Computer PSU low 3.3Vrail, why does this cap charge different

[lordvader88]

I have broken PSU I've worked on before, and had most of it apart. Today I finally measured the 3.3Vrail while it tries to start (w&w/o PC).

So with a Tek2430 scope, and found it only gets to about 1.5V hooked to the PC,  1.7V w/o PC, and just sits there. The supervisor chip waits 75ms before shutting it down.

The trace of this tho does not look like a normal cap charging, it looks more like a parabola. It only takes 1ms to get to about 1.3V, and then it takes even dips a bit for 10ms, then by 30-40ms it at 1.5V (or 1.7Vw/o load)

The 5 and 12V rail look like normal exponential charging curves. The 3.3V trace starts slow, like a parabola, with a normal vector pointing to the upper-left. Sounds strange to me but I'm pretty green.


I actually I had most the cap's out and did ESR tests, so IDK. I'm about to map the 3.3V section.

[lordvader88]

I mapped it out and it uses a tl431 to control a shunt transistor, so I recorded all the voltages last night. And it wasn't working for sure. And it hasn't the many many times I've turned it on over the last few months.


Then today it's MAGICALLY working, so I tried it on the PC and indeed it is working fine.

Since last night I might have moved the shunt transistor to read it better. And a 15Ohm resistor between V33 and V5 . So maybe it was loose? That doesn't account for the voltages tho.

Before when I injected 3.3V after the V33rectifier diode it would use 340mA, which was crazy. Now it's at about 40mA which is about right.

I've cleaned this thing multiple times because I de-soldered a lot of stuff as a learning project, testing it all.


Worst thing is I really can't say why it didn't work when I bought it. Since then I fried 1 MOSFET by accident, replaced it.

Then I replaced a CM6800, but I didn't understand it was being latched off. I have to find the original and test it again out of circuit again...it didn't seem to work out of circuit so IDK.

I did replace a broken PFC gate SMD transistor, but I might have fried that with my iron.

And now today it magically works? Did some solder ball fall off since last night?