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Old 05-04-2021, 04:30 AM   #1
Join Date: May 2017
City & State: Belgrade
My Country: Serbia
Line Voltage: 230V 50Hz
I'm a: Knowledge Seeker
Posts: 37
Default DTK 400W PSU Power Good signal 0V fault

Hello. I have this old PSU for my old computer. DTK PTP-4007P.
The computer shuts down quickly when it boots up. Sometime in the middle of the boot. The computer is old, AMD Athlon 64 2800+ based setup (it has same low end graphics card, low power), the PSU is more than capable of powering it.

I have noticed that when the computer was running, the Power Good (PG) signal from the PSU was high (above 4V), but when the computer shuts itself off, the PG signal goes to 0V.

The PSU provides all voltages, but the PG is 0V even when the PSU is on (PS-ON shorted to GND).

The only thin I have noticed is that the fan inside the PSU barely turns. I have measured the voltage at the fan connector on the board and it only gives about 7V. I have tried the better fan, and it turns better, but of course the voltage is still 7V.
I am guessing this is also monitored, and that is why PG signal goes low.
But I have traced the fan's connector pins a bit, and it seems that it goes from 12V (yellow) to one of the pin directly. And the other pin via the diode goes to a 5V rail (red). That means that the 7V is correct. But that is very strange. Although it does not have the original fan, I am pretty sure that it uses a standard 12V fan. And I remember years before, the fan was working normally, like a regular 12V fan. So I really have no idea.

The PSU is built around standard TL494 controller and it uses an LM339 Op-Amp for other functions (including PG signal output).
The PG output is on the pin 13 of the LM339. The input pins are 10 (-) and 11 (+). The voltage on the pin 13 is of course 0V. The voltages on the pin 10 is 2.5V and the pin 11 is 0.4V. The pin 13 goes to the base of a BJT, that goes to the PG wire.
The Op-Amp pins are very hard to trace, they go all over the place, in circles, via various resistors and so on.

I have checked the input caps, they are fine. I used the LCR meter. I checked all the diodes and transistors and resistor around the LM339. I just can not find the problem.

I can not find schematics anywhere. I have only found some forum topic from years ago, where the same PSU is mentioned, but it is the topic that I started...

Here are the pics:
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