Re: Compaq PDP-121P - Baked MOV
Probably not.
The DC voltages on your input caps looked good and didn't seem to drop out / dip when turning On the PSU... so I suspect this isn't the issue.
Nonetheless, it does need to be addressed just in case. Just because I don't *think* it is a *likely* issue doesn't mean that it can't be one.
Yes.
Though there should be windings inside. This is the PPFC inductor, after all.
Yes.
Why would that be an issue?
I've seen a lot of PSUs with low-value load resistors... hence measuring much lower resistance on the 12V rail. So I don't really think the resistance alone can tell you anything.
^ This.
If the PSUs were the same, I'd definitely try to see why there was such a wild mismatch between the resistances. But...
That pretty much invalidates the resistance comparison... or at least as far as showing any information that may suggest a problem.
Exactly!
I suppose no harm in posting the resistances. Though when things measure above a few hundred Ohms on the output, then generally that means at least there is nothing wrong with the big power delivery components (i.e. rectifiers or their snubbers being shorted.) But it doesn't really amount to much of anything else. Open feedback resistors can essentially go completely unnoticed, due to their relatively high resistance value. So a resistance check on the voltage rail won't tell you if one went bad or not. This can only be found through careful component testing and perhaps a schematic or application diagram of the supervisor chip to understand what is going on.
That being said... and recently trying out an experiment with a non-working PSU that helped me find the problem... maybe this troubleshooting trick could work here too:
- Take a working ATX PSU and backfeed each rail into the non-working PSU, one at a time only, then power-up the non-working PSU too and see if it stays turned On.
So for example, let's start with the 3.3V rail: connect the 3.3V of both PSUs together and turn On the working PSU. Verify that 3.3V is back-feeding into the non-working PSU. Then turn On the non-working PSU with a load and see if it can stay turned On without shutting down. If not, repeat this experiment for the 5V rail (after disconnecting the 3.3V rail.) Then do the 12V rail and -12V rail. Since the 5VSB is derived from a different circuit, don't do this experiment for the 5VSB. Also don't connect any of the signal lines on the two PSUs, such as PS-ON or PG. However, for the specific test of the 3.3V rail, make sure the 3.3V "return" / "sense" (if there is one) is connected on both PSUs. Without 3.3V sense, 3.3V rail could be off-value on one or both PSUs.
If you do find that the non-working PSU stays turned On when one of these rails is connected, then there may be something wrong with the output of that rail on the PSU.
The only tricky part with the PSU from this thread is that it has a 12.8V rail... so not sure if we should be back-feeding this one or just the regular 12V rail. But I suppose you could try both. Nothing should really smoke or burn, so long as both PSUs are decent brands (i.e. have good working protections.) I would avoid using a "cheapie" gutless wonder for this test.
Originally posted by socketa
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The DC voltages on your input caps looked good and didn't seem to drop out / dip when turning On the PSU... so I suspect this isn't the issue.
Nonetheless, it does need to be addressed just in case. Just because I don't *think* it is a *likely* issue doesn't mean that it can't be one.
Originally posted by socketa
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Though there should be windings inside. This is the PPFC inductor, after all.
Originally posted by socketa
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Originally posted by sam_sam_sam
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I've seen a lot of PSUs with low-value load resistors... hence measuring much lower resistance on the 12V rail. So I don't really think the resistance alone can tell you anything.
Originally posted by sam_sam_sam
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If the PSUs were the same, I'd definitely try to see why there was such a wild mismatch between the resistances. But...
Originally posted by socketa
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Originally posted by socketa
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I suppose no harm in posting the resistances. Though when things measure above a few hundred Ohms on the output, then generally that means at least there is nothing wrong with the big power delivery components (i.e. rectifiers or their snubbers being shorted.) But it doesn't really amount to much of anything else. Open feedback resistors can essentially go completely unnoticed, due to their relatively high resistance value. So a resistance check on the voltage rail won't tell you if one went bad or not. This can only be found through careful component testing and perhaps a schematic or application diagram of the supervisor chip to understand what is going on.
That being said... and recently trying out an experiment with a non-working PSU that helped me find the problem... maybe this troubleshooting trick could work here too:
- Take a working ATX PSU and backfeed each rail into the non-working PSU, one at a time only, then power-up the non-working PSU too and see if it stays turned On.
So for example, let's start with the 3.3V rail: connect the 3.3V of both PSUs together and turn On the working PSU. Verify that 3.3V is back-feeding into the non-working PSU. Then turn On the non-working PSU with a load and see if it can stay turned On without shutting down. If not, repeat this experiment for the 5V rail (after disconnecting the 3.3V rail.) Then do the 12V rail and -12V rail. Since the 5VSB is derived from a different circuit, don't do this experiment for the 5VSB. Also don't connect any of the signal lines on the two PSUs, such as PS-ON or PG. However, for the specific test of the 3.3V rail, make sure the 3.3V "return" / "sense" (if there is one) is connected on both PSUs. Without 3.3V sense, 3.3V rail could be off-value on one or both PSUs.
If you do find that the non-working PSU stays turned On when one of these rails is connected, then there may be something wrong with the output of that rail on the PSU.
The only tricky part with the PSU from this thread is that it has a 12.8V rail... so not sure if we should be back-feeding this one or just the regular 12V rail. But I suppose you could try both. Nothing should really smoke or burn, so long as both PSUs are decent brands (i.e. have good working protections.) I would avoid using a "cheapie" gutless wonder for this test.
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