I am going to make some assumptions : 1) The fuse holds if you unplug the transformer connector from J6 on the mainboard (if not, then you've got shorted turns in the transformer or something like that. 2) You have schemaitcs. I only have the 9915A ones, which I think I gave you a copy of. If not, you can get them (reverse-engineered as always) OK, what I'd do next : Pull JP6, JP3, JP5 (disconnect the main PSU outputs from the logic). Fuse still fails? Check CR19 (1N5401, connected form pin 1 on T2 to the +ve side of C46. If that's shorted, the crowbar wull necessarily fire and blow the fuse. While you're about it, check Q4 (crowbar SCR) and the zener connected to its gate. You'd feel really stupid if a short in the crowbar was the cause of all this. Fuse still blows? Remove the diode between pin 6 of T2 and the -12V line and the diode between pin 4 of T2 and the V+ line (RH side of sheet 1 of my CPU board schematic). With those lifted, the supply is reduced to a simple 12V one driving no load. If the fuse will hold now, I'd supsect (a) a short in one of the output regulators _and_ (b) a problem with the current sense circuit (round U31, LM311). Do you have an adjustable bench supply? If so, remove the transformer connector from J6, conenct the supply across C46, and slowly ramp it up. See what the +12VH output does (across C58). Does it just track the input, even when the input is just over 12V? Is U30 oscillating at all (look at TP2)? You removed Q5 (the PIC 645 chopper). Did the fuse hold with that out? If not, then you know there's a short either in the crowbar, or between the windings of T1 (current sense transformer), or CR19, but you should have checked that by now. If the fuse will hold with Q5 out, you could see if U30 is oscillating. Provide a temporary pull-up to TP2 (9V battery and resistor?), do you get drive pulses here? -tony