Bally Diagnostics

[Carl Spiby]

Could someone look this over and make sure I'm understanding it correctly?

I'm trying to verify if the display interrupt generator is working as the game slows down randomly, its possible that the display is stealing too much CPU time and making the rest of the game run slow.

From the diagram it looks like +4.5VDC is pulsed to pin 40 of U11, if this is stuck high when I check it, would that point to C16 being faulty? I haven't got a logic probe.

http://mirror2.ipdb.org/files/1639/Bally_1982_Mr_Mrs_Pac_Man_Schematics.pdf

Thanks

 

[pinballmania]

The voltages on Bally schematics suggest what you should expect when measured with a meter. What you should see at that point if you used an oscilloscope would be a trace mainly at five volts, with pulses down to zero. The meter translates this as an average dc reading of 4.5 volts. What you might infer is that if you measure 5v it is stuck high, or if you measure low volts, it is low for more time than it should be.

 

[Carl Spiby]

Only just got around to having a look at this over the last weekend, the MPU didn't have a grounding screw, it was held in with those plastic pinch clips so we added a screw now which didn't change anything.

I checked the capacitor which controls the display interrupt (Using A/C 20v on the meter) and it reads between 0.12 and 0.29v depending on what solenoids are firing, there's no link to the voltage read and the slowdown occurring.

What we discovered is that more often than not, when the right and left PACMAN drops are down, the game tends to be in the state where its 'slow'. Also, when the ball enters the saucer when its lit for video mode, a bong is played which normally signifies that you don't have enough moves to play the maze! Releasing the saucer switch doesn't interrupt the countdown to the maze.