Voltage issues SY-5SSM, where's the problem? Help needed

[rattlesnake]

Hello everyone,

I'm trying to fix this SY-5SSM motherboard (Socket 7).



When the PSU is connected it powers on by itself but there's nothing on the POST card, no beeps.
The BIOS EEPROM is ok (latest BIOS was flashed).
All the jumpers are set ok.
All the 1000µF 6.3v (14) and 1500µF 16v (2) caps are in spec.
There are two 76129D MOSFETs (datasheet). I took them out and tested them and they are ok (no continuity from drain to source) in off state.
But the voltage in the gate of the second 76129D fluctuates between 1.468-1.649, and it should be 2.2v (with a K6-2 installed, properly configured).

The voltages are:

76129D (closer to IDE interface)
G 0.033v
D 5.08v
S 0.033v

76129D (closer to the edge of the motherboard)
G 1.468-1.649v
D 0.033v
S 0v

What should I check next? What could be the problem?
Any suggestions would be appreciated.

[FriedFred]

So the main power supply switch on by itself?? This is not normal, I think. Usually, you have just the "Always on" voltages, like 3.3V and/or 5V. Nothing else. Then the power supply is waiting for the power-button to press.
Do you have any coils shorted to GND? Do you have any coils with a reasonable voltage? Do you have a "Always on" voltage on the board anywhere?

[rattlesnake]

Hello Fred...

I already fixed it.

Coils were fine. I checked every electrolytic capacitor. I checked the VRM transistors, the buck converter IC (HIP6004). I couldn't detect any failure. I even replaced the transistors just in case. Then close to the edge of the board I noticed a missing SMD cap "C718". It can be seen in the pictures I attached. So I started figuring out how the circuit works as far as I could. I removed and measured some capacitors that were already in circuit and added one of a similar value (0.1µF).

And violà, like magic, the motherboard posted! It was very rewarding. The voltages are now fine.

I'm disappointed that I only get an answer after almost a month, but I appreciate you trying to help, that's why I'm posting this answer to let you know that it's already fixed.

I don't know what happened but this forum is not what it used to be...

[FriedFred]

Nice you fixed it! Usually, The replies for older boards are less, I think. I usually answer the questions with really less or not any answer at all. To give a helping hand. I did not recognise this request was a month old! But I know the feeling, if you don't get any answer at all...