Compaq 1997 "ATX" PSU

[Rainbow]

Got this PSU for repair - it stopped working:

Oh, one of the big caps is bulged and leaked - note the electrolyte on the heatsink:



Overall look - only good caps:

Both big caps removed - and dried electrolyte on the PCB:

Two resistors connected in paralle with the caps - they're different! According to the colors, one is 51K and the other one 61K. The 61K one was connected in parallel with the blown cap and both resistor and the cap were open - thus the PSU didn't work at all. Why the resistors weren't the same? A mistake or intention?

Replaced both resistors with 100K ones and installed new caps (only 330uF as I didn't have anything better):


After repair, I was not able to power it up. +5VSB worked but that was all. Then I found out that the PS_ON signal is active high (unlike standard ATX) - must be connected to +5VSB to turn on, not GND. Then it worked fine. It needs some load on the rails and the voltages weren't nice at all - looks like it's a feature as it's designed for specific Compaq system only (the voltages are probably good with that specific load).

[gg1978]

Re: Compaq 1997 "ATX" PSU

The 51K and 61K resistors are probably bleed resistors, to discharge the caps when the PC is turned off.. Are they the same size capacitance wise?? Then again, the size of a bleed resistor doesn't matter much, as long as it's big enough to not dissipate much power when it's on, and having the RC time constant be low enough to discharge the caps decently quickly..

[japlytic]

Re: Compaq 1997 "ATX" PSU

What appears to be dried electrolyte could be glue.

I can see what appears to be electrolyte on the top of the capacitor and the heatsink.

Some capacitors have a plastic top covering the vents; in some cases, it appears that the capacitor is bulging. Removing the plastic top can show wherether the capacitor is really bulging (or leaking) or not.

I would only recommend using 105C units for such an application as well.

[Rainbow]

Re: Compaq 1997 "ATX" PSU

There is both glue and electrolyte on the PCB - glue is around the cap, electrolyte near one of the holes.
That one cap was really bad (completely open) - the electrolyte sprayed out of it and ended on both heatsinks.
The resistors are also responsible for dividing the input voltage equally between the caps. If they're different, the voltage is not divided equally and one of the caps is stressed more.

[yanz]

Re: Compaq 1997 "ATX" PSU

After repair, I was not able to power it up. +5VSB worked but that was all. Then I found out that the PS_ON signal is active high (unlike standard ATX) - must be connected to +5VSB to turn on, not GND. Then it worked fine. It needs some load on the rails and the voltages weren't nice at all - looks like it's a feature as it's designed for specific Compaq system only (the voltages are probably good with that specific load).

pretty strange psu you have

I would only recommend using 105C units for such an application as well.

strange is that the 85c version has lower esr of the same size/capacitance. example is SMH (85C) versus KMH/KMM (105C).

The resistors are also responsible for dividing the input voltage equally between the caps. If they're different, the voltage is not divided equally and one of the caps is stressed more.

if that the only real cause for that blown primary cap, than it is certainly something to think about. i still think that the existence of those twin resistor are not very important. but it turn out that it's also important for them to be exactly the same.