Location: Highlands Ranch, Colorado.
Symptoms: Display not working, dim playfield lights, needed tune-up and testing.

The player 1 display only had a single digit working (10,000’s).   The first thing I did was swap it with the player 4 display to make sure the problem was with the display and not the MPU board or a connection problem.  The problem moved to the player 4 display after swapping, indicating the problem was with the display board itself.

With my oscilloscope, I verified the signals going to the display and the outputs of the driver transistors.  The other digits were working fine but were not illuminating.  The 10,000’s digit had a shorted transistor (Q11, 2N5401) which kept that digit on all of the time.  Normally, only one digit is turned on at a time, but it is done so fast that the human eye doesn’t detect it, giving the illusion that all digits are on.

In this case, with one digit shorted ON, the other digits stop working.  I’m guessing it has to do with ionization of the neon gas.  I replaced the transistor, and the other digits started working normally.

The controlled lamps on the playfield were dim and when in test mode, the power supply in the backbox would start buzzing when all of the lamps flashed on. I checked the voltage at TP1 and it was around 3V, which is too low.  The symptoms of buzzing and the voltage being about half, indicated a bad bridge rectifier (BR1).  This rectifier fails on many Bally machines of this era.  Once replaced, the playfield lights were at normal brightness and the buzzing stopped.