Bally 7 Digit Display Fix

Here’s a fix on two, seven digit displays for a Bally Cybernaut. These original displays are getting harder to come by and unless the glass itself failed, they are easily fixed.

I’ve said it before and I’ll say it again. The early Bally systems from ‘77 to ‘84 is my favorite of the early systems. One of my favorite features is that the displays all run independent of each other. If one fails you can replace just one with an LED display and leave the others as original.

Bally pinball seven digit display

The principal of operation of the displays is simple. There’s just three main sections. One is the BCD decoder, it’s the only chip on the board. It takes the binary data from the logic board and converts it to the actual seven segment digit you see on the glass. The second are the seven transistors that drive each segment on the glass. Third is the digit enable transistor, one for each digit, six for a six digit display and seven for a seven digit display. There’s also a transistor to drive the comma.

PCB side of a Bally seven digit display

Each digit enable transistor is also tied to a level shifter transistor. Level shifters allow devices operating at different voltages to work with each other. They allow the 5 volt logic to control the 190 volt display glass.

In this situation I had two displays that had the same segment out for each of the seven digits. The first one had the top row out, segment “A” and the second display had the bottom row out, segment “D”. The pic of the display schematic below shows the letter designation for each segment of the seven segment display.

Bally pinball display schematic

A few different things can cause this problem, the BCD chip, the segment driver transistor (MPS-A42) or the glass itself. Also, the pin connecting the PCB to the glass can break off, sometimes this can be fixed and sometimes it can’t.

In both cases with these displays the MPS-A42’s were bad on each one. When I measured the offending transistor with my multimeter on the first display it gave me different readings compared to the other ones. I changed it out and all the segments worked.

On the second display all of the segment driver transistors read the same. To be thorough I measured each of the associated resistors to look for any that might be open, all checked out. I also made sure the associated pin for the missing segments wasn’t broken or pulled off the glass, again everything checked out.

Before going any further or throwing a BCD chip in I decided to replace the MPS-A42. I tested the display and the missing segments now work.

The takeaway from this is that when troubleshooting you need to be aware that a part can measure good but be bad. There’s a difference between taking static measurements on the transistor and expecting it to do work. A component can test good but fail under a load.

I can’t tell you how many times I’ve spun my wheels trying to track down a problem, especially when fixing a board someone else threw a bunch of parts at. Even though someone replaced parts prior to you attempting to fix a board, never assume they are good parts or weren’t blown up for some reason. Been there done that, had a Data East logic board from a Back to the Future that had a ton of problems and after spinning my wheels for a while I decided to check components previously replaced and found one bad.

Hopefully this will help you when tracking down a difficult problem.

Until next time, Frank

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