PSU OK but does not turn on when connected

[grichf]

Computer HP Slimline s5000 System NY511AA#A2L
Motherboard MS7525 Ver. 1.0

I received this computer totally dead. Nothing turns on.
When opened it was full of dust/dirt and I cleaned it up.
-First I removed the disc drive, hard disc and all other periferals except my monitor, one at a time, trying to turn on each time. Nothing happened.
-Next I removed the power supply and checked voltages with PSon grounded to com. All voltages OK. Reconnected and noted the power supply does not turn on when ON switch pressed.
-Checked continuity of ON/OFF switch OK.
-Jumpered PSon to com on plugged in main connector. Connect AC and no result.
- Power supply works with 4pin 12V connector plugged in alone but not the 24 pin alone or both together.
- replaced motherboard with new unused one. Same problem. Hope I did not damage it.
- Ram in&out same problem.
-Close inspection of capacitors on both boards. All look OK.

Now I am looking for what else could prevent the PSU from turning on with a verified OK switch, power cord checked etc.

- Tried anther CPU but not sure of its condition. No result.
- measured resistance on each of the 24 pins to com. on the motherboard to look for abnormalities. No shorts. Detect capacitor charging on each active pin. Nothing unusual.

- now I am just scratching my head.
Would appreciate any input.

[JJR]

Re: PSU OK but does not turn on when connected

Have you tried another PSU?

[tom66]

Re: PSU OK but does not turn on when connected

Could be bad caps on PSU outputs. I've had power supplies that would run under no load but as soon as you connected more than a fan or a hard disk to them, they'd cut out due to the under-voltage protect or control loop getting out of whack.

[grichf]

Re: PSU OK but does not turn on when connected

I need to find another PSU.
Forgot to mention. I put a 1K resistor load on each active pin of the PSU, one at a time, to try to create the problem. No affect.

Will post when I try another PSU.

[grichf]

Re: PSU OK but does not turn on when connected

Borrowed a power supply and it works. Purchased a higher wattage replacement.

[grichf]

Re: PSU OK but does not turn on when connected

Opened up PSU. 5 bad caps. Lesson learned. Checking voltages unloaded not good enough.

[Agent24]

Re: PSU OK but does not turn on when connected

I need to find another PSU.
Forgot to mention. I put a 1K resistor load on each active pin of the PSU, one at a time, to try to create the problem. No affect.

Will post when I try another PSU.

A 1K resistor will draw so little current that it will be about as useful as testing without any load at all.

Even on the 12v rail that is only 12mA, which for a power supply designed to supply several amps on each rail is nothing.

[grichf]

Re: PSU OK but does not turn on when connected

I chose that value because the measured resistance at the pins on the board varied from 11K down to 1.7K. Anyway the 1K on one pin at a time was not sufficient load.

[Agent24]

Re: PSU OK but does not turn on when connected

I chose that value because the measured resistance at the pins on the board varied from 11K down to 1.7K.

That would make sense for a purely passive, resistive load - but in this case you have a complex circuit; a mixture of components with properties that change depending on their operation state and simply trying to measure the overall resistance with a meter won't get you anywhere, except perhaps tracking down short circuits.

[grichf]

Re: PSU OK but does not turn on when connected

That would make sense for a purely passive, resistive load - but in this case you have a complex circuit; a mixture of components with properties that change depending on their operation state and simply trying to measure the overall resistance with a meter won't get you anywhere, except perhaps tracking down short circuits.

That is the likely reason. I was trying everything I could without having a spare PSU. I would have been better off opening up the PSU and looking for faults.