I like the Bally -17 and -35 games. I guess its a good thing too, as I do get a lot of service calls on these games. Most calls begin with, “It was working fine and lights up, but won’t start a game now and I see anything on my displays.” This almost immediately triggers my mind to a locked on or none working MPU.
Most of the time the locked on MPU is a result of corrosion damage from the Ni-Cad rechargeable battery mounted directly on the MPU board. Over time, these batteries leak and corrode the board with a base that damages traces and eats component legs. If you have a Bally or Stern game from 1977-1985 that is working, do yourself a favor and make sure your Ni-Cad is long off this board. The Ni-Cad battery should be either completely absent or replaced with some kind of remote back or coin battery.
This Spy Hunter machine was bought by Bill Fleming, who was the long time Bally guy in the Winston-Salem area. I’ve never met Bill before, but heard many stories of his Bally era expertise from my friend and fellow tech, Jack. Bill’s been out of the game for some time now and enjoying retirement, but he kept his games and boards nice. This machine had a remote battery on it and no corrosion at all, which is very rare for me to come across on these Bally boards.
He had also already upgraded the solenoid driver board of this game and everything looked really good. However, the MPU was locking on. Over time, even without corrosion, other components can fail, sockets can lose their continuity and chips can fail. After all, many of these components are now over 30 years old.
To go to our bare bones boot sequence, I removed all chips but the U6 ROM, U9 6800 CPU chip and U11 PIA. These are the only three chips (plus U2 if its a Stern) that are needed to get the initial flicker we want to see from the LED. When a board is really clean, but still locking on, the three primary things I check are: Ground paths to make sure there’s not a break in the ground throughout the board, +5vdc path between the J4 connector, capacitors in line and test point, and the three primary active components in the reset circuit, which are the Q1, Q2 and Q5 transistors.
Getting good continuity between ground and +5vdc all around, I replaced Q1, Q2 (both 4401) and Q5 (4403). After replacing these three transistors, I booted the board with my test fixture and got the initial flash. After that, I installed the rest of the chips back in the board and booted again revealing the six flashes you need to see on the test fixture (a seventh flash is needed for full game boot, but the test fixture doesn’t provide the needed +43vdc for the seventh flash).
Here’s a video of the boot up on the bench after all chips were re-installed: