Need help for repairing PSU with only +5vsb output

[stefos]

Hello friends,

It's been almost two years since I had joined this wonderful forum and until today i have learned alot in electronics. As newbie in repairing atx psu need some help for resolving the problem in T&P-DHK 350W PSU which outputs only +5vsb. No burned components at all just suddenly stopped working. So far I have checked the components in the sections that I have labelled in attached photo.

I have a working same type and model psu and compared the values with my multimeter. Also exchange and resolder many components from the working one, but the problem remains. The second (working state) psu with the parts from the faulty one still works fine.

I have attached some photos and hope in your kindness to help me solve the problem. I have tested diodes, power IC's, transistors, capacitors, resolder almost half parts rectifiers, transformers, IC's etc, but still no success. :-(

For your information: When I soldered reversed the two transistors near the power IC, I got all volts output but the bulp test shows me as expected that short exists, so I put it in right order again and got +5vsb. Unfortunate I don't have an oscilloscope to make further checks, just a multimeter, DIY in circuit transistor tester, PC ATX SATA PSU HDD Power Supply Diagnostic Tool Tester and a bulp tester.

The attached schematic looks quite same model. I have ordered an esr tester with some other parts needed for another psu (PSU Trust 420W PW-5210) that had some burned items and same problem (only +5vsb) too.

Will you please help me by marking the necessary steps - checks on the photos?

Thanks in advance for your help .

Some photos are:

[senz_90]

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

I don't know if this helps, but i have take a time to measure all IC pin voltage from 2-3 good PSUs just a guide for me when need to fix this common cheap PSUs.
The IC number maybe same as you LM339 and 7500B.

LM339
1. 4.26V 8. 1.18V
2. 0.03V 9. 2.86V
3. 4.97V 10. 2.57V
4. 1.24V 11. 4.57V
5. 0.77V 12. GND
6. 0.05V 13. 5.13
7. 2.42V 14. 4.56

7500B
1. 4.29V
2. 4.30V
3. 2.87V
4. 0.04V
5. 1.65V
6. 3.66V
7. GND
8. 1.97V
9. 0
10. 0
11. 1.98V
12. 22.9V
13. 4.97V
14. 4.97V
15. 4.97V
16. 1.85V

All 3-5% tolerance looks likely no problem when I compare each others. Maybe this would give some sign.

[redwire]

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

The pictures help a lot. There are schematics here that are closer to yours (see 200W): ATX power supply schematics

I would check the KA7500 has power on pin 13,14 (5V?) and pin 4 above 3V is usually the PS_ON signal. It's easier to troubleshoot with all IC voltages if you measure them.

You are jumpering the PSU green wire to COM, to turn on the PSU?

[stefos]

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

I don't know if this helps, but i have take a time to measure all IC pin voltage from 2-3 good PSUs just a guide for me when need to fix this common cheap PSUs.
The IC number maybe same as you LM339 and 7500B.

LM339
1. 4.26V 3.810v
2. 0.03V 3.914v on working psu was 81.4mv
3. 4.97V 4.952v
4. 1.24V 1.250v
5. 0.77V 3.476v
6. 0.05V 0.01v
7. 2.42V 2.418v
8. 1.18V 1.318v
9. 2.86V 559mv
10. 2.57V 2.607v
11. 4.57V 133.5mv
12. GND 0v
13. 5.13 0.4mv
14. 4.56 3.8mv

7500B
1. 4.29V 4.5mv
2. 4.30V 4.718v
3. 2.87V 54.7mv
4. 0.04V 3.361v
5. 1.65V 1.631v
6. 3.66V 3.661v
7. GND 5.8mv
8. 1.97V 2.257v
9. 0 0.2mv
10. 0 0.2mv
11. 1.98V 2.295v
12. 22.9V 13.31v
13. 4.97V 4.953v
14. 4.97V 4.954v
15. 4.97V 4.952v
16. 1.85V 371mv

All 3-5% tolerance looks likely no problem when I compare each others. Maybe this would give some sign.

Dear Rudi and Redwire thank alot for your replies. In blue color are the values from the non working psu. The other values compared to the one which works, was on 3-5% tolerance except from pin 2 on LM339 IC.

You are jumpering the PSU green wire to COM, to turn on the PSU?

To turn on psu and check it I am using a power supply tester.

[redwire]

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

I don't know your PSU schematic exactly, but going off this one, from: http://danyk.cz/s_atx01b.png (My URL in prev. post #3 is wrong somehow: ATX PSU schematics )

Looking at those voltages, the 7500B is remaining off due to the logic high (voltage) on its pin 4. That takes us back to the LM339 output pin 2, which then goes back to LM339 pin 5 where the problem is- assuming that matches your PSU... can you verify 7500B pin 4 goes to LM339 pin2 through resistor.

I think the circuit on LM339 pin 5 looks at all the PSU rails (+5V, +12V, -5V, -12V) - which are obviously zero, and it also monitors power for the 7500B. It might be too high at 22.9V?
I can't help much without knowing if the schematic matches

[stefos]

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

Thank you for your consideration.

I have marked in new photos the requested info. I hope this closer view images will help found the problem.

Since I have another working same type model psu, can you please drive me what to check and compare the values?

[stefos]

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

Some more information. I measure the 15K registor conected to pin 3 of LM339N in comparison with the working psu. Out of the circuit both registors gave me around 14.85K. In circuit on the faulty psu i count only 3.496K. Also checked C35 non polarity cap (102) out of circuit and was ok.

[senz_90]

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

I don't know 22.9V is really high redwire, maybe this chinese cheap PSUs different with yours. I don't have a faulty PSUs recently to fix now and don't have time to compare it because some faulty CRT TV from my friend, but the result IC voltage is from 2-3 PSUs that I have compared, and all voltage is same within tolerance 3-5%. maybe stefos PSUs is different.

You need take a time to compare schematic with redwire given stefos, maybe that would help or give you some idea.

The resistor read low when "in-circuit" because some resistor is connected parallel with it. I have used to remove resistor just when I checked "in-circuit" and give a high reading than resistor code color.

I don't like to acting likes expert but you need a good understanding to test components to fix something, because that is really crucial, if you miss one, you will never found the culprit. are you test all caps with your ESR Meter? TL431? optocoupler? schottky diode? just read article from some site about testing this (I suggest jestineyong site) I don't suggest you to buy the book and I am not salesman but you could find some article free from his site about testing this components. and you will get more experience.

[ben7]

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

The KA7500 datasheet says they are rated for 40V (42V max).

[redwire]

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

I looked at the schematic again. The LM339 does many functions and it is complicated.

LM339 pin 5 looks like a delayed latch for overvoltage and drive transformer overcurrent. Takes us back to Pin 16 on the KA7500, which is not up.
I thought it was monitoring the Vcc (22V) but they are watching the xfmer drive current for some reason.

Check the parts between pin 16 and pin 12 (Vcc) on the KA7500, then next is to follow KA7500 pin 16 or 12 up to the transformer. Check the diodes, 4 transistors and capacitor from the driver transistors C945's to gnd for being open. Small chance the capacitor from LM339 pin5 to gnd for being open.

[stefos]

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

As u can see on image at KA7500B pins 3 and pin 5 are connected with mylar film capacitor 2A103J (0.01uF 100V 5%). I compared them with those on the working psu and found one was faulty and change it but still getting +5vsb output only.

While testing again voltages on KA7500B ic, I shorted for a while pins 3 (Feedback) and 4 (DTC) with my multimeter red probe and got all outputs in psu tester correctly, but the tester also produced a high pitch sound.

I will do all checks that Redwire suggested and let you know.

Thank you.

[Escort Eagle]

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

You have encountered with Difficult Problem !!

[stefos]

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

You have encountered with Difficult Problem !!

I hope with help from this great forum members will solve the problem. So far i have test in circuit from secondary side almost all components (caps, registors, diodes) comparing the value from the working one psu. I 've found differences values on zener diode marked as ZD3 on pcb.

[Escort Eagle]

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

1. What is the voltage on the KA7500 IC PIN No. 4, 8 & 11 ?

2. Are those switching transistors C945 OK ? [Shown in "Image-01"]

3. If the switching Transistors C945 are OK, then Check those components Resistors, Diodes . Are they OK or Not ? [Shown in "Image-02"]

4. What is the voltage in T2 Transformer PIN in both condition - PS_Without switching state (Green without Black) and PS_ON state (Green + Black) [Shown in "Image-03"]and KA7500 IC's PIN 8 & 11 voltage also in both condition .

[stefos]

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

1. What is the voltage on the KA7500 IC PIN No. 4, 8 & 11 ?

PS_ON state

-------------PIN 4 --- PIN 8 --- PIN 11
GOOD PSU : 0.85mv --- 2.104v --- 2.113v
FAULTY PSU: 3.360v --- 2.290v --- 2.290v

PS_Without switching state

-------------PIN 4 --- PIN 8 --- PIN 11
GOOD PSU : 3.299v --- 2.270v --- 2.270v
FAULTY PSU: 3.307v --- 2.274v --- 2.274v

2. Are those switching transistors C945 OK ? [Shown in "Image-01"]

They are OK, because I have tested with my second working same model and type psu as most of the componenets so far, with no luck.

3. If the switching Transistors C945 are OK, then Check those components Resistors, Diodes . Are they OK or Not ? [Shown in "Image-02"]

See attached image (in comparison with working psu are all OK)

4. What is the voltage in T2 Transformer PIN in both condition - PS_Without switching state (Green without Black) and PS_ON state (Green + Black) [Shown in "Image-03"]and KA7500 IC's PIN 8 & 11 voltage also in both condition .

For pins 8, 11 mentioned above

But for T2 transformer I got differenent volts in comparison with good psu as following:

--------------PS_ON state --- PS_Without switching state

GOOD PSU on all 3 pins got 2.410v --- all 3 pins got 1.530v
FAULTY PSU GOT ONLY 1.530v --- equal to good psu (1.526v)

[Escort Eagle]

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

Now Directly Short PIN 4 with Ground(GND) . Is the PSU turning on ??

If the PSU is not turning on by shorting with GND, then Change D19, D20, **R9** and R10 with a **new one** . Specially R9(1.475K) . R9 is the vital point for Vdd supply . If you do not find 1.4K, then you can replace it with 1K . Then try again by shorting PIN 4 with GND .

And Let us know - what is the result .

[stefos]

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

Now Directly Short PIN 4 with Ground(GND) . Is the PSU turning on ??

YES, the psu is turning on with my psu tester. Is this overriding the LM339 IC or something other component?

[Escort Eagle]

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

1. Now remove PIN 4 From Ground .

2. Now Is it turning on in normal mode Green + Black ??

[stefos]

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

1. Now remove PIN 4 From Ground .

2. Now Is it turning on in normal mode Green + Black ??

NO I got only +5vsb output only as expected.

[Escort Eagle]

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

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

Now Check that Transistor C945 and 4 Diodes carefully by removing from the PCB . May be The transistor OR One of 4 Diodes could be Faulty . [Shown in "Image-04"] You can also give a quick check to other Crystal Diodes surrounding of LM339 IC .

If you find that Transistor and Diodes are OK, then Change the LM339 IC with a new one .