(Topic ID: 202755)

Bally -35 display issues

By CadillacMusic

1 year ago

Topic Stats

  • 4 posts
  • 3 Pinsiders participating
  • Latest reply 1 year ago by CadillacMusic
  • Topic is favorited by 1 Pinsider


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#1 1 year ago

Got a Harlem Globetrotters that tdiddy is bringing me a transformer for, and I'm trying to get a working cpu board together for it. I've got a board that plays great (in our Star Trek), but the displays are strangely messed up. They display only 8s or 9s. I'd say it was a bad U20 chip, but I've replaced that and everything is still broken. I guess I don't fully understand where something could be going wrong. It seems like the only chip that really effects display is that u20, with u14 and u19 playing a small role. Can someone step-by-step me through the display process so I understand it better?

#2 1 year ago

So it sounds like you may be trying to repair an MPU in a Star Trek that may not be broken.

While the MPU could cause display problems, it may not be the cause... I apologize if you know this but a single blown/bad display can affect other how ALL displays work in a Bally -35 game. I would start by testing each of the 5 displays individually in the Star Trek.

#3 1 year ago

The output signals PA4-PA7 from the U10 PIA are used to select different numbers to be displayed. Are these signals pulsing at the displays (pins 19, 18, 17 and 16 at the display connectors)?
You might have bad connections at the MPU J1 connector (pins 25 - 28 respectively) where they exit the MPU board.

The U20 chip is involved in selecting between displays. U14 and U19 are for the blanking signal. These are unrelated to the symptom of only showing 8s and 9s.

First check snyper2099 suggestion above of determining if there's any bad displays. Then do the logic tests on the PA4-PA7 signals which contain the display BCD (Binary Coded Decimal) data.

#4 1 year ago

Ah, thanks Quench! My pin 28 was broken off and I didn't even notice it. Hate those connectors! I knew the displays were fine because the normal CPU works fine. Shoulda said that, sorry.

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