Electronics > Repair
Help with dead AT PC Power Supply
naujoks:
As requested. What are we looking for?
Harry_22:
Here we are!
Please see the PCB drawing. Only the DIP8 chip (bottom left) remains unknown.
The thyristor protects against overvoltage. It simply short +5V rail.
Apply the voltages marked in red from an external power source one by one and check that there is no short circuit. Then measure the power on the controller (marked in yellow).
The source file can be viewed in Paint.NET
https://drive.google.com/file/d/1oZoXP_6CauAqUHhR-VeVlyOd5lAP2KV1/view?usp=sharing
naujoks:
My external PSU can only do positive voltages. And did you really mean -7 and -15V, or -5 and -12V?
Wouldn't any shorts also show up without any external power supplied?
Do you want me to measure the voltage at the yellow points when the PSU is powered normally, i.e. no external power supplied?
naujoks:
So I turned on the PSU as normal and measured at the yellow points.
I'm getting 91mV. Yes, millivolts.
Harry_22:
--- Quote from: naujoks on October 08, 2024, 11:52:06 am ---My external PSU can only do positive voltages. And did you really mean -7 and -15V, or -5 and -12V?
--- End quote ---
Just swap the positive and negative wires. Yes I mean -7 and -15V to get -5 and -12V after installed voltage stabilizers. Both are placed on PCB.
Please check -5 and -12V at output connector.
--- Quote ---Wouldn't any shorts also show up without any external power supplied?
--- End quote ---
Yes. We do it rail by rail to check PS functionality.
--- Quote ---Do you want me to measure the voltage at the yellow points when the PSU is powered normally, i.e. no external power supplied?
--- End quote ---
Yes you already did that. You got basically zero voltage but expected around 13-15VDC.
Let's move on.
Please carefully check the voltage between the big capacitor pins. Expected 320 VDC.
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