After a lamp matrix MOSFET upgrade every lamp in column 1 burns very bright all the time, only dimming slightly as the attract mode flashes them, though they look normal in lamp test. Refitting the original transistors, replacing the PIA and checking for solder bridges did not help.
Lamp column 1 stuck at full brightness after a MOSFET mod: bad pre-driver Q15
Cause: The small pre-driver transistor for column 1 (Q15) had failed. It tested good with a meter both on and off the board but failed under load, holding the column driver on.
Checks
- Swap the suspect pre-driver with the one from a working column: if the fault moves, the transistor is bad.
- Remember that on System 9 the lamp MOSFET upgrade is not the same simple swap as on System 7; check the instructions for this board before modifying it.
Fix
- Replace the column 1 pre-driver transistor Q15.
Systems
Games
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Source: Pinball Repair help group!! (Facebook group)