A4000 PSU - KABOOM!!!!

[Castellen]

The main switching transistor (Q1) going short circuit would be a likely cause of the AC fuse blowing and IC1 being damaged.  The fault current may have also damaged the bridge rectifier as well.

These power supplies aren't that complicated, so should be repairable with a bit of time and effort.  I don't have one of those supplies apart on the bench at the moment, so not sure what the part number of IC1 is.  It's the switchmode controller and being a DIP8 package, it's certainly not anything common such as a TL494.  Someone might be able to supply a photo to identify the part.

If you can source a replacement IC1, I'd be inclined to replace both Q1 and the bridge rectifier as a start.  Also do a quick check across each diode and transistor to look for any obvious short circuits anywhere else.  I doubt if this would be a capacitor problem, but it would be worth replacing all of the electrolytics once you get the supply running.

Another tip for working on switchmode supplies is to use a series test lamp (100W - 200W incandescent/halogen) on the AC input.  That way if you have a catastrophic short circuit fault somewhere, worst case you'll just make the lamp light up instead of causing more damage.

 

[Castellen]

Took a closer look around Q1 and sure enough, black marks on the PCB. They were a little obscured by R9. R9 also exploded I took out Q1 and just for fun, tested it, shorted. R9 was open. Best my tired eyes can tell, R9 is Red, Purple, Silver, Gold which decodes to a 0.27 ohm resistor. Seems like a weird value (not the order of magnitude so much but the 0.27), but ok. I've attached a photo that shows the bands if anyone wants to look and tell me what they see.

IC1 is a Unitrode SK-8085. Seems no one can find a data sheet or application note. But at the vendor where I found some, there was a description and it was described as a variable voltage regulator. not a PWN Modulator, does that make sense? Q1 is a Toshiba K2038 (SK2038) power MOSFET. I found a bunch of these, too.

The question I have now is, what fried IC1? Something on the MB or did something else in the PSU cause Q1 to explode?

A quick search suggests that the TI part UCC38085 is probably the same thing.  Looks as though it's obsolete and no longer available from the usual vendors (Digi-Key, Mouser), but you might be able to source something from Ebay, Ali, etc.

Yes, the value of R9 looks correct at 0.27 Ohms.  You can see in the circuit that it's used in a current detection arrangement.  I'd expect the controller IC (pin 3) measures the voltage across R9 to determine the current flow.  I would fully expect R9 to be open circuit, it would have been carrying a lot of fault current with Q1 shorted, so it's acted like a fuse.

To answer your question, Q1 has probably died first, which isn't uncommon with high voltage transistors.  So instead of silicon junctions in the transistor, you've now got a shorted mess.  That means you'll have around (110VAC/SIN(45)) = 156VDC on the drain terminal of Q1, and this is now connected directy to the gate terminal.  I don't know what the maximum voltage on the drive output pin of IC1 is, but probably something like 10V.  And you've now got 156V on it (via R6).  You've already seen what happens next....

Which reminds me, you should sanity check R6 as well to make sure it's not open circuit.  If it's a low-ish value (under 100 Ohms) then it might have been damaged.

The A4000 main board is probably fine.  Looks as though there's sufficient protection in the supply design to prevent anything too nasty from appearing on the DC output with this kind of failure.

 

[Castellen]

You should be good with that part from Mouser.  Not sure how available the SK2038 high voltage FET is, a quick search suggests it's not.  Won't be hard to find an equivalent part by comparing data sheets and using parameter based filters with Digi-Key/Mouser.  You're a bit spoiled for choice with what's available these days.

Datasheet for the original part is here: https://www.alldatasheet.com/datasheet-pdf/pdf/30600/TOSHIBA/2SK2038.html

Another suggestion is to replace capacitor C6 when you replace IC1 and Q1, before you do the initial run-up, as it might have been damaged at the same time as IC1 failed.  If you're unlucky, then the optocoupler (IC3) might have been damaged as well.  You could try it as it, or replace it if you wanted to be sure.

I'm not convinced that schematic is correct either.  It shows pin 7 (un-used push-pull output) connected to what's obviously the power supply for the PWM controller.  I suspect it should say pin 8 instead of 7 and you'll find that pin 7 isn't connected to anything on the PCB.

When you do the run-up, don't connect it to the computer, but instead connect a 4.7 Ohm wire wound resistor of at least 5W between +5V and GND (yellow and blue wires) to give the supply a 1A load, else you might get some unexpected results.  You might need the fan connected to the +12V supply also, some power supplies won't run correctly without at least a small load on the +12V supply as well.

If it all looks good, then you might as well replace the rest of the electrolytics while you have it apart, then call it done.

[Castellen]

Nice to hear you have the power supply working again.  At least it's a simple design that's easy to work on, unlike the A3000T power supply.

Someone was asking me the other day about how to fine tune the +5V output on the A4000D Skynet power supply, the same one as in this thread.  Not sure what the specification is, I'd expect around 3% (4.85V - 5.15V).  My notes on adjusting the output voltage:

Output voltage is set by ratio of fixed value resistors R12//R14 and R13//R15
R12 = 5.6k
R13 = 3.9k
R14 is not normally fitted
R15 = 220k
Increasing the R12//R14 total resistance increases the 5V output voltage
Increasing the R13//R15 total resistance decreases the 5V output voltage

Or to explain another way, leave the lower value resistors (R12, R13) as they are and add higher values (typically 100k or more) at R14 or R15 to fine tune the output voltage.


And just to state the obvious, it's not (practically) possible to adjust the +12V or -12V output independently of the +5V output.  What you see is what you get.