Cooler Master 500W atx psu repair. Total cannot start at all

[anikin73]

I am attempting to repair this. Circuit is quite simple, but i a quite stuck at it for a long time. so today i try to ask for new ideas. my challenge is to under stand what mistake i made. I had repaired not lesson than 50 pcs of various brands ATX psu . main pwm chip is Cm 6805 Sip 9 .
5VSB is fine. When I short Pson, totally nothing happen. I check , the Cm 6805 VCC pin2 did have 16v, But totally nothing happen. I changed to a good chip from another good PSU. Same thing.
What other pin should I check such that it will result in no start up at all ?

[anikin73]

To add, the pwm to the mosfet has 2 transistors Kn 222A and Kn 2907 A . I had checked and all normal.
Pin 9 IAC i had check and add addtional resistors but still no start at all. I am curious what mistake i made. I check through all components but seems nothing is spoilt. Any idea will be appreciated.

[anikin73]

the actual picture of the components as follow. This PSU seems so new that i suspect easy job but .......

[CapLeaker]

There are a few variants of these… what exact model number is this? Post straight shot high resolution pictures if this PSU. Post that schematic too…

[harp]

What voltage is on green wire, while grounding it, is signal reach controller ic, like changing startup state on ic leg?

[stj]

do you have a scope or a multimeter that can display frequency?
i would look at the pins on the chip to see if it tries to output the squarewaves to drive the power-stage

[anikin73]

Short green line and the opto couple will cause vcc to have 16v. This part is normal. No square wave from chip as I measure .

[McWhitePink]

Can you please post the schematic here?

[anikin73]

The schematics is not the actual one. But from another psu. So may not directly apply. But very similar.
VCC have 16 v. But not Pwm to the main mosfet. The mosfet seems ok . The chip is the 9 pin type.

[anikin73]

Model as shown below. But is 500w. I suppose it is the same design since only 100w difference.
1) VCC has 16 V . It’s from the opto coupler. Which is activated by pson green wire.
2) no output Pwm to the 2222A and 2907 transistors.
3) transistors I measure are fine. All components seems fine after measurements.
4) I recently repaired psu using same chip. Gamemax 500w to 800w also use the same chip.
5 ) so it must be other pins unable to fulfill the requirements and hence no start up.
6) it is a low voltage 500w and hence not expensive and hence i did not waste a lot of time on it. But I was sadden by my in capability to solve such a simple issue considering the amount of psu I repaired.
7) I would have thought pin 9 IAC is important for the start up . It is the connected to the bridge rectifier by 6M Resistor. I measure and the input is about 1. 6V . About same as gamemax psu..
8) what are others check points.

[CapLeaker]

look at the supervisor IC. It will shut the primary down. You can disable this protection (FPO pin) and try forcing the PSU on. FPO pin = high: means fault protection in on. Check U7 on the secondary side and compare each leg to GND (secondary). If the voltages are similar on these legs, the supervisor has shut the PWM down.

[anikin73]

look at the supervisor IC. It will shut the primary down. You can disable this protection (FPO pin) and try forcing the PSU on. FPO pin = high: means fault protection in on. Check U7 on the secondary side and compare each leg to GND (secondary). If the voltages are similar on these legs, the supervisor has shut the PWM down.

FPO will on u5 opto coupler and then on 2907 Q7, then VCC 16 V will appear. It has VCC. So it is not FPO. I will check on u7 as u suggested. Thanks for the Input.

[CapLeaker]

FPO will on u5 opto coupler and then on 2907 Q7, then VCC 16 V will appear. It has VCC. So it is not FPO. I will check on u7 as u suggested. Thanks for the Input.

Yes, absolutely verify that the u5 opto is working as it should. Verify VDC Pin 9 (for the startup) with the pec sheet.

[McWhitePink]

The schematics is not the actual one. But from another psu. So may not directly apply. But very similar.
VCC have 16 v. But not Pwm to the main mosfet. The mosfet seems ok . The chip is the 9 pin type.

Does anyone know how much voltage should the optical coupler output? I have two PSU, don't find any short, but it turns off instantly. I'm suspecting the feedback circuit, but don't know what value should I expected.

[anikin73]

Yes, absolutely verify that the u5 opto is working as it should. Verify VDC Pin 9 (for the startup) with the pec sheet.

I tried but it is not this issue. opto couple was not activated at all. even i lift the opto couple up also there is no response from psu. I actually did tried this in the past. the problem is more strange than any other psu that i had encounter.

any other have some idea how to trouble shoot this particular chip ?

[CapLeaker]

I tried but it is not this issue. opto couple was not activated at all. even i lift the opto couple up also there is no response from psu. I actually did tried this in the past. the problem is more strange than any other psu that i had encounter.

any other have some idea how to trouble shoot this particular chip ?

If you have nothing coming out from the optocoupler on the primary side, you have a few possibilities that need to be on the secondary side.

a) both pins are “high” something like 5V compared to GND. Which means that pin 9 on the supervisior IC is high and it detected a fault.
b) you don't have any voltage on any of these 2 pins on the secondary.
c) the opto is dead.

On the secondary one pin of the optocoupler is getting fed voltage, The other pin is tied to fault pin on the supervisor. For that LED isnde the optocoupler to shine there has to be like 3V difference if you compare both pins from the opto on the secondary side to GND.

[harp]

opto couple was not activated at all.

Fast in-circuit check if optoic is responding...
I use a probe of LCR tester, who briefly change polarity... and monitor other side. With this you can check both side at once.

https://m.youtube.com/watch?v=U3qYXnbVhuo

Also, what is voltage on every single pin before PSON applyed, and what voltages are while is to be powered on? Can you compare with working psu who have same ic...

[anikin73]

Thanks for all your input, at last I decided to change item by item. When I changed the 2 npn transistor, the psu works. I measured both in diode mode, they are normal..
so anyone can tell what is a better method to test npn transistor or to buy a tester ?
as mention from start I am more keen to explore what I do not know. Whether this pc got repaired make no difference to me.

[Don4321]

What transistor you changed? I have the same problem.