Need help for repairing PSU with only +5vsb output

[stefos]

Re: Need help for repairing PSU with only +5vsb output

**I hope - You have at least 3 Volt present in Green Cable .**new one .

I got 4.943v on green cable vs 4.914v on the good one psu

[Escort Eagle]

Re: Need help for repairing PSU with only +5vsb output

4.94 Volt is OK . What about the Transistor C945 and Diodes ? Are they OK ??

[stefos]

Re: Need help for repairing PSU with only +5vsb output

4.94 Volt is OK . What about the Transistor C945 and Diodes ? Are they OK ??

Yes all are ok... I wanted to be 100% sure so i used all IC's and almost all diodes from secondary side, LM339N and 7500B from the good one psu and vise versa. The good one psu working ok and the faulty still no go just +5vsb. , Also checked all caps with ESR tester.

[redwire]

Re: Need help for repairing PSU with only +5vsb output

I don't want to interrupt the flow you guys have going.

I think the KA7500 is off (7500-pin 4) due to the LM339's shutdown latch (LM339-pin2/4/5) tripping. If you jumper KA7500 pin 4, it will then force the PSU on at full output.
I am looking at what makes the LM339 comparator trip (LM339-pin2/4/5). It's complicated, one comparator is doing a bunch of functions. Taking 339-pin5 high trips the latch. Then you have to remove power to reset it.

That takes me back to KA7500 pin 16. Why is the voltage wrong there?

I would compare readings around D14, C23, D37, R12.

[Escort Eagle]

Re: Need help for repairing PSU with only +5vsb output

Wow, You have a nice strange problem !!! But One thing is strange to me - LM339 IC's PIN 2 is connected with +5VSB . If it is connected with +5VSB, then Can that marked place be 0 Volt after switching ? [Shown in Image-05] However PSU ran in this design before . Is the other Good PSU same design ??

Now it seems to me that One of the component is failing at Load time OR there could be a bad soldering joint . I will suggest you to give a try by replacing those component with New One . [Shown in Image-06]

After changing the components if it does not run, then put the PCB under 100W bulb and keep heating about 30 to 45 minutes . If it favors the luck, then it could be run again !!!!!

[stefos]

Re: Need help for repairing PSU with only +5vsb output

Is the other Good PSU same design ??

The second working psu is exactly the same.

[Escort Eagle]

Re: Need help for repairing PSU with only +5vsb output

Measure the voltage in those Red circle Point in both Switching and Non-switching Condition in Both Good and Bad PSU and try to find out which component is failing at load time .

[stefos]

Re: Need help for repairing PSU with only +5vsb output

I would compare readings around D14, C23, D37, R12.

Are you referring on the attached schematic points? Is it possible to mark proposal checks on the real psu image please?

Thank you alot.

[stefos]

Re: Need help for repairing PSU with only +5vsb output

Measure the voltage in those Red circle Point in both Switching and Non-switching Condition in Both Good and Bad PSU and try to find out which component is failing at load time .

Hello !!!

I made proposal checks on both psu (good and bad) and got same results for NON-switching condition. As for PS_ON state the measurements were as the following :

........BAD PSU.......GOOD PSU
-----------------------------
P1___3.846 v______3.880 v
P2___0.661 v______520 mv
P3___36,6 mv______34,9 mv
P4___3.910 v______82,5 mv
P5___4.952 v______4.928 v
P6___0 v__________0 v
P7___0 v__________0 v
P8___3.363 v______8.5 mv

Sorting for 2-3 seconds D1 and D2 or C945 Emitter with D1 pins the psu switch on and works until unplug the cord (press switch off-on button).

I have changed the C945 transistor, near registors and diodes D1-D2-D3-D4 from the working psu and vise versa. Still no go

[Escort Eagle]

Re: Need help for repairing PSU with only +5vsb output

It is very difficult to repair when all the parts are OK and PSU does not run !! Because one parts fails at load time and it is the hardest thing to find that culprit component .

However - C945 Transistor's which you are indicating as Emitter (E), that must be Base (B) . If it is not Base(B), then PSU will not run . Emitter(E) is directly connected with Ground .

Finally - I will tell you to replace 3 Resistors From Good PSU [Shown in Imge-08] . Replace them and see what happens . If it runs then well, otherwise I have no solution left to help you !!

Best of luck Dude .....

[stefos]

Re: Need help for repairing PSU with only +5vsb output

However - C945 Transistor's which you are indicating as Emitter (E), that must be Base (B) . If it is not Base(B), then PSU will not run . Emitter(E) is directly connected with Ground

You are right. It's typo fault. Here is the correct one.

[stefos]

Re: Need help for repairing PSU with only +5vsb output

It is very difficult to repair when all the parts are OK and PSU does not run !! Because one parts fails at load time and it is the hardest thing to find that culprit component .

However - C945 Transistor's which you are indicating as Emitter (E), that must be Base (B) . If it is not Base(B), then PSU will not run . Emitter(E) is directly connected with Ground .

Finally - I will tell you to replace 3 Resistors From Good PSU [Shown in Imge-08] . Replace them and see what happens . If it runs then well, otherwise I have no solution left to help you !!

Best of luck Dude .....

I have changed and tested 3+2 near registors.
NO LUCK AT ALL

In such cases proffesionals what procedure -tools are they using?

PS: Thank you very much for your time spending to verify the problem.

[stefos]

Re: Need help for repairing PSU with only +5vsb output

Hello @ll !!!

It's weekend so learning bench time started again. Took me some hours checking every single component on secondary side on bad psu and comparing results with the good psu. Some of them changed from good to bad and vice versa. Still got only +5vsb output to bad one.

As friend Escort Eagle mentioned already it is very difficult to repair when one part fails at load time. All this time I have learned a lot of things about smps.

While testing and comparing voltages between god and bad psu, I figured out that shorting 3-4 seconds (+) side of two signas diodes the psu power on and works fine. Also on both psu if LM339N pin 4 is shorted with ground, both returns to +5vsb only output.

1. Can someone please explain the logic of this status?
2. How safe is it if i solder the two diodes points permanently on bad psu?

Best regards

[stefos]

Re: Need help for repairing PSU with only +5vsb output

Wow, You have a nice strange problem !!! But One thing is strange to me - LM339 IC's PIN 2 is connected with +5VSB . If it is connected with +5VSB, then Can that marked place be 0 Volt after switching ? [Shown in Image-05]

I 've decided to disconnect the +5vsb from LM339 ic PIN 2 and psu was working but the 2pin filter coil in secondary side was smelling like burned.

So I replaced it with another one and tried again. The psu was running ok again but this time nothing started to burn. So I soldered again +5vsb with pin2 of LM339 IC and guess what.

THE PSU FIXED AND RUNNING ALL OUTPUTS CORRECTLY as seen on image.

I would like to thank everyone in this great forum and specially the friends who helped me with thei suggestions.

[ron350]

Re: Need help for repairing PSU with only +5vsb output

Good job you guys don’t give up.

[goodpsusearch]

Re: Need help for repairing PSU with only +5vsb output

Congratulations!!

So, the 3.3V torroid coil had shorted turns or its core saturated permanently?

[stefos]

Re: Need help for repairing PSU with only +5vsb output

Congratulations!!

So, the 3.3V torroid coil had shorted turns or its core saturated permanently?

I believe the glue got conductive. Also measured the temperature in this coil and was around 70 C. So thought that the coil conductors were insulated and change it. Temperature dropped down immediately around 35 C and psu worked.

[Escort Eagle]

Re: Need help for repairing PSU with only +5vsb output

Many many Congrats .... !! At last a lot of efforts come to a happy end . It is a strange problem and a strange Solution !! PSU creates all kinds of SMPS Problem than any other SMPS devices . The more you efforts, the more you will learn .

Congrats again for your Success .....

- Escort Eagle

[c_hegge]

Re: Need help for repairing PSU with only +5vsb output

Interesting. I've seen the 3.3v toroid coil burn up once before (on a Bestec ATX-300-12Z), but it was quite obvious in my case - the wire was completely black, and the PCB immediately around where it was soldered in was quite darkened as well and the caps around it were cooked. The funny part is, though that the PSU still ran (although I can only imagine how bad the 3.3v rail would have looked on the load tester)

[mikey5791]

Re: Need help for repairing PSU with only +5vsb output

I 've decided to disconnect the +5vsb from LM339 ic PIN 2 and psu was working but the 2pin filter coil in secondary side was smelling like burned.

So I replaced it with another one and tried again. The psu was running ok again but this time nothing started to burn. So I soldered again +5vsb with pin2 of LM339 IC and guess what.

THE PSU FIXED AND RUNNING ALL OUTPUTS CORRECTLY as seen on image.

I would like to thank everyone in this great forum and specially the friends who helped me with thei suggestions.

Hi guys,
Got to revive this old thread as this seems to be the one related to my psu problem. My psu also only has 5vsb and all the other components seems to be not shorted or open.

Stefos,
Did your psu running all outputs after just changing LM339 ic? Fyi, on my LM339 pin 2, i got 0v, pin 9,11,13,14=0v (i wonder whether this ic is bad).