When servicing your early Bally power driver board, always make sure to check the display output voltage *before* removing it for service. I forget to do this sometimes and end up having to take the board back out despite the displays working.
It can fail “working” and you won’t even know it which means the high voltage circuit isn’t regulating the +230v down to +190v, it’s just passing it through right to the displays. The displays seem to be able to take it, but who knows for how long.
Save yourself a bunch of trouble and check display voltage first. What happens is Q21 (the transistor on the small heat sink) goes short. This is usually caused by Q23 (the round transistor with the little fins on it) going short.
Also make sure to check Q22, typically it’s ok but in this case it was shorted as well. As I always say, “when in doubt, throw it out.” Just replace Q22 and Q23 if Q21 is bad. You really don’t want to replace failed parts only to find they’ve blown again because of another bad part. Ask me how I know.
You can also replace R51 which usually gets hot and burns the board. When resistors fail, they go open so if it’s getting hot its resistance will only increase over time until it fails. Mount it high as shown in the pic below for better air circulation and to prevent further burning of the board.
When everything is done and the board is back in, check the voltage again. I also lower it down to +170v. This puts less pressure on the glass. On a rare occasion you may have a display that acts goofy when doing that. In that case raise the voltage up enough to a point that the display is stable again. Check out the pics below.
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