Old PC Power Supply

[fzabkar]

AISI, the Vcc pin must be connected to the positive terminal of one of the bulk capacitors in the voltage doubler via a resistor.

[xavier60]

The negative side of the small capacitor goes to the positive side of one of the large capacitors and one winding of the transformer. The positive side is fed from the drain of the FET through a diode. The big 5W resistor is in parallel with the small capacitor

Ok, that's a snubber circuit.
You will need to trace where the start current comes from either by tracing away from the UC3844's VCC or finding the source which can be one of the mains phases, the junction of the HVDC capacitors or from the HVDC rail. A common trick is to use the bottom bleed resistor as the start resistor also.

[RyanG]

This is what I have traced out so far. Will do more tomorrow.

[xavier60]

It uses a charge pump from one of the mains phases. The X2 capacitor is now suspect.

[xavier60]

Now is a good time to note the values of the MOSFET's Source resistors in case of a future accident.
The current through the lower bleeder resistor could be diverted to the VCC rail for testing the PSU if a new X2 capacitor isn't available.
This has to be done properly, allowing safe clearances.

More: the grounded end of the lower resistor just needs to be moved to the adjacent empty hole that's on the VCC track.

[RyanG]

I moved the resistor to the empty hole and not much changed. The voltage on the VCC pin was still under 2.5v

[xavier60]

Is the resistor good?

[RyanG]

Yes, the resistor is good

[xavier60]

What voltages are there across the 1K resistors drawn on the photo in Reply #27?

[RyanG]

0v

[xavier60]

Time to suspect leakage to ground. Try disconnecting things on the VCC track in turn.
Start with the 39Ω resistor. Double check the polarity of the VCC capacitor or remove it.
As a precaution, solder a jumper wire between the MOSFET's Gate and Source.

Added: Allow enough time for the HVDC capacitors to discharge before doing any work to it.

[RyanG]

Before removing anything from the VCC circuit the resistance across the rail to ground is 570 ohms.

I removed(one at a time and replaced before testing next):
39 ohm resistor
Both transistors
The first 1k resistor
The diode from the charge pump
The resistor that goes to the optocoupler

The resistance stayed at 570 ohms

Should I remove the UC3844?

[xavier60]

That's not leaving much. Try de-soldering pin 7 pin the UC3844. The MOSFET should be safe with the Gate connected to Source.
Also look for any carbon tracking on the PCB, the MOSFET's heatsink is live with HVDC plus 600V peak chopper waveform.

[RyanG]

With pin 7 desoldered, I am reading >20Meg, slowly counting down, from VCC to GND

[xavier60]

The UC3844 was likely damaged by leakage from the heatsink through the glue.
The 100K resistor might as well be put back to where is came from.

[RyanG]

Looks like that was the problem. I had an UC3845 in my spare parts bin. I put that in and it worked. I have all the voltage rails working.

I did actually swap that out already for the same one I put in it now, but it was before I cleaned the glue. It didn't work then, but it does now.

I should have known because I think I remember Dave making a video about that exact thing.

[xavier60]

It's lucky that the MOSFET survived it all, a big problem because of the extra damage that occurs and often not being able to see the  values of damaged Source resistors.
I would have a think about replacing the 15Ω Gate drive resistor, it looks corroded,
And the snubber capacitor being so close to the heatsink and resistor.

[RyanG]

I will check everything out and replace the parts you listed if they seem corroded. Thanks for all the help.