Black Jack Pinball Machine (Bally, 1977)

Location: Cheyenne, WY
Symptom: Wouldn’t boot up.  Battery leaking.

Before powering up, the first thing I did was to remove the leaking NiCad battery from the MPU board.  Fortunately, it hadn’t damaged the PCB traces.  I replaced the RAM with an anyPin NVRAM module.

The machine wouldn’t boot when power was turned on. I checked the power supply first, since the power supplies in this era of Ballys are notorious for failing.  All voltages were good.

The light on the MPU board flashed 7 times on power-up which indicated that most of the boot sequence was executing, but it was stopping just short of going into “Attract Mode”.

With my oscilloscope, I started probing around the MPU board.  The processor was running and there was activity on the address and data buses.  There was no activity on the IRQ (interrupt) line (pin 4 on U9).  There are two sources (that I’m aware of) for interrupts.  One is the display, the other is the AC zero crossing detector.

I checked the display interrupt generator, which is a 555 timer at U12.  There were pulses on pin 3.

Next I checked the zero crossing detector and found no pulses there.  The problem ended up being the 2.0K resistor (R113) at the input to the board, and is the top part of a voltage divider in conjunction with another 2K resistor at R16.  Fortunately, Radio Shack still carries resistors (although the guys working there have no idea what a resistor is or what it does).  We were able to get a 2.2K which is close enough.

Once the resistor was replaced, the machine booted up just fine.