I've added mosfets to the lamp columns in WMS boards from Sys 3-11 with zero problems.
My process involves removing the current limiting resistors and installing zero ohm jumpers in their place, and then replacing the TIP42s with IRF9Z34 mosfets.
The purpose of this - if you aren't familiar, is to reduce the heat on the board. In extreme cases, the 27 ohm resistors bake the board to the point where the solder crystalizes, the boards burn, and the resistors even fall off in rare cases.
Never a problem on a WMS board, but I did my first Data East board last night, as I had one come in with a really baked lamp column section.
Well, it didn't go so well. As soon as the lamp matrix fired up, a couple of the 6427s exploded.
I double and triple checked my work. MOSFETS installed correctly. The board is identical to the WMS lamp matrix as used from Sys 3-11. But something has to be different - something I've missed perhaps.
Worse case scenario, I can return to the original configuration, but this board has seen a lot of heat in that section, so I'd prefer to make the MOSFETs work.
Any thoughts? I'm trying to get another DE MPU I have here to boot, so I can take some voltage measurements and compare them against a working sys11 board. If the 2n6427s are exploding, I have to think I'm getting voltage back fed somehow.
My work is clean. I didn't find any solder bridges, or other mistakes.