Atari Pole Position. Where's the road gone?

So, my Pole Position board had been working perfectly for a year since its last repair, it's had regular play every week and in my eyes it wasn't a scary event turning the cab on, expecting the a bad outcome.

A few weeks ago I play a game on the cab entered my initials at the high score page and moved onto another game, 10 minutes later I took a peek at the screen and something had gone wrong... The attract mode scoreboard and the title screen all worked fine, but during the screen showing the cars, the track was missing and the background and clouds we gone :(

Grass tracking instead of GP racing.

All the scores, timings, signs and other cars were present, the track had gone walkabout... After a couple of re-powers and checking of test mode for RAM & ROM failures, I knew the board had thrown its dummy out of the pram and would need some attention.

Test setup for testing.Power and video only at this point.

Well after being on the test rig for a very short time, the image deteriorated even more than when it failed in the cab.

Things getting worse...

To me it looked like the 'Green' video signal in the track elements were now missing, the signs and the cars had notably changed colour.

After some extensive google searching for possible faults I came across one post on KLOV arcade forum saying that if the custom IC's 5L or 6L from the Video PCB are removed, you will get the the exact fault that my board originally had.

I did have a spare Video board that had never been powered on, my first test was to swap the two custom IC's around, luckily they are socketed so that didn't take more than five minutes to find I still had the same problem with them swapped out, it would have been unlikely all four were bad so I moved on to some 'real' fault finding.

Video circuit showing the two Custom IC's mentioned above

With a logic probe, all the input and output signals looked busy pulsing away, signals 'PIC1-4' feed into an 74LS157 Multiplexer at location 8C.

74L157 at location 8C on the Video PCB

With a logic probe I moved through the input signal and everything looked well, moving to the four output signals pin four was stuck low while the other three outputs were pulsing away. I fired up my scope to look that what was coming out of the output pins, and even on the three outputs that were pulsing away, there wasn't much data on the signal and the output on pin four was definitely dead.

I did a quick piggyback test and when fires up the green grass seemed to be back and I could see an outline of the track but it still looked pretty messed up. Anyway, I removed the IC, put a socket in and pressed in a new 74157. With finger crossed I powered the game up and was gladly met with the game fully working again. :)

Pole Position lives again

With the game fully working I did confirm that removing either custom at 5L or 6L will exhibit the same fault my boar had when it failed.

Boom! Thanks for reading, more PP repairs to come in the future. I've got a CPU board to look at, it doesn't boot, and I also have a video board that has a graphic fault in the mountains and clouds.