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IBM PS/2 P70 power supply issue

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markgm

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Nov 2, 2017
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Plymouth, CT
I was curious if anyone has any pointers in bringing a P70 power supply back to life. With the power supply disconnected from the unit I get no voltage on any of the lines going to the motherboard (that seems really strange to me). The fuse is good, and the fan twitches when the power switch is switched on (maybe a complete rotation, and only when powered on after sitting for some time). I'm stumped as I don't think I've ever had a power supply that didn't put out anything on any of the rails. Nothing looks obviously wrong (photos here: https://photos.app.goo.gl/Yz7Tufi6oEc6iyxr6).
 
Just a quick update for anyone else who runs into this, the solution isn't to replace all of the capacitors! My guess is there's a short somewhere within the PSU, so my next step is to start desoldering components.
 
Remember that many SMPS designs back in the day couldn't run without a sufficient load on them. Running them unloaded can potentially cause damage to the supply, but it usually just results in unstable operation where the outputs are wildly out of spec.

But assuming a load is on the supply, and the fuse isn't blown, I'd suspect something on the secondary side of the supply. Check all of the power rails for shorts to ground with nothing plugged into the PSU and it being unplugged and turned off. If you do find a short or a low resistance, I'd start checking all of the diodes and transistors/mosfets for shorts. Another thing to check for are open resistors and/or resistors that have drifted out of their tolerance range.

If you don't find anything amiss, move over to the primary side. Since the fuse isn't blown, there's not going to be a short, but there could be an open resistor, or one that has drifted far off value causing issues. Other components are suspect. If you still don't find anything, you may try to replace the optocouplers (basically an LED and a receiver in a epoxy package) because they can eventually get weak enough to not provide proper feedback.
 
GiGaBiTe said:
Remember that many SMPS designs back in the day couldn't run without a sufficient load on them. Running them unloaded can potentially cause damage to the supply, but it usually just results in unstable operation where the outputs are wildly out of spec.
Click to expand...

Lacking ANY documentation on the P70 PSU [or any PS/2 PSU, for that matter...] lets me indulge in a fantasy that I "know" something... I'm unaware of any PS/2 that needed a load on the PSU to run correctly. Any hard facts from this probulation needs to be posted to this forum...

You can see forever on the shoulder of a giant...
 
Probing the power supply connector to ground I get a connectivity beep from my multimeter for a second or two until the resistance starts to climb on the 5V rail. Hopefully that actually narrows down the components to remove!
 
markgm said:
Probing the power supply connector to ground I get a connectivity beep from my multimeter for a second or two until the resistance starts to climb on the 5V rail. Hopefully that actually narrows down the components to remove!
Click to expand...

That doesn't tell you anything. The reason it beeps before the resistance starts climbing is the capacitors were charging up. When a capacitor is drained, it will pull heavy current for a very brief time and look like a short until it charges up.
 
Then I'll start removing and testing components. It'll be easier than tracking down another PSU for it!
 
markgm said:
Then I'll start removing and testing components. It'll be easier than tracking down another PSU for it!
Click to expand...
Did you have any luck solving this? I just just got a P70 with the same symptoms. The seller said it powered on with no display but after shipping it no longer powers on. My fan does the same twitch that you describe.
 
I did! I had already desoldered (and ordered) a bunch of components so I didn't know which component specifically fixed it, but if I had to do it again I'd start with the STR81159.
 

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There was some discussion above about if the power supply would work without a load. Can you confirm that it runs when taken out of the computer? Thanks
 
EXTRAordinary said:
There was some discussion above about if the power supply would work without a load. Can you confirm that it runs when taken out of the computer? Thanks
Click to expand...
it's generally a good idea to have a load on a PSU when testing it. If you have an old HDD (that you don't care too much about), hook that up and then check the voltages.
 
Mark,
I wonder what part was bad that you replaced that fixed the problem. i have the same issue (same symptoms you stated above). So i replaced the STR81159 regulator so far but no success. I see you desoldered most stuffs in the left side of the psu...did you have to replace them all? Are all parts available to order?
Thanks,
 
Hello, I recapped the board (that was the first thing I did), and I know someone else (I think it was in the FB PS/2 group) had luck replacing the STR81159. That said, I replaced the following: K794, STR81159, D5SB60, AC12F, and L78M05. I think they were all around $29 shipped when I ordered them.
 
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