failing transistors

[bfhamr]

Newbie here. I repair machines for a living, I'm currently working on an EDM. Normally I would contact the manufacturer but I'm told they are no longer in business.
The machine has a set of 6 transistors (Hitachi 2SC1343). These can be switched into and out of the circuit depending on how much power is wanted while machining. The problem is these transistors are failing (shorted) and I'm not sure why. I had replaced the board which contained these and all 6 have failed again. Another board which had been in the machine had 4 of the 6 shorted.
I'm not looking for specifics on this machine but in general what would cause a transistor to fail shorted.

Thanks
Dan

 

[socrace]

Hey bfhamr, if you want something to check out quickly, I would focus in on terminal 12 in the picture. That seems to be the return line for the flyback diodes (verify that); that sad looking resistor r29 in the picture could get overworked like that if the return line has a problem. The return probably connects to the 100v supply somewhere.
Bob D

 

[bfhamr]

Yes, I have checked the return line for the flyback diodes. I don't believe the problem is with the board unless it was designed poorly. Three identical boards have failed. I'm thinking the problem is external to the board.

dan