Test each display one at a time ..... they are daisy chained. Un plug them all and start with player #1, play a few games and bring #2 online, and so on. One bad one could cause all to display bad.

Turn the power off each time before you add one!!!

If you haven't already, reflow the solder under all the header pins on all display assemblies.

Also check the connector on the MPU that outputs display data.

Yes, 1st step is resolder header pins

Read here for a good tutorial:
http://stevekulpa.net/pinball/bally_disp6r.htm

Because the logic coming through all the displays are daisy chained, if one decoder chip fails, it can cause gibberish to display in all of them.

A shorted switch matrix can also cause odd display behavior. You can rule this out by disconnecting J2 and J3 from the MPU. If the problem goes away, then it's switch matrix related. It's fairly common for the diode leads on the slam switch on the coin door to touch ground.

As vdojaq suggested, the decoder chips can be the problem, too. With the power off, disconnect all but one display to see if they get better. Then connect the other displays, one at a time, to see which one is the culprit. CAUTION: Always power down before connecting or disconnecting the displays!

Allan

Clear the memory on the Alltek, then start it again.

The decoder outputs on the display PCBs are what fail 95% of the time, not an input. While an input could short to high or low, this rare. Also if this happened you would get a miscount of digits, not garbage. I think this thought is barking up the wrong tree.

Junk data in the CMOS ram will cause garbage numbers to stored as a high scores, audits, etc... not cause a garbage across all four displays during game play.

Garbage on the displays i would think is more likely a strobe timing issue. Someone mentioned above shorts to the switch matrix. This is actually does happen(i THINK because how the 4502 does multiplexing, anyone got theory on this???)). Most often is going to be seen on the coin door with the credit switch shorting to the grounded coin door.

When the displays go wacky. Pull the two switch connectors on the right side of the MPU, see if it goes away. Also gently flex around MPU J1 (top left MPU plug) and see if the garbage goes in and out moving around that bundle wires.

I serviced a Bally Cybernaut that all displays would go to total gibberish after about 10 minutes of play.

Game itself seemed to still be running properly, just displaying nonsense.

Reboot would again allow about 10 minutes of play, so I was ruling out thermal.

Replaced RAM, game was happy again.

_

I know I've set the switches on the Alltek to clear memory and it fixed other oddball problems

MPU J1 has all the display logic connector pins on it. There is BCD data, encoded digit information for the decoders, four lines. Latch strobes which are basically an enable line for a specific display, one per display, tells a display it is its turn to read the inputs since they are shared. Digit enable, one per digit 0's through 100k or million. Display blanking line sends a high pulse to BLANK out the display. If there is one pin that i would guess to cause your smptoms on MPU j1, it would be the display blanking line. Pin 10. Should be pulsing, i believe at 320 times a second. Make sure this signal leaves the MPU and makes it to the displays.

The CMOS ram is a scratch pad for digit enable memory. If the ram is running to slow or whatever, i guess that makes sense. I want to say i have even see digit problems caused by a too slow 5101. What the heck is altek using for the CMOS ram? I dont own or have never really even messed with one of those pcbs since i can fix the originals.

Show-off!