of course). My guess is that then the bulbs will be dark and the startup
5 p.m.
On Sun, Jun 25, 2023 at 5:52 PM Rob Jarratt <robert.jarratt(a)ntlworld.com> wrote:
Before I confuse matters too much I should point out that I was not using the Rainbow
power switch unit but one from a DECmate. I had erroneously thought there was a fault with
the Rainbow one. I think the fan supply may be different on the DECmate switch, so ignore
the bit about the fan turning.
It appears that there are 2 ways of powering (different types of) fans
with this power supply. Whether one was used in the Rainbow and the
other in the Decmate I know not. But anyway :
A 12V DC fan running off the DC output of the power supply. This is
the one shown in my schemtic for the supply. Note the jumper link in
the fan power plug that means the supply doesn't get mains if you
forget to plug the fan in.
A 115V AC fan connected to the unused pair of pins on the AC input
cable to the power supply (top right of the 'switch/fan assy' in my
schematc. This uses the primary winding of the startup transformer as
an autotransformer on 230V mains.
Note that the latter fan will run if the chopper side of things is
dead, the former won't.
-tony
5:06 p.m.
-----Original Message-----
From: Tony Duell <ard.p850ug1(a)gmail.com>
Sent: 25 June 2023 18:00
To: rob(a)jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>rg>; Brent Hilpert <bhilpert(a)shaw.ca>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
On Sun, Jun 25, 2023 at 5:52 PM Rob Jarratt <robert.jarratt(a)ntlworld.com>
wrote:
That is what I failed to notice and I thought that somehow there was a fault in the switch
unit.
Before I confuse matters too much I should point out that I was not using
the
Rainbow power switch unit but one from a DECmate. I had erroneously
thought there was a fault with the Rainbow one. I think the fan supply may
be different on the DECmate switch, so ignore the bit about the fan turning.
It appears that there are 2 ways of powering (different types of) fans with this
power supply. Whether one was used in the Rainbow and the other in the
Decmate I know not. But anyway :
A 12V DC fan running off the DC output of the power supply. This is the one
shown in my schemtic for the supply. Note the jumper link in the fan power
plug that means the supply doesn't get mains if you forget to plug the fan in.
A 115V AC fan connected to the unused pair of pins on the AC input cable to
the power supply (top right of the 'switch/fan assy' in my schematc. This uses
the primary winding of the startup transformer as an autotransformer on
230V mains.
Note that the latter fan will run if the chopper side of things is dead, the
former won't.
-tony
29 Jun
29 Jun
9:19 p.m.
Thank you to everyone who helped me with this. The PSU if finally fixed. I have posted
about it on my blog: https://robs-old-computers.com/2023/06/29/h7842-finally-fixed/
Regards
Rob
-----Original Message-----
From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Sent: 25 June 2023 18:07
To: 'Tony Duell' <ard.p850ug1(a)gmail.com>om>; rob(a)jarratt.me.uk
Cc: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>rg>; 'Brent Hilpert' <bhilpert(a)shaw.ca>
Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
-----Original Message-----
From: Tony Duell <ard.p850ug1(a)gmail.com>
Sent: 25 June 2023 18:00
To: rob(a)jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>rg>; Brent Hilpert <bhilpert(a)shaw.ca>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
On Sun, Jun 25, 2023 at 5:52 PM Rob Jarratt
<robert.jarratt(a)ntlworld.com>
wrote:
about the
fan turning.
Before I confuse matters too much I should point out that I was not
using
the Rainbow power switch unit but one from a DECmate. I had
erroneously thought there was a fault with the Rainbow one. I think
the fan supply may be different on the DECmate switch, so ignore the bit
It appears that there are 2 ways of powering (different types of) fans
with this power supply. Whether one was used in the Rainbow and the
other in the Decmate I know not. But anyway :
A 12V DC fan running off the DC output of the power supply. This is
the one shown in my schemtic for the supply. Note the jumper link in
the fan power plug that means the supply doesn't get mains if you forget to
plug the fan in.
That is what I failed to notice and I thought that somehow there was a fault in
the switch unit.
>
> A 115V AC fan connected to the unused pair of pins on the AC input
> cable to the power supply (top right of the 'switch/fan assy' in my
> schematc. This uses the primary winding of the startup transformer as
> an autotransformer on 230V mains.
>
> Note that the latter fan will run if the chopper side of things is
> dead, the former won't.
>
> -tony 
25 Jun
25 Jun
3:23 p.m.
On 6/25/23 07:36, Rob Jarratt via cctalk wrote:
This made me suspect that the startup transformer may
have been damaged. So
I checked the output of the startup transformer. On the bad PSU this was
about 2.4VAC, while on the working one it was about 12VAC. I lifted all 4 of
the diodes connected to the startup transformer and they all tested OK.
Well, apply current-limited 12 V DC to the output of the
bridge rectifier and see what current it draws.
Could be a failed capacitor or regulator, or something
downstream of the regulator that has failed.
(Maybe you have already checked this.)
Jon
4:32 p.m.
-----Original Message-----
From: Jon Elson via cctalk <cctalk(a)classiccmp.org>
Sent: 25 June 2023 16:24
To: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Cc: Jon Elson <elson(a)pico-systems.com>
Subject: [cctalk] Re: Rainbow H7842 PSU Fault
On 6/25/23 07:36, Rob Jarratt via cctalk wrote:
Well, apply current-limited 12 V DC to the
output of the bridge rectifier and
see what current it draws.
Could be a failed capacitor or regulator, or something downstream of the
regulator that has failed.
If you mean the 7812 on PSU Sheet 1 then I believe it is fine as I have already tested it
with a bench PSU supplying it with 15VDC and it works fine.
This made me suspect that the startup transformer
may have been
damaged. So I checked the output of the startup transformer. On the
bad PSU this was about 2.4VAC, while on the working one it was about
12VAC. I lifted all 4 of the diodes connected to the startup transformer and
they
all tested OK.
(Maybe you have already checked this.)
Jon
10:33 a.m.
What can/will go wrong in either case, and why?
Ulli
Am Di., 30. Mai 2023 um 12:30 Uhr schrieb Christian Corti via cctalk <
cctalk(a)classiccmp.org>gt;:
On Thu, 25 May 2023, Rob Jarratt wrote:
I am thinking I may put it back together and test
with a light bulb in
series.
SMPUs don't like that, and don't even try a variable transformers.
Christian
10:41 a.m.
On Tue, May 30, 2023 at 11:33 AM Hans-Ulrich Hölscher via cctalk
<cctalk(a)classiccmp.org> wrote:
What can/will go wrong in either case, and why?
The normally-given reason is that an SMPSU approximates to being a
constant power load and draws more current if the input voltage is
reduced. This can cause the chopper transistor to fail from having to
handle too high a current.
That said, I have run many, many SMPSUs with a light bulb in series
with the mains input and had no problems at all. I run them with a
minimal load (say 1A on the 5V line, so 5W power going there) and put,
say, a 100W filament bulb in series with the mains. The voltage drop
across the latter is <20V (I've measured it), given the local mains
here is around 240V, that means the input to the SMPSU is well within
the acceptable voltage range.
And while the seres bulb probably won't save the chopper transistor if
things go seriously wrong on the primary side of the supply, it
probably will save the bridge rectifier, expensive fast-acting fuse
and PCB tracks. Not to mention my nerves.
-tony
11:21 a.m.
On Tue, May 30, 2023 at 11:30 AM Christian Corti via cctalk <
cctalk(a)classiccmp.org> wrote:
SMPUs don't like that, and don't even try a variable transformers.
In the 80s, i worked for a company making just-pre-PC 8086 machines. We
manufactured the SMPSU in-house, designed by a specialist company. I was
involved in training the technicians to repair them and built a debug rig
with the designer's advice.
This may not apply to all SMPUs but worked well for that design, which was
fairly conventional.
There was an isolation transformer followed by a variac. The procedure was
to slowly bring up the variac until the control circuit was functioning,
check the waveforms around that, and then increase the supply voltage while
watching the chopper, control and output signals. This allowed the chopper
to work at low current limited by the circuit resistance and slowly
increase with supply voltage until the control circuit backed it off (which
would happen quite quickly with no output load).
I agree that low input voltage with a loaded output will stress the supply
but done carefully as above it should be possible. I didn't use a lamp, but
the idea there is that the maximum current in the case of a short is
limited to lamp current rather than fuse current. I wouldn't do this with
much of a load, but it's more likely to kill the chopper due to dissipation
rather than peak current since it's rated to chop the full input DC rail
into the transformer. Abuse might cause higher frequency cycling or longer
on-time but shouldn't increase peak current.
I'm happy to learn if I'm missing another mechanism.
1:11 p.m.
On Tue, May 30, 2023 at 12:22 PM Adrian Godwin via cctalk
<cctalk(a)classiccmp.org> wrote:
There was an isolation transformer followed by a variac. The procedure was
to slowly bring up the variac until the control circuit was functioning,
check the waveforms around that, and then increase the supply voltage while
watching the chopper, control and output signals. This allowed the chopper
to work at low current limited by the circuit resistance and slowly
increase with supply voltage until the control circuit backed it off (which
would happen quite quickly with no output load).
I don't normally use a Variac when repairing SMPSUs but I've seen
diagnostic procedures in official HP (etc) service manuals which tell
you to use such a device to increase the input voltage to the supply.
In some cases the the control circuit is separately powered and gets
the full voltage all the time, you just vary the input voltage to the
chopper circuit.
I would assume that an HP service manual of that vintage gave a
sensible procedure for that particular design of SMPSU.
-tony
10 May
10 May
10:17 a.m.
On Tue, 9 May 2023, Peter Coghlan wrote:
Also, this device is a not just a straighforward
transistor. It's data sheet
says it is a package containing two transistors and some other components,
That's a Darlington power transistor with freewheeling diode and biasing
resistors for the second base. I really wonder why that fact has been
unmentioned, yet.
Christian
9 May
9 May
7:28 a.m.
On 2023-May-08, at 12:27 PM, Peter Coghlan via cctalk wrote:
However, Brent's calculations show that the
current trip value for the -12V
line is as low as 1.3mA and I can't see any reason to disagree with his
calculations or his conclusion that this seems very low (except that another
tiny smidgen of current is available from the negative startup supply but
this won't really have any bearing on things). If this is really the case,
then placing something like a 5k6 resistor across the -12V line on the good
PSU should cause enough current to flow for the trip to operate. Finding
this level of leakage in the failed PSU is not going to be easy.
On the other hand, if this test doesn't trip it, then please look very
closely at the resistors and connections to the inputs of E3d and verify
that they are as described on the circuit diagram.
It seems very strange indeed to have a trip value as low as 1.3mA combined
with a shunt regulator whose method of regulation is to pull the voltage
down by drawing current from the supply line. Perhaps the shunt regulator
might be able to pull enough current to cause the trip to operate if the
-12V line was too high (in the negative sense) or if the shunt regulator
was under the mistaken impression that the -12V line was too high?
(This is a bit unlikely but the 115V/230V switch is set correctly, isn't it?)
On the other other hand, if the manual says that the -12V line is supposed
to be able to supply 150mA, then it doesn't make sense for the current trip
to operate at 1.3mA and we must be going wrong somewhere.
I thought I must be off somewhere by 10^n when first doing the calc. The 51Ω is 3 orders
of magnitude away from the 0.01Ω on the other outputs, so a similar diff could be
anticipated on the current sense.
The low current sense might be explained as follows:
This power supply does not follow the more-common design of flyback switching mains
supplies.
This supply is actually 2 (or 3) bucking regulators being fed from a mains-isolation
transformer, with the bucking regulation going to the PWM oscillator then looping back to
the driver on the primary side of the mains-isolation transformer. This is discernible in
the presence of the freewheeling diodes beside the rectifier diodes of the +5 & +12
outputs. DEC seemed to like doing this as the VaxMate supply from jarratt last year was of
similar design. A guess is it may have made the inductor design easier, separating the
flyback operation out to separated inductors rather than in the all in the same mains
transformer.
I'm speculating the unusual shunt regulator in the -12V output is actually a
'controlled freewheeling diode' for a -12V bucking step-down. The choke would be
stepping down the voltage from the -12 output secondary from something higher down to the
actual -12V output. With a higher V from the secondary, a lower current is needed for the
same energy throughput, and thus sense on a lower max current from the secondary. However,
one would anticipate the regulation V sense point in such operation to be on the other
side of the choke than what's shown in the schematic.
5 p.m.
On 2023-May-09, at 12:28 AM, Brent Hilpert via cctalk wrote:
I thought I must be off somewhere by 10^n when first
doing the calc. The 51Ω is 3 orders of magnitude away from the 0.01Ω on the other outputs,
so a similar diff could be anticipated on the current sense.
The low current sense might be explained as follows:
...
I'm speculating the unusual shunt regulator in the
-12V output is actually a 'controlled freewheeling diode' for a -12V bucking
step-down. The choke would be stepping down the voltage from the -12 output secondary from
something higher down to the actual -12V output. With a higher V from the secondary, a
lower current is needed for the same energy throughput, and thus sense on a lower max
current from the secondary. However, one would anticipate the regulation V sense point in
such operation to be on the other side of the choke than what's shown in the
schematic.
Thinking further this morning. The TIP121 is the wrong orientation or polarity to be
acting as a freewheeling conductor (--explanation). However, that's the transistor
section. The 121 has a built-in flyback diode which is the right orientation
(++explanation). Still some things that don't make sense though, like the high EI
ratio needed from the order 100 I difference (--explanation), and how the load current
would be supplied during the charge portion of the cycle (--explanation).
7 May
7 May
10:53 p.m.
On 2023-May-07, at 1:54 AM, Rob Jarratt via cctalk wrote:
The comments about the tolerance of the 7812 were
right, it doesn’t appear to be an issue with the replacement 7812 regulator because when I
tried using the bench PSU to feed exactly 12V to the circuit from the output of the 7812
the comparator still gave the wrong result. It was still wrong if I applied only 11V
I then looked at the value of Vz on the good and bad PSUs, when applying 12V to the 7812
output. That was 5.4V in both the good and bad PSUs. Where I saw a difference was on the
-12V output, it was +0.4V on the good PSU and 0.56V on the bad one (the voltage varied so
this was an average). I checked the voltage drop across the current sense resistor. It is
0.01V on the good PSU and 0.08V on the bad PSU, which would explain the higher positive
voltage on the -12V output and the comparator being turned on.
From earlier measurements and the 45uA calc of current through the 51Ω sense resistor, the
V across the 51Ω ISense-12 resistor should be only 0.002V.
So a question is where is this 0.08V coming from? An unfulfilled -12V supply for the E3
power pin might have been an explanation, as extra current might be drawn out of the
E3d.+in input due to the +in being pulled below (negative to) the
E3 -power pin. But you say that pin is connected to GND, so the source of the 0.08V should
be sought, some more comprehensive measurements around the E3d inputs / ISenseR might
help.
From calculations from design/schematic:
- E3d.+in should = 1/2 the voltage across the 51Ω ISense-12 resistor.
- E3d.-in should = 0.033 V.
- The trip condition to send the E3d output high (shutdown) is: V(ISR-12) > 0.066V
(V across ISense-12 Resistor).
Also: Is that ISense-12 resistor really a 51Ω resistor? Or is it 5.1Ω or 0.51Ω? Again from
calculation, 51Ω would imply a current limit of only 1.3mA, which is rather low.
8 May
8 May
12:03 p.m.
-----Original Message-----
From: Brent Hilpert <bhilpert(a)shaw.ca>
Sent: 07 May 2023 23:53
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Rainbow H7842 PSU Fault
On 2023-May-07, at 1:54 AM, Rob Jarratt via cctalk wrote:
I thought I had answered where the Vcc and GND are connected on the comparator. Vcc is
connected to Vstart and GND to GND. I will look again at all the inputs to check.
The comments about the tolerance of the 7812 were
right, it doesn’t
appear to be an issue with the replacement 7812 regulator because
when I
tried using the bench PSU to feed exactly 12V to the circuit from the output of
the 7812 the comparator still gave the wrong result. It was still wrong if I
applied only 11V
I then looked at the value of Vz on the good and bad PSUs, when applying
12V to
the 7812 output. That was 5.4V in both the good and bad PSUs.
Where I saw a difference was on the -12V output, it was +0.4V on the good
PSU and 0.56V on the bad one (the voltage varied so this was an average). I
checked the voltage drop across the current sense resistor. It is 0.01V on the
good PSU and 0.08V on the bad PSU, which would explain the higher positive
voltage on the -12V output and the comparator being turned on.
From earlier measurements and the 45uA calc of current through the 51Ω
sense resistor, the V across the 51Ω ISense-12 resistor should be only 0.002V.
So a question is where is this 0.08V coming from? An unfulfilled -12V supply
for the E3 power pin might have been an explanation, as extra current might
be drawn out of the E3d.+in input due to the +in being pulled below
(negative to) the
E3 -power pin. But you say that pin is connected to GND, so the source of the
0.08V should be sought, some more comprehensive measurements around
the E3d inputs / ISenseR might help.
From calculations from design/schematic:
- E3d.+in should = 1/2 the voltage across the 51Ω ISense-12 resistor.
I measure 0.08V across the current sense resistor and indeed I found 0.04V on the E3d +IN
- E3d.-in should = 0.033 V.
And that is what I measured (actually 0.034V).
- The trip condition to send the E3d output high
(shutdown) is: V(ISR-12) >
0.066V (V across ISense-12 Resistor).
Indeed. The drop across the resistor is too high causing the comparator to turn off, when
it should be on and sinking current to allow the PWN to operate. I think the comparator is
working correctly, the problem seems to be the voltage drop across the resistor is
genuinely too high, but I can't see why.
Under the same test conditions, I measure a 0.01V drop across the resistor in the good
PSU.
Also: Is that ISense-12 resistor really a 51Ω resistor? Or is it 5.1Ω or 0.51Ω?
Again from calculation, 51Ω would imply a current limit of only 1.3mA, which
is rather low.
It is 51R, I have measured it (in circuit) and the colour coding confirms it. The spec in
the technical manual says the -12V is rated at 150mA max.
Thanks
Rob
PS Will reply to Peter's email later when I have had a chance to understand it and to
try the suggestions.
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Adrian Godwin -
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