Help Needed Repairing a HP PC power supply Urgent

[PWM123]

Hello everyone. recently got a used HP 300w ATX pc power supply for cheap and the main reason to buy it was to use as power supply to power car electronics,lights at home. So when i got it the power supply was very dirty so i opened up and cleaned inside. Then i plugged it in a 220v outlet(double checked that the Power supply switch was set to 230v) and turned it on by shorting green and ground wires on the ATX connector. Instantly i saw fan moved slighty and sparks/smoke . I have a good knowledge about electronics but not able to understand complex circuits. So i opened up the power supply and found 2 varistors( TVR 10241) burned and broken and everything else looked fine even the fuse was good(tested with multimeter). Then headed over to electronic shop to buy replacement varistors. got them and soldered on the board. when i flipped the PS circuit board i found a thick layer of dust on the soldering side of the board. i cleaned it with a brush and assembled everything back. Plugged it once again and shorted the PS on and ground wires. this time no sparks etc and everything looked good. But i noticed fan was not turning on. i double checked input volts and all was good. then i checked if there was any volts at the output side. To my surprise the 12v rail was giving 12.4v , 5v rail was giving 5.2v and 3.3v rail WAS GIVING 1.28V !!!!. So there must be a issue with 3.3v rail.
Turned everything off then on again. checked all voltages and all were same. but the Fan was not spinning. I cut off the fan wires(thought the fan may have shorted) but it was working fine on a 12v battery. checked the output volts going from the PS PCB to the fan and it read 0.28V!!.
Then i checked how the PS performes on load. I used 2 12v bulbs and one wired in 5v line and one in 12v line. shorted the ps green and ground. The bulbs turned on for a split second and then off. shorted the ps green and ground again but same bulbs turn on for split second then PS shuts off.

Now searched on net about it but no one reported similar issues. Then inspected the components inside all looked great all caps are fine. glanced every component but couldnt find any issue. now i need help from you people. I really need help to bring back this PS to life again.

I dont know why it is giving outputs when no load is there but as soon as there is a load. PS shuts off.
5VSB is giving 5.15v
3.3V sense wire is giving 1.28v.

checked all other voltages all are fine i think.

Fan circuit does not work and 1.28v on 3.3v rail and on load PS shuts off.
It has PS224 protection IC and it is installed on a separate small PCB with one other 8 pin IC and fan wires are connected to this board.
Please help me out what to do and what to check/diagnose.
thanks

 

[PWM123]

Pics of the PSU internals

 

[chuckey]

Is fan temperature controlled?
Frank

 

[PWM123]

Is fan temperature controlled?
Frank

Yes i think.. i tested it with a heat gun blowing on the temperature sensor attached on the heatsinks. but the volts on the fan wires rose 0.28v to 1.20v only and then at the 1.20v mark the PS shuts off without any load

 

[PWM123]

Anyone can tell me how to diagnose this problem? I think 3.3v rail is shorted. The PSU model is Liteon PS-6301-9HP

 

[Relayer]

Hello PWM123,
First up, I'd clean all that white gunk off the solder side of the PCB. During my computer monitor and TV repair days, glues used in the factory (not all), resulted in their composition to change over the years, and in many cases, the glue started to conduct.
Next, check any low value electrolytic capacitors in the secondary area of the PSU. Check any close to the transformer or near the optpo-coupler. Also check any resistors connected to the opto, as their values are fairly critical in regards to voltage feedback and regulation.
I'm surprised the varistors didn't take out the main fuse. I don't see them in any of your photos. Are you sure they were varistors and not high voltage caps?
Please let me know how you get on.
Regards,
Relayer

 

[c_mitra]

Please see if any of the electrolytic capacitors are bulged or deformed (at the top) - if yes, please replace them. Also see the power good line- it is supposed to sense all output voltages.

 

[PWM123]

Ok i will clean it. I glanced all components looked fine but will check caps and resistors. Yes the fuse is good. actually in the pic i have removed the varistors as thr ones i replaced were of little different part no. And i saw on net that a PS can work fine without varistord aswell. Will get back with the updates

 

[PWM123]

Not a single cap look bulged..all look fine. But will check in more detail now. Will get back with updates

 

[PWM123]

I checked the PS OK line...it showed zero volts

 

[PWM123]

I also checked Negative 12v rail and found out it is giving only 0.82v.

 

[c_mitra]

Not a single cap look bulged..all look fine. But will check in more detail now. Will get back with updates

Please see the 3V3 filter caps. They have high current ratings and they are prone to failure.

 

[Relayer]

actually in the pic i have removed the varistors as thr ones i replaced were of little different part no. And i saw on net that a PS can work fine without varistord aswell.

The varistors are in there for safety reasons and should be put into circuit. They are there to suppress voltage spikes. Otherwise you risk major destruction if a sufficient spike is generated by your mains voltage.
Regards,
Relayer

 

[D.A.(Tony)Stewart]

really old PC PSU's would not function without a 10% minimum load on 5 V rail. With no load other outputs like +12V can trigger overvoltage protection .

Dust and moisture can induce faults.

PC's of this vintage are usually scrapped.
I would look for another PSU.