Hi Ian, the 50 Amp fuse is between the pass transistors and the bridge rectifier output, in the negative side (- ve floats relative to chassis ground). Would testing at max volts but low amps with say a 5 Amp fuse there save the pass transistors, or is it purely volts that kill them? Right now the bridge is showing 93V DC, with the one of the pass transistors shorted, but with the 50 Amp fuse open. I am pretty sure for the SCR stuff to work it needs to read the OUTPUT of the pass transistors as feedback, so it's probably doing as expected with no output from the pass transistors.
I have just taken delivery of some 140V MJi5003 pass transistors, but obviously i can't just keep buying the damned things
I'm 90% sure the SCR stuff loses control and the output from the bridge into the pass transistors goes to 93V and then BANG, one or more go short.
I was wondering what these devices would make of 93V in, if I bypassed the SCR stuff to run the primary at full mains volts, to do a divide and conquer assessment of where the issue lies. If I ran plenty of volts out the pass transistors wouldn't have a huge drop across them even without the SCR control. I am not sure if I can divide SCR from pass control though...