Hardware Insights Forum

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 ............

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.

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 .

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 ..............

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.

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 ...... !!

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

another suggestion would be the controller IC itself.

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 .............. !!!

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.