Hello everyone

I friend of mine recently bought a TAXI, and is having some display problems. One or more of the individual parts of all segments are always lit. According to him it varies a little hom many "parts" of the segment is lit up. In the video, it is just one (upper right), but it can be even worse as shown in the photo.

He tried reseating the cable a couple of times without any luck. He did however tell me that at one point he took his board out to remove batteries, and solder on a couple of wires to move the batteries to the bottom of the machine. And after that the error was gone for a week or so. But then it came back.

Any ideas? Could it be just a defective cable? Or is it more likely that it is a defective component?

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He tried a brand new cable without any luck.

But he just purchased a Rollergames machine, so we decided to try swapping the displays between the two machines. But the problem stayed with the Taxi machine, even when using the Rollergames display in it. And the Taxi display was working fine in the Rollergames machine.

So i guess that tells us the problem is not with the display or the cable, but somewhere on the CPU / Power board. Does anybody have an idea where to go next?

It's as if it gets a little better when the machine have had some time to warm up properly.

Note: I just asked again about the time he said it was working for a week after removing the CPU board, but he doesn't remember that anymore, so theres a chance that part of the story should be disregarded

Thank you so much for your reply. I actually do have a (crappy and cheap) logic probe, but i'm not sure i would know how to use it to pinpoint the fault. Perhaps one of the datalines would be stuck hi / low?

But if you are pretty certain that it is one of the PIA chips, perhaps i should just try replacing it? They are pretty cheap, if this is the correct one i found:
https://www.ministryofpinball.com/en/ic-mc6821-peripheral-interface-adapter.html

I have struggled to find decent schematics of the CPU board for Taxi online. Would anybody happen to have a link to semething decent i can use? Or actually tel me exactly what specific PIA i should replace, based on the pictures in the original post?

Found the schematics. Don't know why i didn't just check IPDB to begin with

https://www.ipdb.org/files/2505/Williams_1988_Taxi_Schematics.pdf

I am terrible at reading CPU board schematics, so if anyone can help pinpoint which PIA is running top 16 segment display on the machine, i would really appreciate it

There is definitely some acid damage. Most visible on the banks of resistors just above the battery holder. So i have a feeling there is a short or something here. Will start by desoldering those, and cleaning up.

Checked continuity on the datalines between the 6821's and the was no problems here. As far as i can tell, it is the 6821 just below the battery holder that drives the alphanumeric display (Top one of the two displays on TAXI).

Just to follow up on this: The PIAs were fine. It turned out being battery acid doing some shorting on the resistor banks above the battery holder.

I removed the 4 4.7 kOhm resistor packs, neutralized and cleaned up the area, and installed 4 new resistor packs. And the display is working great again.

Thank you everyone for your help and tips