Ok, breakthrough (of sorts).
At first I was tackling the problem by trying to trace things out from the +320 DC supply through the FETs, then the transformer to the secondary rectifiers for +5 and +12. I can see the gates on the two main high voltage switching transistors are both low, and both are controlled by a small transformer that's configured with one primary (signal input) and two secondaries that are 180 out of phase with each other. The secondaries are wired to the gates, the whole thing is an isolation system, and if you put a signal into the transformer one way it activates one FET on and one off, and the other way activates one fet off and one on.
Built in crossover protection. Ok, good. That seems to be powered by ANOTHER set of transistors in a push pull arrangement at which point I thought:
"Screw this, what is calling the shots?"
On the accessory motherboard there is a single chip that has no visible markings. However with the right light I can see it's a MB3759 buck controller that probably gates stuff that gates stuff that eventually hits the FETs. Ok, no biggie. However the important thing is it shows me that ground is on pin 7, VCC is pin 12. NOW I have something.
So I traced the pins around the accessory board I ripped off the bad supply (helpful) and finally figured it out. That's powered by a transformer on the accessory board that has 4 windings. One set goes to a diode then a 7812 regulator for -12, one goes into the vcc and ground of the 3759 through a small diode, and the other two I have no clue where they go. One probably goes to ANOTHER switching supply that is sourced by either mains AC or the +320v raw DC I don't know but something, and the other goes heck knows where.
But. Now that I know where the power is, and that the transformer isn't working, what if I backfed the 3759 circuitry with another power supply? So I hooked up my trusty vicor to ground and the circuit side of the diode, set the current to 0 and the voltage to 15 (recommended voltage) and fired it up.
Voltage 0, current limited. Brought the current up and at 200ma the voltage stabilized. Good! Now we can do something here. With the clips on carefully and the vicor off, I plugged in the naked board, set up a camera to record the fireball, and turned on the main switch.
No output on the +5 and +12. Now turn on the Vicor and wait for the sparks....
No sparks. More interestingly nothing exploded. I turned down the current to zero on the vicor and noted the voltage stayed at 10v. Which meant something was now powering the damn MB3759. Could it be????
Checked the +5 volt rail on the main power supply. Perfect 5v. Likewise the +12 has a perfect 12 volts. -12 is down, but -5 and +5 DC ok were both fine.
Which means the main supply circuitry, the FETs, the drivers, and the control circuitry are all good. The problem is in the snooper circuitry or whatever powers that little transformer. But it does mean that SOMETHING still works on the board, and the big supplies (+5 and +12) are good. At least on this board. It also means that once the board is up the snooper circuit is no longer used, which means something else is also sourcing power from the board, but it's probably not the transformer as the -12 is down.
How many frigging power supplies does this thing have in it?
Anyway I'm going to walk away for awhile and think about this. Something in the primary snooper supply circuitry is bad, I'm not sure where of course, but it's possible I could just put in a pair of small 120/240 to +12 supplies and bypass the whole primary system anyway with connections to the -12 regulator and the 3759 subsystem. Put a blocking diode in to ensure the main supply doesn't somehow toast the custom built snooper and Bob's your uncle....
Progress. Well on one of the supplies. I haven't tried this with the other one yet, it could still explode in a new and exciting way. But it is interesting.
So I powered