31 May
2010
31 May
'10
3:50 p.m.
Probably the lowest effort option also makes the fewest changes to the
original: A small external "boost" autotransformer to give maybe 10 or 15%
extra headroom.
This could be done without using a big transformer. What you do is use
a say 20VAC 3A transformer secondary in series with the primary, wired
to that you end up with 140VAC with 120VAC in. This can use a physically
much smaller transformer than a 140VAC 3A isolation transformer.
I'm guessing the AC current at the input to the power supply is circa a
very few amps or so (why I suggest a 20VAC 3A unit above).
Tim.
1 Jun
1 Jun
7:20 a.m.
Good suggestion Tim and thanks to others for their suggestions too.
I'll give this a try and see how it pans out. I can sure test the
theory with a VARIAC. The original transformer is rated at 7.25V @
10A so I think the current capability is sufficient-- just too low of
a secondary voltage.
On Monday (05/31/2010 at 06:50PM -0400), Tim Shoppa wrote:
Probably the lowest effort option also makes the
fewest changes to the
original: A small external "boost" autotransformer to give maybe 10 or 15%
extra headroom.
This could be done without using a big transformer. What you do is use
a say 20VAC 3A transformer secondary in series with the primary, wired
to that you end up with 140VAC with 120VAC in. This can use a physically
much smaller transformer than a 140VAC 3A isolation transformer.
I'm guessing the AC current at the input to the power supply is circa a
very few amps or so (why I suggest a 20VAC 3A unit above).
Tim.
--
Chris Elmquist
2 Jun
2 Jun
11:10 a.m.
Good suggestion Tim and thanks to others for their suggestions too.
I'll give this a try and see how it pans out. I can sure test the
theory with a VARIAC. The original transformer is rated at 7.25V @
10A so I think the current capability is sufficient-- just too low of
a secondary voltage.
If I had a penny for every time I 'thought it was sufficient/OK/not the
fault/,,,' I would have no problem attending a VCF in the States :-)
More seriously, I much prefer to make mesaurements and do tests before
changing anything. That way you know what is going on, you know what the
problem really is, and you can correct it. Not do something that seems to
work now and whcih will cause problems later.
Since in this case the first test is an easy one to do (namely measuring
the AC voltage from the transformer secondary winding), I wouldn't even
think of not doing it.
-tony
4:44 p.m.
On Wednesday (06/02/2010 at 07:10PM +0100), Tony Duell wrote:
Yes Tony... I have done all of that. The secondary is dropping to nearly
6 VAC with only 5A load on the DC side of the bridge and 120VAC supplied
to the primary. The ripple on the DC side is in excess of 1V with that
same 5A load. I have swapped in alternative filter caps temporarily
and see little improvement. I checked the bridge for shorted diodes or
excessive loss and it looks OK.
I just received two new transformers to consider as a replacement for
the original. The problem with putting a booster on the primary of the
original is that it also raises the 24VAC secondary resulting in the +/-
12VDC rails going well above 16V. There's quite a few things in the
machine that use the 12V rails directly without regulators and I think
that's too high for some of them. So, I am going to put in a seperate
24VAC CT transformer for the +/- 12V supply and an 8VAC 10A transformer
for the +8V supply. This was a mod often done to the machine so I'm
not making it too much of a Frankenstein.
In restrospect, I think I am like you and will likely never sell this
machine... so, for now, it's about making it run right and having some
fun (again) with it.
Chris
--
Chris Elmquist
Good suggestion Tim and thanks to others for their suggestions too.
I'll give this a try and see how it pans out. I can sure test the
theory with a VARIAC. The original transformer is rated at 7.25V @
10A so I think the current capability is sufficient-- just too low of
a secondary voltage.
If I had a penny for every time I 'thought it was sufficient/OK/not the
fault/,,,' I would have no problem attending a VCF in the States :-)
More seriously, I much prefer to make mesaurements and do tests before
changing anything. That way you know what is going on, you know what the
problem really is, and you can correct it. Not do something that seems to
work now and whcih will cause problems later.
Since in this case the first test is an easy one to do (namely measuring
the AC voltage from the transformer secondary winding), I wouldn't even
think of not doing it. 
3 Jun
3 Jun
6:35 a.m.
From: chrise at pobox.com
---snip---
I just received two new transformers to consider as a replacement for
the original. The problem with putting a booster on the primary of the
original is that it also raises the 24VAC secondary resulting in the +/-
12VDC rails going well above 16V. There's quite a few things in the
---snip---
Hi
Is there an air gap between the outside of the windings
and the frame core? If so, you could add a few turns for the
8V winding.
Often there is a thick cardboard like cover over the winding that
can be carefully cut from between the core and the outside of the
windings and the core. One just has to be careful not to
break the primary windings as they are quite fine.
The windings typically go under this outer cover and then are
spliced with the insulated wire to feed out of the transformer.
Dwight
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5:18 p.m.
On Thursday (06/03/2010 at 06:35AM -0700), dwight elvey wrote:
Hi Dwight. No, the transformer is completely enclosed in a metal shell.
No access to any of the windings.
--
Chris Elmquist
From: chrise at pobox.com
---snip---
I just received two new transformers to consider as a replacement for
the original. The problem with putting a booster on the primary of the
original is that it also raises the 24VAC secondary resulting in the +/-
12VDC rails going well above 16V. There's quite a few things in the
---snip---
Hi
Is there an air gap between the outside of the windings
and the frame core? If so, you could add a few turns for the
8V winding.
Often there is a thick cardboard like cover over the winding that
can be carefully cut from between the core and the outside of the
windings and the core. One just has to be careful not to
break the primary windings as they are quite fine.
The windings typically go under this outer cover and then are
spliced with the insulated wire to feed out of the transformer.
11:30 a.m.
Since in this
case the first test is an easy one to do (namely measuring
the AC voltage from the transformer secondary winding), I wouldn't even
think of not doing it.
Yes Tony... I have done all of that. The secondary is dropping to nearly
6 VAC with only 5A load on the DC side of the bridge and 120VAC supplied to the primary. The ripple on the DC side is in
excess of 1V with that
same 5A load. I have swapped in alternative filter caps temporarily
and see little improvement. I checked the bridge for shorted diodes or
excessive loss and it looks OK.
I assume youy've also checked for open diodes in the bridge. If it was
running as a half-wave rectifier you might have problems.
-tony
5 Jun
5 Jun
10:52 a.m.
Since in this
case the first test is an easy one to do (namely measuring
the AC voltage from the transformer secondary winding), I wouldn't even
think of not doing it.
Yes Tony... I have done all of that. The secondary is dropping to nearly
6 VAC with only 5A load on the DC side of the bridge and 120VAC supplied
to the primary. The ripple on the DC side is in excess of 1V with that
same 5A load. I have swapped in alternative filter caps temporarily
and see little improvement. I checked the bridge for shorted diodes or
excessive loss and it looks OK. 
6 Jun
6 Jun
4:21 a.m.
New subject: [SPAM] - Re: Authenticity opinions - Bayesian Filter detected spam
Philip Belben wrote:
That sounds very low indeed. I wouldn't expect a
transformer
secondary to drop below rated voltage with only half load on it! Are
you sure you haven't got a shorted turn somewhere?
I agree with Tony that you should do some more tests to see if there's
something wrong with the transformer or rectifier. For a start, look
at the secondary voltages and current drawn from the supply with all
the secondaries open circuit. Excessive no-load current is a
reasonable indicator of a shorted turn.
If you eventually do need to upgrade the power supply, could you fit
just one switcher in the cabinet with that transformer? Use it for
the 8V line, and keep the old PSU for the rest?
Just a suggestion: I am running a Pentium III system which probably
needs its internal
power supply fixed. When I attempt to boot the system with the 3 hard
disk drives
on the internal power supply, the system crashes. I now use an external
power
supply for the 3 hard disk drives and the internal power supply for
everything else.
When I run my PDP-11/83, the 3 ESDI hard disk drives probably place a strain
on the internal power supply. I use the same solution and run the 3
ESDI drives
on an external power supply along with a fan for each to keep them
cool. As it
happens, I purchased about a dozen ESDI hard drives about 15 years ago when
they were no longer in demand, but still plentiful. I seem to remember
that they
arrived 4 in a box with additional hardware to support a RAID
configuration of
some kind. Each box had its own power supply. My requirement was to use
just the ESDI hard drive for the PDP-11/83 on a Sigma RQD11-EC ESDI
controller, actually 3 hard drives on the controller. The power supply
and the
fans in those boxes are still in use after 15 years with one power
supply now
used with the Pentium III to run 3 ATA 160 GB drives as opposed to the
original 4 * 600 MB Hitachi ESDI drives that were in each of the RAID
boxes.
Its amazing how the same solution solves the problem on such different
hardware.
The VT103 PDP-11/73 system that I run once a decade also uses an external
power supply to run the hard disk drives.
Jerome Fine
7:23 a.m.
From: philip at axeside.co.uk
That sounds very low indeed. I wouldn't expect a transformer secondary
to drop below rated voltage with only half load on it! Are you sure you
haven't got a shorted turn somewhere?
I agree with Tony that you should do some more tests to see if there's
something wrong with the transformer or rectifier. For a start, look at
the secondary voltages and current drawn from the supply with all the
secondaries open circuit. Excessive no-load current is a reasonable
indicator of a shorted turn.
If you eventually do need to upgrade the power supply, could you fit
just one switcher in the cabinet with that transformer? Use it for the
8V line, and keep the old PSU for the rest?
Philip
Hi
I should note that a transformer rated at 5A AC will not provide
5A DC. Because the rectifiers only take power on the peaks of the
cycle the power in the transformers windings are much greater
than if it was a continuous load. One needs to derate the transformer
some.
Dwight
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Since in
this case the first test is an easy one to do (namely measuring
the AC voltage from the transformer secondary winding), I wouldn't even
think of not doing it.
Yes Tony... I have done all of that. The secondary is dropping to nearly
6 VAC with only 5A load on the DC side of the bridge and 120VAC supplied
to the primary. The ripple on the DC side is in excess of 1V with that
same 5A load. I have swapped in alternative filter caps temporarily
and see little improvement. I checked the bridge for shorted diodes or
excessive loss and it looks OK. 
7:49 a.m.
dwight elvey wrote:
Unless you have big choke hiding in the circuit. :)
From: philip at axeside.co.uk
That sounds very low indeed. I wouldn't expect a transformer secondary
to drop below rated voltage with only half load on it! Are you sure you
haven't got a shorted turn somewhere?
I agree with Tony that you should do some more tests to see if there's
something wrong with the transformer or rectifier. For a start, look at
the secondary voltages and current drawn from the supply with all the
secondaries open circuit. Excessive no-load current is a reasonable
indicator of a shorted turn.
If you eventually do need to upgrade the power supply, could you fit
just one switcher in the cabinet with that transformer? Use it for the
8V line, and keep the old PSU for the rest?
Philip
Hi
I should note that a transformer rated at 5A AC will not provide
5A DC. Because the rectifiers only take power on the peaks of the
cycle the power in the transformers windings are much greater
than if it was a continuous load. One needs to derate the transformer
some.
Dwight Since in
this case the first test is an easy one to do (namely measuring
the AC voltage from the transformer secondary winding), I wouldn't even
think of not doing it.
Yes Tony... I have done all of that. The secondary is dropping to nearly
6 VAC with only 5A load on the DC side of the bridge and 120VAC supplied
to the primary. The ripple on the DC side is in excess of 1V with that
same 5A load. I have swapped in alternative filter caps temporarily
and see little improvement. I checked the bridge for shorted diodes or
excessive loss and it looks OK.
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8:52 a.m.
On Sunday (06/06/2010 at 08:49AM -0600), Ben wrote:
Ya... thanks guys. The transformer is rated at 7.25V @ 10A. But it's
not even able to do 7.25V at 5A so I think it is the culprit.
I can't find any hidden choke either so they weren't relying on any magic.
Good chance this transformer has been "bad" since the day I built the
machine.
I think I have converged on my solution. I have two new Triad
transformers that when bolted to a piece of aluminum angle stock,
suitably cut to size, will fit stacked in the same footprint as the
original transformer. Therefore, I won't need to drill any new holes
in the chassis, will have a "period" accurate modification and the thing
should work as was originally intended.
Thanks for playing along.
Chris
--
Chris Elmquist
I should note that a transformer rated at 5A AC will not provide
5A DC. Because the rectifiers only take power on the peaks of the
cycle the power in the transformers windings are much greater
than if it was a continuous load. One needs to derate the transformer
some.
Dwight
Unless you have big choke hiding in the circuit. :)
7 Jun
7 Jun
11:21 a.m.
Ya... thanks guys. The transformer is rated at 7.25V
@ 10A. But it's
not even able to do 7.25V at 5A so I think it is the culprit.
Does it have just a simple 2-terminal secondary winding, or is it
centre-tapped? What sort of rectifier are you using (bridge, 2 diode
biphase, etc)?
Does the thransformer 'look big enough'? If it is 7.25V at 10A, that's
72.5VA. Is the core comparable in size to other 72VA transformers? No,
this isnot an exact thing, but in general the size of a small transformer
core (small being less than a couple of kVA...) gives a good idea as to
its VA rating.
I am wondering just what sort of fault could cause this other than a
mislabelled transformer.
I think I have converged on my solution. I have two
new Triad
transformers that when bolted to a piece of aluminum angle stock,
suitably cut to size, will fit stacked in the same footprint as the
original transformer. Therefore, I won't need to drill any new holes
in the chassis, will have a "period" accurate modification and the thing
should work as was originally intended.
Sounds like a good modification. Totally reversable, and it keeps the
circuit design much the same.
I just hope the problem is just the transformer...
-tony
5:02 p.m.
On Monday (06/07/2010 at 07:21PM +0100), Tony Duell wrote:
The 8V supply uses a bridge in a chassis mount package. The +/- 12V
supply uses a 24V CT secondary of the same transformer feeding a bridge
built from discrete diodes on a PCB.
Here's a picture of the whole shbang,
http://www.swtpc.com/mholley/MP_P/PowerSupply_Photo.jpg
as well as all the docs for the (trivially simple) supply, SWTPC MP-P,
http://www.swtpc.com/mholley/MP_P/MP_P_Index.htm
and Micheal Holley documents his modification to improve the supply here,
http://www.swtpc.com/mholley/MP_P/M6809/Power_Supply_Mods.htm
I have indentified two transformers that can be stacked in the same footprint
as the single original so I will not have to drill new holes to mount the
second transformer near the power distribution board nor will have I have
to mount the power distribution board on its side as Micheal did.
Well, there's only two other parts (a bridge and giant cap) to swap after
that so I will get there pretty quickly if it's not the transformer ;-)
Chris
--
Chris Elmquist
Ya... thanks
guys. The transformer is rated at 7.25V @ 10A. But it's
not even able to do 7.25V at 5A so I think it is the culprit.
Does it have just a simple 2-terminal secondary winding, or is it
centre-tapped? What sort of rectifier are you using (bridge, 2 diode
biphase, etc)? 
Does the thransformer 'look big enough'? If it
is 7.25V at 10A, that's
72.5VA. Is the core comparable in size to other 72VA transformers? No,
this isnot an exact thing, but in general the size of a small transformer
core (small being less than a couple of kVA...) gives a good idea as to
its VA rating.
It seems small to me but that's a very subjective measurement.
I am wondering just what sort of fault could cause
this other than a
mislabelled transformer.
I'd offer that it was an under designed supply and it could never
really source the kind of current they claimed. The transformer has no
markings on it so not possible to identify the manufacturer or actual
specifications. The specs I am operating against are those in the SWTPC
MP-P documentation, where the claim of 7.25V at 10A is made.
I think I have
converged on my solution. I have two new Triad
transformers that when bolted to a piece of aluminum angle stock,
suitably cut to size, will fit stacked in the same footprint as the
original transformer. Therefore, I won't need to drill any new holes
in the chassis, will have a "period" accurate modification and the thing
should work as was originally intended.
Sounds like a good modification. Totally reversable, and it keeps the
circuit design much the same.
I just hope the problem is just the transformer... 
6 Jun
6 Jun
10:53 a.m.
Ben wrote:
.. and in which event one has to lower the voltage expectations.
Here, for example, is how Hammond derates some of their common
low-voltage transformers:
Ratios of DC output from filter to AC transformer specs
-------------------------------------------------------
Voltage Current
DC/AC DC/AC
Bridge feeding cap-input filter 1.25 0.56
Bridge feeding choke-input filter 0.88 1.0
Also to note, simply adding capacitance to a cap input filter cannot be relied
on as a solution, as it reduces the conduction angle of the rectifiers (the
portion of a cycle for which they conduct), leading to more transformer losses.
dwight elvey wrote:
>
>> From: philip at axeside.co.uk
...
>>>
>>> Yes Tony... I have done all of that. The secondary is dropping to nearly
>>> 6 VAC with only 5A load on the DC side of the bridge and 120VAC supplied
>>> to the primary. The ripple on the DC side is in excess of 1V with that
>>> same 5A load. I have swapped in alternative filter caps temporarily
>>> and see little improvement. I checked the bridge for shorted diodes or
>>> excessive loss and it looks OK.
>>
....
I should note that a transformer rated at 5A AC will not provide
5A DC. Because the rectifiers only take power on the peaks of the
cycle the power in the transformers windings are much greater
than if it was a continuous load. One needs to derate the transformer
some.
Dwight
Unless you have big choke hiding in the circuit. :)
10:54 a.m.
That sounds very low indeed. I wouldn't expect a
transformer secondary
to drop below rated voltage with only half load on it! Are you sure you
haven't got a shorted turn somewhere?
My experience is that a single shorted turn in a mains transformer will
cause enough primary-side current to be drawn to blow the fuse. Of course
that's assuming there is a primary-side fuse -- but if there isn't there
darn well should be :-)
Still, it can't hurt to measure the primary current with no load on the
secondaaries. If you have a wattmeter, masure the power consumed by the
trransformer with no secodnary load (note that this is _not_ the product
of the primary corrent and the mains voltage, they will not be in phase).
If there is significant power drawn on no-load, then suyspect a shorted turn.
I agree with Tony that you should do some more tests
to see if there's
something wrong with the transformer or rectifier. For a start, look at
the secondary voltages and current drawn from the supply with all the
secondaries open circuit. Excessive no-load current is a reasonable
indicator of a shorted turn.
I should have read on before writing the above :-)
If you eventually do need to upgrade the power supply, could you fit
just one switcher in the cabinet with that transformer? Use it for the
8V line, and keep the old PSU for the rest?
My personal view (if I had such a machine) is that as the originial PSU
wasn't a switcher, you shouldn't use a switcher to do any form of upgrade
or repair. But it's really up to you.
-tony
3:05 p.m.
On Sunday (06/06/2010 at 06:54PM +0100), Tony Duell wrote:
Yes... this is a fuse but it has not blown... ever. The machine got
pretty extensive use from early 1976 until 1984 or so and even with the
low 8V rail, it did heroic efforts for me.
I completely agree... although, I am now prepared to replace the transformer
so once I have desoldered the original I suspect I'll be putting back the
new ones.
I agree with this too absolutely.
I have another homebrew S100 machine from the same era and it too has
a marginal power supply... but I won't think twice about rebuilding
that one with a set of switchers. It has no pedigree or authentity to
be preserved... other than it is built in a wooden box with a bright
orange Formica front panel... a piece that was left over from my mom's
new kitchen in 1979 ;-)
Chris
--
Chris Elmquist
That sounds
very low indeed. I wouldn't expect a transformer secondary
to drop below rated voltage with only half load on it! Are you sure you
haven't got a shorted turn somewhere?
My experience is that a single shorted turn in a mains transformer will
cause enough primary-side current to be drawn to blow the fuse. Of course
that's assuming there is a primary-side fuse -- but if there isn't there
darn well should be :-) Still, it can't hurt to measure the primary
current with no load on the
secondaaries. If you have a wattmeter, masure the power consumed by the
trransformer with no secodnary load (note that this is _not_ the product
of the primary corrent and the mains voltage, they will not be in phase).
If there is significant power drawn on no-load, then suyspect a shorted turn.
Yup. Have a wattmeter and can make this measurement... however only
after desoldering all the connections from the transformer to the
chassis mounted bridge and the bridge mounted on a PCB since none of
this is connectorized.
I agree with
Tony that you should do some more tests to see if there's
something wrong with the transformer or rectifier. For a start, look at
the secondary voltages and current drawn from the supply with all the
secondaries open circuit. Excessive no-load current is a reasonable
indicator of a shorted turn.
I should have read on before writing the above :-) If you
eventually do need to upgrade the power supply, could you fit
just one switcher in the cabinet with that transformer? Use it for the
8V line, and keep the old PSU for the rest?
My personal view (if I had such a machine) is that as the originial PSU
wasn't a switcher, you shouldn't use a switcher to do any form of upgrade
or repair. But it's really up to you. 
7:40 p.m.
Chris Elmquist wrote:
Have you tried the simplest bridge rectifier test of all - simply hook
your voltmeter to the outputs of the bridge and then using a single
diode of sufficient voltage and amperage you then jumper each internal
diode with the external one. If/when the voltage jumps up, then you have
proven the bridge rectifier is defective. Static tests (power off -
voltage drop across the bridge) doesn't catch bridges that fail under load.
Further to that - if you are not clear about what I mean by jumpering -
you take your external diode and bend the leads so you can place one
lead on an AC terminal and the other to either the '+' or '-' terminals
of the bridge. Taking EXTREME care to always have the banded end of the
external diode connecting to the more positive side of the bridge under
test!!!
I hope that is clear enough to enable you to check your bridge
recitifier which I (and others here) believe is the actual problem, not
the transformer!
I completely agree... although, I am now prepared to replace the transformer
so once I have desoldered the original I suspect I'll be putting back the
new ones.
Please don't do this until you have tested the bridge. If the bridge is
OK, then the next suspect is the primary filter capacitor for your 8VDC
system. Again, bridge it with a known to be good capacitor or similar or
less capacity and watch to see if the voltage goes up...
I agree with this too absolutely.
I have another homebrew S100 machine from the same era and it too has
a marginal power supply... but I won't think twice about rebuilding
that one with a set of switchers. It has no pedigree or authentity to
be preserved... other than it is built in a wooden box with a bright
orange Formica front panel... a piece that was left over from my mom's
new kitchen in 1979 ;-)
Chris
Switchers are great for protecting against power supply failures,
however some reset circuits assume linear supplies and don't work
reliably with switching supplies unless you modify them so the voltage
rails go up as the system expects.
If you decide against switches get TRANSORPS for the critical voltage
lines to clamp the voltage (and blow fuses) if the linear supply voltage
rises due to failure.
John :-#)#
PS - my experience is in servicing tube and solid state equipment since
the 1960s - since the mid 70s as a living (flippers.com ;-).
On Sunday (06/06/2010 at 06:54PM +0100), Tony Duell
wrote:
Yes... this is a fuse but it has not blown... ever. The machine got
pretty extensive use from early 1976 until 1984 or so and even with the
low 8V rail, it did heroic efforts for me.
Transformers are extremely simple beasts - they either work or they
don't. When they fail you really know it - they blow fuses and/or start
smoking with goop leaking out and making a general smelly mess or a
winding opens and you get no output at all. A transformer does NOT get
weak.
That
sounds very low indeed. I wouldn't expect a transformer secondary
to drop below rated voltage with only half load on it! Are you sure you
haven't got a shorted turn somewhere?
My experience is that a single shorted turn in a mains transformer will
cause enough primary-side current to be drawn to blow the fuse. Of course
that's assuming there is a primary-side fuse -- but if there isn't there
darn well should be :-)
Still, it can't hurt to measure the primary
current with no load on the
secondaaries. If you have a wattmeter, masure the power consumed by the
trransformer with no secodnary load (note that this is _not_ the product
of the primary corrent and the mains voltage, they will not be in phase).
If there is significant power drawn on no-load, then suyspect a shorted turn.
Yup. Have a wattmeter and can make this measurement... however only
after desoldering all the connections from the transformer to the
chassis mounted bridge and the bridge mounted on a PCB since none of
this is connectorized.
I agree with Tony that you should do some more tests
to see if there's
something wrong with the transformer or rectifier. For a start, look at
the secondary voltages and current drawn from the supply with all the
secondaries open circuit. Excessive no-load current is a reasonable
indicator of a shorted turn.
I should have read on before writing the above :-)
If you eventually do need to upgrade the power supply,
could you fit
just one switcher in the cabinet with that transformer? Use it for the
8V line, and keep the old PSU for the rest?
My personal view (if I had such a machine) is that as the originial PSU
wasn't a switcher, you shouldn't use a switcher to do any form of upgrade
or repair. But it's really up to you.
7 Jun
7 Jun
3:58 a.m.
On Sunday (06/06/2010 at 07:40PM -0700), John Robertson wrote:
Capacitor has been replaced with no improvement. The original transformer
with a new bridge and new filter cap are unable to keep 6VDC on the
DC side with 5A load. That's half the rating of the supply.
Transformers are extremely simple beasts - they either work or they
don't. When they fail you really know it - they blow fuses and/or start
smoking with goop leaking out and making a general smelly mess or a
winding opens and you get no output at all. A transformer does NOT get
weak.
I completely understand and I never believed it "got weak". I believe
it has been defective from the beginning. Probably not a failure but
a manufacturing defect or was just plain too small for the job.
I am not the first person to find the 8V rail being low on a SWTPC 6800
system.
Have you tried the simplest bridge rectifier test of
all - simply hook
your voltmeter to the outputs of the bridge and then using a single
diode of sufficient voltage and amperage you then jumper each internal
diode with the external one. If/when the voltage jumps up, then you have
proven the bridge rectifier is defective. Static tests (power off -
voltage drop across the bridge) doesn't catch bridges that fail under
load.
Yup. I've done the even simpler test which is to replace the entire bridge.
I have been using a dummy load to test the supply and in one of the tests
I used the original transformer and filter cap but substituted a new bridge.
Same inability to source 5A.
I then also swapped the filter capacitor... same inability to source 5A.
Further to that - if you are not clear about what I
mean by jumpering -
you take your external diode and bend the leads so you can place one
lead on an AC terminal and the other to either the '+' or '-' terminals
of the bridge. Taking EXTREME care to always have the banded end of the
external diode connecting to the more positive side of the bridge under
test!!!
I hope that is clear enough to enable you to check your bridge
recitifier which I (and others here) believe is the actual problem, not
the transformer!
An abundance of clearness. Thank you. I too have been doing this
stuff for 35+ years so the actual tests, circuit modifications, etc are
not a problem.
My original question was seeking input on how much religion there is
about making modifications to original equipment or not. I had pretty
much already determined that the transformer was the problem.
I completely
agree... although, I am now prepared to replace the transformer
so once I have desoldered the original I suspect I'll be putting back the
new ones.
Please don't do this until you have tested the bridge. If the bridge is
OK, then the next suspect is the primary filter capacitor for your 8VDC
system. Again, bridge it with a known to be good capacitor or similar or
less capacity and watch to see if the voltage goes up... Switchers are great for protecting against power
supply failures,
however some reset circuits assume linear supplies and don't work
reliably with switching supplies unless you modify them so the voltage
rails go up as the system expects.
Understood. This system has a 555 timer that resets it sometime a few
SECONDS after 5V goes valid. I think we'll be OK.
If you decide against switches get TRANSORPS for the
critical voltage
lines to clamp the voltage (and blow fuses) if the linear supply voltage
rises due to failure.
A good plan. Thank you.
PS - my experience is in servicing tube and solid
state equipment since
the 1960s - since the mid 70s as a living (flippers.com ;-).
Great. I appreciate the input but I think unfortunately, in the case
of my SWTPC 6800, it does need a new transformer (or two).
Chris
--
Chris Elmquist
4:02 p.m.
From: chrise at pobox.com
---snip---
My original question was seeking input on how much religion there is
about making modifications to original equipment or not. I had pretty
much already determined that the transformer was the problem.
---snip---
Hi Chris
One other thing may have happened over time, in the transformer.
Many transformers were originally manufactured with 2 primary windings.
This way, they could be easily configured for 220 or 110.
In cases used for 110, the windings would be paralleled inside
the case and then only one pair of wires run external.
It is possible that from stress or corrosion that one of the wires
has gone open. This would match the problem that you are
seeing.
Dwight
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8 Jun
8 Jun
11:10 a.m.
Transformers
are extremely simple beasts - they either work or they
don't. When they fail you really know it - they blow fuses and/or start
smoking with goop leaking out and making a general smelly mess or a
winding opens and you get no output at all. A transformer does NOT get
weak.
I completely understand and I never believed it "got weak". I believe
it has been defective from the beginning. Probably not a failure but
a manufacturing defect or was just plain too small for the job. Switchers are
great for protecting against power supply failures,
however some reset circuits assume linear supplies and don't work
reliably with switching supplies unless you modify them so the voltage
rails go up as the system expects.
Understood. This system has a 555 timer that resets it sometime a few
SECONDS after 5V goes valid. I think we'll be OK. If you decide against switches get TRANSORPS for
the critical voltage
lines to clamp the voltage (and blow fuses) if the linear supply voltage
rises due to failure.
A good plan. Thank you.
7 Jun
7 Jun
11:40 a.m.
Still, it
can't hurt to measure the primary current with no load on the
secondaaries. If you have a wattmeter, masure the power consumed by the
trransformer with no secodnary load (note that this is _not_ the product
of the primary corrent and the mains voltage, they will not be in phase).
If there is significant power drawn on no-load, then suyspect a shorted turn.
Yup. Have a wattmeter and can make this measurement... however only
after desoldering all the connections from the transformer to the
chassis mounted bridge and the bridge mounted on a PCB since none of
this is connectorized. My personal
view (if I had such a machine) is that as the originial PSU
wasn't a switcher, you shouldn't use a switcher to do any form of upgrade
or repair. But it's really up to you.
I agree with this too absolutely.
I have another homebrew S100 machine from the same era and it too has
a marginal power supply... but I won't think twice about rebuilding
that one with a set of switchers. It has no pedigree or authentity to
5:04 p.m.
On Monday (06/07/2010 at 07:40PM +0100), Tony Duell wrote:
You bet. Will report on what I find.
True... but they also rarely used the horrendous kludge of a supply the
thing has it now so I am not bothered by this sort of change for this
particular machine. The end result is that the thing will be many,
many pounds lighter ;-)
Chris
--
Chris Elmquist
Yup. Have a
wattmeter and can make this measurement... however only
after desoldering all the connections from the transformer to the
chassis mounted bridge and the bridge mounted on a PCB since none of
this is connectorized.
As you're considering replacing the transformer anyway, I think I would
disconnect the secondary leads and do the test just for fun. I like to
know exactly what's failed and how :-) I have another
homebrew S100 machine from the same era and it too has
a marginal power supply... but I won't think twice about rebuilding
that one with a set of switchers. It has no pedigree or authentity to
I probably wouldn't just because S100 boxen rarel used SMPSUs. But it's a
personal choice. 
2 Jun
2 Jun
10:50 p.m.
On 06/02/2010 11:10 AM, Tony Duell wrote:
More seriously, I much prefer to make mesaurements and
do tests before
changing anything.
That's crazy talk! Everyone knows that the way to repair
equipment is
to swap all the parts first!
Reminds me of Feynman's story about repairing someone's radio when he
was young. The guy was getting impatient because Feynman was just
sitting around doing nothing. Then Feynman swapped two tubes, and the
radio worked. They guy was amazed. "He fixes radios by *thinking*!"
3 Jun
3 Jun
8:05 a.m.
From: eric at brouhaha.com
On 06/02/2010 11:10 AM, Tony Duell wrote:
Hi
It was 201A's not 212A's.
Dwight
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More seriously, I much prefer to make
mesaurements and do tests before
changing anything.
That's crazy talk! Everyone knows that the way to repair
equipment is
to swap all the parts first!
Reminds me of Feynman's story about repairing someone's radio when he
was young. The guy was getting impatient because Feynman was just
sitting around doing nothing. Then Feynman swapped two tubes, and the
radio worked. They guy was amazed. "He fixes radios by *thinking*!"
11:33 a.m.
On 06/02/2010 11:10 AM, Tony Duell wrote:
Fortunately I am not 'everyone'. And I am proud to be 'crazy':-)
More seriously, I much prefer to make
mesaurements and do tests before
changing anything.
That's crazy talk! Everyone knows that the way to repair
equipment is
to swap all the parts first!
Reminds me of Feynman's story about repairing someone's radio when he
was young. The guy was getting impatient because Feynman was just
sitting around doing nothing. Then Feynman swapped two tubes, and the
radio worked. They guy was amazed. "He fixes radios by *thinking*!"
I have been compared to Feynman in that story several times before. I
take it a a great honour to be compared to such a man (even though I know
I am nowhere near up to his standard in physics).
And yes, that is how I fix things. Do a few tests and _think_ about it.
-tony
1:37 p.m.
On 3 Jun 2010 at 19:33, Tony Duell wrote:
I have been compared to Feynman in that story several
times before. I
take it a a great honour to be compared to such a man (even though I
know I am nowhere near up to his standard in physics).
Hmmm, I've been compared to Ted Bundy...
--Chuck
2:33 p.m.
----- Original Message -----
From: "Chuck Guzis" <cclist at sydex.com>
To: "General Discussion: On-Topic and Off-Topic Posts"
<cctalk at classiccmp.org>
Sent: Thursday, June 03, 2010 4:37 PM
Subject: Re: Authenticity opinions
On 3 Jun 2010 at 19:33, Tony Duell wrote:
Why, are you a serial "port" killer or something?
I have been compared to Feynman in that story
several times before. I
take it a a great honour to be compared to such a man (even though I
know I am nowhere near up to his standard in physics).
Hmmm, I've been compared to Ted Bundy...
--Chuck
3:48 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Fred Cisin
Sent: Thursday, June 03, 2010 2:50 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Authenticity opinions
Howard Stern as Zaphod - brilliant! Scary.
Hmmm, I've been compared to Ted Bundy...
I've been compared to Al Bundy...
(I've always thought that Ed O'Neill would be a great actor for the
role
of Arthur Dent. Jacqueline Pearce for Trillian, Howard Stern for
Zaphod
Beeblebrox)
4 p.m.
> I've been compared to Al Bundy...
> (I've always thought that Ed O'Neill would be a great actor for the
> role of Arthur Dent. Jacqueline Pearce for Trillian, Howard Stern for
> Zaphod Beeblebrox)
On Thu, 3 Jun 2010, Ian King wrote:
Howard Stern as Zaphod - brilliant! Scary.
Roseanne Barr as the Vogon captain
(maybe Tom Arnold and John Goodman as Vogon guards?)
Paul Darrow as Ford Prefect?
I'm torn between Sean Connery and Christopher Lloyd as Slartibartfast.
4:10 p.m.
New subject: Recasting HGttG (was RE: Authenticity opinions)
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Fred Cisin
Sent: Thursday, June 03, 2010 4:00 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: RE: Authenticity opinions
Too scary - and too obvious. Could we just go with Anthony Hopkins in
"Hannibal Lector mode"?
> I've been compared to Al Bundy...
> (I've always thought that Ed O'Neill would be a great actor for the
> role of Arthur Dent. Jacqueline Pearce for Trillian, Howard Stern
for
> Zaphod Beeblebrox)
On Thu, 3 Jun 2010, Ian King wrote:
Howard Stern as Zaphod - brilliant! Scary.
Roseanne Barr as the Vogon captain
(maybe Tom Arnold and John Goodman as Vogon guards?)
Paul Darrow as Ford Prefect?
I'm torn between Sean Connery and Christopher Lloyd as Slartibartfast.
Lloyd, hands down. IMHO Connery doesn't have the (blunt-edged) tool-of-fate role in
him - he's too much a mover, not a shaken.
5092
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32 comments
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ard＠p850ug1.demon.co.uk -
bfranchuk＠jetnet.ab.ca -
cclist＠sydex.com -
chrise＠pobox.com -
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dkelvey＠hotmail.com -
eric＠brouhaha.com -
hilpert＠cs.ubc.ca -
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pinball＠telus.net -
shoppa＠trailing-edge.com -
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