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PC Power supply Troubleshooting

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MisaghSepehr

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Hello everybody;
I have a PC power supply in which when I connect Green wire to GND wire the power supply fan just turns for 20 degree and then stops.TL494 and rectifier diodes are healthy but I don't know how to remove the trouble of this power supply.further, +5V stand by is live too.what should I do?
tnx
 

look at Vin
look at Vout

look at FET gate drive
look at feedback signal

-go with scope probe "round the pins" of the pwm controller ...check each one
 

PC supplies usually have a least one output that requires a DC
load, or they will cut out. Whether it is the 5V or the 3.3V,
seems to depend on vintage and maker.

There are numerous web pages on using PC supplies standalone,
which indicate this. Try connecting a 10 ohm resistor from 5V
main to GND, then 3.3V to GND, see if that helps.
 

tnx for your replies;
I have done all these you mentioned, but TL494 and LM339 are healthy.power transistors driver is healthy too. but yet nothing!!!!!!!
I've connected 2A load to +12 and Gnd but in this case, the slight 20 degree revolution of the fan disappears!!!!!!!! in addition with no output voltage!
any other idea???
 
Did you try loading +5V and +3.3V like I suggested? I have
converted two PC supplies for bench use, one needed the
+5V and the other (newer) one, the +3.3V loaded.
 

After the feedback from the optocoupler on the Ac section you'll find some transistors which will control the oscillator in respect to the volatges monitored on the secondary side. You may check the opto coupleres and those transistors whcih may be faulty.
Cheers
 

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