Hello,
I have a ATX PSU that has a weird issue with how it handles the PS_ON signal.
I know these are cheap and can easily be replaced, but I tend to try and repair them, as there is always something to be learned while doing it.
The symptoms:
PSU, provides VSB fine without issue, it also turns on normally when PS_ON pin is grounded.
However, when it sits turned off, and plugged in to a motherboard. It starts to "leak" voltage on the power rails. Like 7V on the 12 and 3.5 on the 5v.
It is easy to blame the motherboard here, but checking the PS_ON voltage with it unplugged compared to it plugged in I get a voltage drop from 4v down to 3.5v.
This is far from from TTL 0v that is in the spec for PS_ON signal, so the PSU should absolutely not start to present this voltage during these conditions.
I did experiment a bit on the PSU and when loading it with different resistors I can make this happen without a motherboard connected. And it is analog in a sense. Different resistance on the PSON signal, will result in different voltage on the rails.
However, this should function as a switch as far as I know.
I did some digging around in the PSON circuit on the PSU,
Schematics so far:
PSON pin straight to the base of a C945 transistor.
Base connected in parallel with GND through a 150kOhm resistor and also a 33kOhm resistor to pin 13,14,15 of the PWM controller DBL494.
It probably goes further than those pins, this was just as far as I have come on my last session.
Was hoping that the C945 was the culprit, so i pulled it, but it checked out on the standard diode multimeter test.
Voltage on the transistor when sitting idle in standby:
Emitter 3.3388
Collector 5.028 (This comes from VSB)
Base 3.994
The reason for me to suspect either the transistor or circuit around it is that I can make this problem occur by only messing with the PSON pin, and this guy is the only one connected directly through it.
Might be completely incorrect ofcourse.
At first i was working with the theory that the internal pullup for PSON in the psu was incorrect. But thinking about it, we do have clearly voltage above TTL 0 when it starts to turn on.
Before continuing on my hunt, I wanted to check if I'm on the right track or if another theory might be more probable?
The PSU is one of these standard ATX PSUs from late 90s early 2000s.
Brand: ATNG (Might be ALTNG, the logo is not really clear)
Model: AT-230S