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Looking for some help with a dead power supply...

S

Stone

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This one's for all you PSU gurus.

I've got an AT PSU that I use in a test bed. Occasionaly I put a card in a slot and the PSU shuts down immediately after power up. I think the card has a bad cap that shorts the 12v line to ground. So I see why the PSU immediately shuts down. But after several minutes it always comes back to life when I turn it back on after having removed the suspect card from the slot. The last time this happened was the last time the PSU worked. After that there was nothing. I've tried it with various loads and it's just DEAD. Any ideas where to look for the problem? I've inspected the PSU's components visually but that's as far as I've gone. What should I do next?
 
Stone said:
This one's for all you PSU gurus.

I've got an AT PSU that I use in a test bed. Occasionaly I put a card in a slot and the PSU shuts down immediately after power up. I think the card has a bad cap that shorts the 12v line to ground. So I see why the PSU immediately shuts down. But after several minutes it always comes back to life when I turn it back on after having removed the suspect card from the slot. The last time this happened was the last time the PSU worked. After that there was nothing. I've tried it with various loads and it's just DEAD. Any ideas where to look for the problem? I've inspected the PSU's components visually but that's as far as I've gone. What should I do next?
Click to expand...

Just off hand, if the PSU is shutting down (not smoking anything), it would indicate that the crow bar is working. If a quick visual of the caps is okay, you might look at the SCR and all rectifier bridges (10-1 front to back but not accurate in circuit - lift one end). Another quick check (not all conclusive) if you have access to a scope is to check for AC on the power transistor(s) collectors (less than 100 mvac). If your anything like me you'll probably say the hell with it. P.S. I'm working on one right now for a Tandy Sensation! and I'm thinking I've better things to do with my time.
 
It could well be an out-of-spec power resistor as well. When those age, their resistance tends to increase, raising the voltage drop across them and giving fake overcurrent indications, triggering the over-current circuitry.
 
Possibly a HV cap on the HT side, in the switching circuit. They can fail silently and just stop the switching. You could start from the mains input and work forward to see where the power stops, but checking collector and base on the switching MOSFETs will also indicate which side of the circuit has the problem. AT PSUs are not that easy to replace anymore.
Rick
 
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