28 Aug
2014
28 Aug
'14
10:53 a.m.
I have started playing with the rtVAX 1000 I collected last year. It was
running OK when then was a sudden bang and the machine stopped. Not even the
fans will run. There was no smell and no smoke, so I knew immediately that
it can't be the mains filter capacitors, which always fail on these power
supplies.
I have opened it up and cannot see any damage in the PSU, the CPU, the
memory, or any of the other boards. I am sure it must be the power supply,
but I really can't see any damage. The power supply is one of the ASTEC ones
common in the MicroVAX II.
My first step is going to be to test the PSU with a dummy load using light
bulbs (still haven't built a board for a dummy load) and with the fans
connected, but nothing else. Will also check for ripple at the same time.
There are two outputs from the PSU that I don't fully understand, one is P
OK, I am guessing that means *something* is OK, but not sure what, power
perhaps, but what power?. The other is called LTC, I have no idea what that
one means. Can anyone enlighten me?
Any other ideas?
Regards
Rob
28 Aug
28 Aug
11:59 a.m.
On Thu, Aug 28, 2014 at 10:53 AM, Robert Jarratt
<robert.jarratt at ntlworld.com> wrote:
There are two outputs from the PSU that I don't fully understand, one is P
OK, I am guessing that means *something* is OK, but not sure what, power
perhaps, but what power?. The other is called LTC, I have no idea what that
one means. Can anyone enlighten me?
LTC is the Line Time Clock (50 or 60 Hz) signal that connects to the
BEVNT(L) signal on the H9278A backplane in the BA23.
12:06 p.m.
On 28/08/2014 18:53, Robert Jarratt wrote:
There are two outputs from the PSU that I don't
fully understand, one is P
OK, I am guessing that means *something* is OK, but not sure what, power
perhaps, but what power?. The other is called LTC, I have no idea what that
one means. Can anyone enlighten me?
'P OK' is indeed power OK. This signal will be asserted once all the
power supply outputs have reached their normal operating voltage. The
Qbus devices will be held in reset until this signal is asserted.
'LTC' is the mains derived line time clock (50hz here in the UK).
Have any of the internal fuses (or circuit breakers) blown? Don't
replace then until you find the real problem. I would probably start by
checking the bridge rectifier and main switching transistor for shorts.
Matt
1:19 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Matt Burke
Sent: 28 August 2014 20:07
To: cctalk at classiccmp.org
Subject: Re: Mysterious rtVAX 1000 Failure
On 28/08/2014 18:53, Robert Jarratt wrote:
devices
There are two outputs from the PSU that I
don't fully understand, one
is P OK, I am guessing that means *something* is OK, but not sure
what, power perhaps, but what power?. The other is called LTC, I have
no idea what that one means. Can anyone enlighten me?
'P OK' is indeed power OK. This signal will be asserted once all the power
supply outputs have reached their normal operating voltage. The Qbus will be held in reset until this signal is asserted.
'LTC' is the mains derived line time clock (50hz here in the UK).
Have any of the internal fuses (or circuit breakers) blown? Don't replace
then
until you find the real problem. I would probably
start by checking the
bridge
rectifier and main switching transistor for shorts.
Matt
I can't find any fuses. There is a circuit breaker, but it doesn't seem to
have tripped (it would be sticking out, right?). I notice that the light in
power switch comes on when I turn it on, if it was the fuse/circuit breaker,
I expect even the switch's light would not come on.
There are a number of candidates for the main switching transistor, I think.
Is there a printset somewhere? I have never been able to find one, and it
would help in identifying components.
Regards
Rob
3:08 p.m.
If it actually made a 'bang' sound the most likely culprit is an
electrolytic capacitor. A visual inspection should find it, although I've
known them to fail pretty amazingly leaving the case intact except for the
circle on the top where the plates came flying out and landed somewhere
remotely.
The system is waiting for P_OK (power OK) before it will fire up, and the
fans are on the non-mains side of that PSU as I recall. Is it the BA23
case? (most common case) there was an issue with the power distribution
harness on those where DEC used several wires in parallel to carry the 5V
current. Except they were not all the same length and the short one would
some times vaporize. The 'fixed' harness you can see all the wires are the
same length which makes it harder to tuck in behind the back plane but it
doesn't blow up.
--Chuck
On Thu, Aug 28, 2014 at 1:19 PM, Robert Jarratt <robert.jarratt at ntlworld.com
wrote:
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:
cctalk-bounces at classiccmp.org]
On Behalf Of Matt Burke
Sent: 28 August 2014 20:07
To: cctalk at classiccmp.org
Subject: Re: Mysterious rtVAX 1000 Failure
On 28/08/2014 18:53, Robert Jarratt wrote:
power
There are two outputs from the PSU that I
don't fully understand, one
is P OK, I am guessing that means *something* is OK, but not sure
what, power perhaps, but what power?. The other is called LTC, I have
no idea what that one means. Can anyone enlighten me?
'P OK' is indeed power OK. This signal will be asserted once all the supply outputs have reached their normal
operating voltage. The Qbus
devices
will be held in reset until this signal is
asserted.
'LTC' is the mains derived line time clock (50hz here in the UK).
Have any of the internal fuses (or circuit breakers) blown? Don't replace
then
until you find the real problem. I would probably
start by checking the
bridge
rectifier and main switching transistor for
shorts.
Matt
I can't find any fuses. There is a circuit breaker, but it doesn't seem to
have tripped (it would be sticking out, right?). I notice that the light in
power switch comes on when I turn it on, if it was the fuse/circuit
breaker,
I expect even the switch's light would not come on.
There are a number of candidates for the main switching transistor, I
think.
Is there a printset somewhere? I have never been able to find one, and it
would help in identifying components.
Regards
Rob
29 Aug
29 Aug
12:40 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Chuck McManis
Sent: 28 August 2014 23:09
To: General Discussion: On-Topic Posts Only
Subject: Re: Mysterious rtVAX 1000 Failure
If it actually made a 'bang' sound the most likely culprit is an electrolytic
capacitor. A visual inspection should find it, although I've known them to fail
pretty amazingly leaving the case intact except for the circle on the top where
the plates came flying out and landed somewhere remotely.
The system is waiting for P_OK (power OK) before it will fire up, and the fans
are on the non-mains side of that PSU as I recall. Is it the BA23 case? (most
common case) there was an issue with the power distribution harness on those
where DEC used several wires in parallel to carry the 5V current. Except they
were not all the same length and the short one would some times vaporize.
The 'fixed' harness you can see all the wires are the same length which makes
it harder to tuck in behind the back plane but it doesn't blow up.
Yes, it is a BA23 and I do have the "bad" wire harness. In fact I think all my
BA23 machines have the bad one, but I don't run them that often, and they are not
particularly loaded, so hopefully that will be OK.
The fans are powered from the half of the power supply that contains the input filter and
the rectifier board. The bulk of the power supply is in the other half and does not appear
to have any involvement with the fans. As the fans don't work, this would suggest that
the fault is either on the input filter board or the rectifier board.
Is it safe to connect up the first half of the PSU to the mains, switch it on and try to
probe around to see if I can see where the fault lies?
Regards
Rob
--Chuck
On Thu, Aug 28, 2014 at 1:19 PM, Robert Jarratt <robert.jarratt at ntlworld.com
wrote:
>
>
> > -----Original Message-----
> > From: cctalk-bounces at classiccmp.org [mailto:
> cctalk-bounces at classiccmp.org]
> > On Behalf Of Matt Burke
> > Sent: 28 August 2014 20:07
> > To: cctalk at classiccmp.org
> > Subject: Re: Mysterious rtVAX 1000 Failure
> >
> > On 28/08/2014 18:53, Robert Jarratt wrote:
> > > There are two outputs from the PSU that I don't fully understand,
> > > one is P OK, I am guessing that means *something* is OK, but not
> > > sure what, power perhaps, but what power?. The other is called
> > > LTC, I have no idea what that one means. Can anyone enlighten me?
> >
> > 'P OK' is indeed power OK. This signal will be asserted once all the
> power
> > supply outputs have reached their normal operating voltage. The Qbus
> devices
> > will be held in reset until this signal is asserted.
> > 'LTC' is the mains derived line time clock (50hz here in the UK).
> >
> > Have any of the internal fuses (or circuit breakers) blown? Don't
> > replace
> then
> > until you find the real problem. I would probably start by checking
> > the
> bridge
> > rectifier and main switching transistor for shorts.
> >
> > Matt
>
> I can't find any fuses. There is a circuit breaker, but it doesn't
> seem to have tripped (it would be sticking out, right?). I notice that
> the light in power switch comes on when I turn it on, if it was the
> fuse/circuit breaker, I expect even the switch's light would not come
> on.
>
> There are a number of candidates for the main switching transistor, I
> think.
> Is there a printset somewhere? I have never been able to find one, and
> it would help in identifying components.
>
> Regards
>
> Rob
>
>
4:03 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 29 August 2014 08:40
To: 'General Discussion: On-Topic and Off-Topic Posts'; 'General Discussion:
On-
Topic Posts Only'
Subject: RE: Mysterious rtVAX 1000 Failure
Answering my own question, I decided it was safe to connect up the filter/rectifier half
of the PSU, and indeed it was. I have found the following:
The large capacitors, which I think are smoothing the output of the rectifier, are
charging up. The V+ and V- connector that goes to the other half of the PSU has about 320V
across it. However, there is another connector that output +15V and -15V, these are both
at zero.
The LM317T linear regulator appears to have no input and no output, it could be that I am
measuring it incorrectly though. I connected one probe on my DMM to the chassis and then
tried the three pins on the regulator, all at zero.
So I am guessing the output of the rectifier is not reaching the regulator, which I am
guessing runs the fans. It would seem that the rectifier itself, and the input filter are
all fine.
I still can't see any failed components, but the next step is going to be see if I can
work out the schematic by examining the board, so that I can narrow down where the failure
might be.
Regards
Rob
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Chuck McManis
Sent: 28 August 2014 23:09
To: General Discussion: On-Topic Posts Only
Subject: Re: Mysterious rtVAX 1000 Failure
If it actually made a 'bang' sound the most likely culprit is an
electrolytic capacitor. A visual inspection should find it, although
I've known them to fail pretty amazingly leaving the case intact
except for the circle on the top where the plates came flying out and landed
somewhere remotely.
The system is waiting for P_OK (power OK) before it will fire up, and
the fans are on the non-mains side of that PSU as I recall. Is it the
BA23 case? (most common case) there was an issue with the power
distribution harness on those where DEC used several wires in parallel
to carry the 5V current. Except they were not all the same length and the
short
one would some times vaporize.
The 'fixed' harness you can see all the
wires are the same length
which makes it harder to tuck in behind the back plane but it doesn't blow up.
Yes, it is a BA23 and I do have the "bad" wire harness. In fact I think all my
BA23 machines have the bad one, but I don't run them that often, and they are
not particularly loaded, so hopefully that will be OK.
The fans are powered from the half of the power supply that contains the input
filter and the rectifier board. The bulk of the power supply is in the other half
and does not appear to have any involvement with the fans. As the fans don't
work, this would suggest that the fault is either on the input filter board or the
rectifier board.
Is it safe to connect up the first half of the PSU to the mains, switch it on and try
to probe around to see if I can see where the fault lies?
1:27 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 29 August 2014 12:04
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: Mysterious rtVAX 1000 Failure
I have done some reverse engineering on the board that houses the rectifier and which has
the outputs to the fans. I had been assuming that the fans were powered by this board
alone, possibly through that linear regulator, and not through the SMPS, which led me to
think that if the fans don't work it must be this board rather than the SMPS (I am not
an electronics person, so pardon me if that was a silly thought).
However, I now see that it is impossible for this to be the case. The rectifier portion of
the board is isolated from the portion which powers the fans by an optocoupler, so the
power is not coming from the rectifier. In which case it must be coming from the SMPS on
the other half of the clam shell, through a connector which I had assumed was an output
from the rectifier board, but appears to be an input.
As power is getting sent to the SMPS half of the clam shell from the rectifier, that means
the fault has to lie in the SMPS side of the clam shell. This fills me with dread, as the
board is much more complex on this side, I don't have schematics, I am no PSU expert,
and I still can't see any physical damage, although there *must* be some.
Regards
Rob
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 29 August 2014 08:40
To: 'General Discussion: On-Topic and Off-Topic Posts'; 'General
Discussion: On- Topic Posts Only'
Subject: RE: Mysterious rtVAX 1000 Failure
up.
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Chuck McManis
Sent: 28 August 2014 23:09
To: General Discussion: On-Topic Posts Only
Subject: Re: Mysterious rtVAX 1000 Failure
If it actually made a 'bang' sound the most likely culprit is an
electrolytic capacitor. A visual inspection should find it, although
I've known them to fail pretty amazingly leaving the case intact
except for the circle on the top where the plates came flying out
and landed
somewhere remotely.
The system is waiting for P_OK (power OK) before it will fire up,
and the fans are on the non-mains side of that PSU as I recall. Is
it the
BA23 case? (most common case) there was an issue with the power
distribution harness on those where DEC used several wires in
parallel to carry the 5V current. Except they were not all the same
length and the
short one would some times vaporize.
> The 'fixed' harness you can see all the wires are the same length
> which makes it harder to tuck in behind the back plane but it doesn't blow
Yes, it is a BA23 and I do have the "bad" wire harness. In fact I
think all my
BA23 machines have the bad one, but I don't run them that often, and
they are not particularly loaded, so hopefully that will be OK.
The fans are powered from the half of the power supply that contains
the input filter and the rectifier board. The bulk of the power supply
is in the other half and does not appear to have any involvement with
the fans. As the fans don't work, this would suggest that the fault is
either on the input filter board or the rectifier board.
Is it safe to connect up the first half of the PSU to the mains,
switch it on and try to probe around to see if I can see where the fault lies?
Answering my own question, I decided it was safe to connect up the
filter/rectifier half of the PSU, and indeed it was. I have found the following:
The large capacitors, which I think are smoothing the output of the rectifier,
are charging up. The V+ and V- connector that goes to the other half of the PSU
has about 320V across it. However, there is another connector that output +15V
and -15V, these are both at zero.
The LM317T linear regulator appears to have no input and no output, it could
be that I am measuring it incorrectly though. I connected one probe on my
DMM to the chassis and then tried the three pins on the regulator, all at zero.
So I am guessing the output of the rectifier is not reaching the regulator, which
I am guessing runs the fans. It would seem that the rectifier itself, and the input
filter are all fine.
I still can't see any failed components, but the next step is going to be see if I
can work out the schematic by examining the board, so that I can narrow down
where the failure might be.
Regards
Rob
31 Aug
31 Aug
8:53 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 29 August 2014 21:27
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: Mysterious rtVAX 1000 Failure
regulator, all
at zero.
I have compared a known good PSU with the bad one and I can see that all the outputs from
the PSU, including P OK and DC OK are shorted to Common, whereas on the good one they are
not. Any suggestions where to start looking in that case?
Regards
Rob
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 29 August 2014 12:04
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: Mysterious rtVAX 1000 Failure
board.
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 29 August 2014 08:40
To: 'General Discussion: On-Topic and Off-Topic Posts'; 'General
Discussion: On- Topic Posts Only'
Subject: RE: Mysterious rtVAX 1000 Failure
up.
>
>
> Yes, it is a BA23 and I do have the "bad" wire harness. In fact I
> think all my
> BA23 machines have the bad one, but I don't run them that often, and
> they are not particularly loaded, so hopefully that will be OK.
>
> The fans are powered from the half of the power supply that contains
> the input filter and the rectifier board. The bulk of the power
> supply is in the other half and does not appear to have any
> involvement with the fans. As the fans don't work, this would
> suggest that the fault is either on the input filter board or the rectifier -----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Chuck McManis
Sent: 28 August 2014 23:09
To: General Discussion: On-Topic Posts Only
Subject: Re: Mysterious rtVAX 1000 Failure
If it actually made a 'bang' sound the most likely culprit is an
electrolytic capacitor. A visual inspection should find it,
although I've known them to fail pretty amazingly leaving the case
intact except for the circle on the top where the plates came
flying out and landed
somewhere remotely.
The system is waiting for P_OK (power OK) before it will fire up,
and the fans are on the non-mains side of that PSU as I recall. Is
it the
BA23 case? (most common case) there was an issue with the power
distribution harness on those where DEC used several wires in
parallel to carry the 5V current. Except they were not all the
same length and the
short one would some times vaporize.
> The 'fixed' harness you can see all the wires are the same length
> which makes it harder to tuck in behind the back plane but it
> doesn't blow
Is it safe to connect up the first half of the PSU to the mains,
switch it on and try to probe around to see if I can see where the fault lies?
Answering my own question, I decided it was safe to connect up the
filter/rectifier half of the PSU, and indeed it was. I have found the following:
The large capacitors, which I think are smoothing the output of the
rectifier, are charging up. The V+ and V- connector that goes to the
other half of the PSU has about 320V across it. However, there is
another connector that output +15V and -15V, these are both at zero.
The LM317T linear regulator appears to have no input and no output, it
could be that I am measuring it incorrectly though. I connected one
probe on my DMM to the chassis and then tried the three pins on the
So I am guessing the output of the rectifier is not reaching the
regulator, which I am guessing runs the fans. It would seem that the
rectifier itself, and the input filter are all fine.
I still can't see any failed components, but the next step is going to
be see if I can work out the schematic by examining the board, so that
I can narrow down where the failure might be.
Regards
Rob
I have done some reverse engineering on the board that houses the rectifier
and which has the outputs to the fans. I had been assuming that the fans were
powered by this board alone, possibly through that linear regulator, and not
through the SMPS, which led me to think that if the fans don't work it must be
this board rather than the SMPS (I am not an electronics person, so pardon me
if that was a silly thought).
However, I now see that it is impossible for this to be the case. The rectifier
portion of the board is isolated from the portion which powers the fans by an
optocoupler, so the power is not coming from the rectifier. In which case it
must be coming from the SMPS on the other half of the clam shell, through a
connector which I had assumed was an output from the rectifier board, but
appears to be an input.
As power is getting sent to the SMPS half of the clam shell from the rectifier,
that means the fault has to lie in the SMPS side of the clam shell. This fills me
with dread, as the board is much more complex on this side, I don't have
schematics, I am no PSU expert, and I still can't see any physical damage,
although there *must* be some.
Regards
Rob 
1:08 p.m.
I have compared a known good PSU with the bad one and I can see that all the
outputs from the PSU, including P OK and DC OK are shorted to Common, whereas
on the good one they are not. Any suggestions where to start looking in that
case?
That's very bizarre. It's hard to come up with a single failure that could
account for that.
Could it be something like the output wiring harness trapped against a sharp
metal edge causing it to dig through the insulation and short the conductors?
Other than that, maybe pick one of the outputs, trace it back to find the
rectifier and smoothing components that produce it, unsolder one end of
each and check them for shorts. If the components check ok and the short
still remains on the output in question, see if there is anything else in
connected to it that could be disconnected to further narrow the suspects.
Regards
Peter Coghlan
2:51 p.m.
On 31/08/2014 16:53, Robert Jarratt wrote:
I have compared a known good PSU with the bad one and
I can see that all the outputs from the PSU, including P OK and DC OK are shorted to
Common, whereas on the good one they are not. Any suggestions where to start looking in
that case?
I can think of three possibilities (all on the output side of the PSU):
- Failure of the over voltage crowbar circuit
- Shorted rectifier diode
- Shorted capacitor
I'm surprised you didn't get the clicking sound from the PSU as it tries
to recover from over current. I had a PDP-11/84 with a shorted rectifier
diode that did this a few years ago.
I haven't had chance to look at my H7864 yet but for this kind of
problem I would start by removing the main output capacitor and checking
if the short still exists. If that was OK then I would remove the
rectifier diodes and so on.
Matt
28 Aug
28 Aug
3:22 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 28 August 2014 21:19
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: Mysterious rtVAX 1000 Failure
have
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Matt Burke
Sent: 28 August 2014 20:07
To: cctalk at classiccmp.org
Subject: Re: Mysterious rtVAX 1000 Failure
On 28/08/2014 18:53, Robert Jarratt wrote:
devices
There are two outputs from the PSU that I
don't fully understand,
one is P OK, I am guessing that means *something* is OK, but not
sure what, power perhaps, but what power?. The other is called LTC,
I have no idea what that one means. Can anyone enlighten me?
'P OK' is indeed power OK. This signal will be asserted once all the
power supply outputs have reached their normal operating voltage. The
Qbus will be held in reset until this signal is
asserted.
'LTC' is the mains derived line time clock (50hz here in the UK).
Have any of the internal fuses (or circuit breakers) blown? Don't
replace
then
until you find the real problem. I would probably
start by checking
the
bridge
rectifier and main switching transistor for
shorts.
Matt
I can't find any fuses. There is a circuit breaker, but it doesn't seem to
tripped (it would be sticking out, right?). I notice
that the light in
power switch
comes on when I turn it on, if it was the fuse/circuit
breaker, I expect
even the
switch's light would not come on.
There are a number of candidates for the main switching transistor, I
think.
Is there a printset somewhere? I have never been able
to find one, and it
would
help in identifying components.
Regards
Rob
Just tested the rectifier, while in-circuit, and it appears to be fine, no
shorts anyway.
Regards
Rob
3:40 p.m.
On 28/08/2014 21:19, Robert Jarratt wrote:
I can't find any fuses. There is a circuit
breaker, but it doesn't seem to
have tripped (it would be sticking out, right?). I notice that the light in
power switch comes on when I turn it on, if it was the fuse/circuit breaker,
I expect even the switch's light would not come on.
There are a number of candidates for the main switching transistor, I think.
Is there a printset somewhere? I have never been able to find one, and it
would help in identifying components.
Regards
Rob
I'm not sure if a print set is available for the H7864. I'll have to
look at one of my power supplies to see if I can identify the major
components.
Matt
3:32 p.m.
I have started playing with the rtVAX 1000 I collected last year. It was
running OK when then was a sudden bang and the machine stopped. Not even the
fans will run. There was no smell and no smoke, so I knew immediately that
it can't be the mains filter capacitors, which always fail on these power
supplies.
I have opened it up and cannot see any damage in the PSU, the CPU, the
memory, or any of the other boards. I am sure it must be the power supply,
but I really can't see any damage. The power supply is one of the ASTEC ones
common in the MicroVAX II.
Is it one of those 115V/230V PSUs that autosenses the applied voltage rather
than has a voltage selector switch? I've had an autosensing PSU that got
itself stuck in 115V mode and the supply here is 230V. The result was one of
a pair of VDRs in black rubber sleeves failed with a loud bang at switchon.
The debris was contained within the sleeve and it took quite a while to figure
out what had gone bang then a lot of headscratching to figure out why...
Another possibility might be an exploding tantalum electrolytic decoupler.
They can leave little evidence that they were ever there but it is hard to see
how one could permanently take out all outputs from the PSU in one go. It
is a good match for a failure some time after switchon though.
Some of the DEC PSU's I've come across have to be left switched off for
a few minutes before they reset after an overload but I doubt you've had it
switched on all the time since the problem happened either.
Regards,
Peter Coghlan.
4:45 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Peter Coghlan
Sent: 28 August 2014 23:32
To: General Discussion: On-Topic Posts Only
Subject: Re: Mysterious rtVAX 1000 Failure
rather
I have started playing with the rtVAX 1000 I collected last year. It
was running OK when then was a sudden bang and the machine stopped. Not
even the fans will run. There was no smell and no smoke, so I knew
immediately that it can't be the mains filter capacitors, which always
fail on these power supplies.
I have opened it up and cannot see any damage in the PSU, the CPU, the
memory, or any of the other boards. I am sure it must be the power
supply, but I really can't see any damage. The power supply is one of
the ASTEC ones common in the MicroVAX II.
Is it one of those 115V/230V PSUs that autosenses the applied voltage than has a voltage selector switch? I've had an
autosensing PSU that got
itself
stuck in 115V mode and the supply here is 230V. The
result was one of a
pair
of VDRs in black rubber sleeves failed with a loud
bang at switchon.
The debris was contained within the sleeve and it took quite a while to
figure
out what had gone bang then a lot of headscratching to
figure out why...
Mine has the selector switch. It did occur to me that the switch could have
been in the wrong position, it is actually hard to tell which position it is
in, although it looks to be on 220-240, which is right for me here. I would
expect the failure to be immediate if it was in the wrong position, or is
that not necessarily so? I will look inside the sleeved components to check
for anything there.
Another possibility might be an exploding tantalum electrolytic decoupler.
They can leave little evidence that they were ever there but it is hard to
see
how one could permanently take out all outputs from
the PSU in one go. It
is a
good match for a failure some time after switchon
though.
I couldn't see any Tantalum caps when I thought of this. I have a working
PSU which I can dig out with a bit of difficulty, so I may be able to
compare and see if I can spot something.
Some of the DEC PSU's I've come across have to be left switched off for a
few
minutes before they reset after an overload but I
doubt you've had it
switched
on all the time since the problem happened either.
Indeed, some time passed between switch ons.
Thanks
Rob
3532
days inactive
3535
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cctech＠beyondthepale.ie -
chuck.mcmanis＠gmail.com -
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matt＠9track.net -
robert.jarratt＠ntlworld.com