Problem with ATX 450W PSU

[Galib]

Hi,
I get a ATX PSU 450W for Repairing one of my customer . But This PSU have all parts OK . I am getting 5V PS-ON and 5VSB . The Power Supply is not running . But this PSU is Running - When I make short Pin 4 of TL494 with GND and this time I do not need to short PS-ON with Ground . Just Short Pin 4 with Ground and PSU is running Fine !!! What is Problem in the Control Section of this PSU ?? What have I to check in the control section ?? This Pin 4 is DTC ( Dead time Control ) of TL494 and it is showing 4.2V .

This Power supply have two IC - TL494 and LM339 .

Waiting for your quick reply ............

[Th3_uN1Qu3]

I don't really understand you. Does it only work when you connect pin 4 to ground, not when you connect PS_ON? If so you need to check those TO-92 transistors (usually 2SC945 or similar) and the 1N4148 diodes going to pin 4 of the TL494. They form the protection circuit in this type of power supply. If any of them is blown it will keep pin 4 high (like you have noted) and prevent the PSU from working properly. Also check all outputs of the LM339.

[Galib]

Thanks Th3_uN1Qu3 . Thanks ... thanks a lot for your reply and valuable tips . Let me to check it . I will reply later what happens . Among the 22 Power supplies - I have just repair only 4, still I have to Repair 18 PSU !!! It's is long duty dude and PSU Repairing seems very tough to me .......... !!! Please don't leave me ..... I will come back soon .

[Galib]

Th3_uN1Qu3 you will be amazed - what I am going to say . I have De-soldered from PCB and soldered again all the Resistors, Capacitors, Diodes and Transistors in the controller section ... !!! But sorry to say - I have not found any single defective/faulty parts in the controller section . Every parts are OK !! Even I have changed all the electrolytic Capacitors there with new ones . But still the same Problem . What will I do now ??

Waiting for your Reply ..............

[c_hegge]

Did you actuallty replace any of the parts that The_uN1Qu3 mentioned? I would still try if even if they seem OK, since the problem has to be there somewhere.

[Galib]

Thanks C_hegge for your reply . But you do not need replace any good parts except Capacitor . It will be just waste of money and time !! Because your Orginal Problem is in another place, not with that good parts ...... !!

[Th3_uN1Qu3]

If the protection is being tripped then obviously something isn't good.

[c_hegge]

another suggestion would be the controller IC itself.

[Galib]

But I have checked Every Single Parts by De-Soldering them !!! I have 9 Controller IC TL494 and 11 Comparator IC LM339 . I have checked with these both Controller and Comparator IC one by one . But destination seems to be very very far .............. !!!

[c_hegge]

Sorry, you didn't mention that you tested the ICs in your last POST. How are you testing them? The only possibility I can think of that the components aren't being tested correctly. If all the components were really working, then the PSU should run.