'82 Bally Display problem - missing digits

dasvis

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I am in the process of bringing an '82 Mr & Mrs Pacman pin back from the dead & need some help with the displays. Score displays are 7-digit.
Have rebuilt the rectifier board, the Solenoid driver board, the Squawk & talk board, & the battery damaged MPU board. All voltages are now within spec.
The pin will now boot up, & I can run it thru self diagnostics. When I get to the display test, it will flash & count up on only the 100, 1000, & 10000 digits -- the 1's, 10, 100000, & 1000000 digits are blank on all displays.
I have swapped displays around to different positions, checked the J1 header pins on the MPU board for corrosion (found none, but lightly sanded the pins anyway just in case), reflowed the solder connections on the MPU J1 connector & on the #1 display, I have tested the wiring harness for continuity to the display & it checks ok. I am at a loss on this.
I was getting some weird behavior from the center 6-digit credit display, so I replaced the 100k ohm resistors & it solved the odd segments lighting up, but still it too will only flash the 3 digits.
I have ordered an MPU connector kit & plan on replacing both the male & female side of all of the connectors just in case - but I doubt that is the solution.

Can anyone help shed some light on my problem? I am a pinball repair newbie, but so far it has been a lot of fun!
Mike
 
Ummm, uhhh, yeah......

Just like the real bible those are just guidelines ;)

EDIT:

I would start by checking the digit enables with a logic probe at the displays. They're common so you should only need to check one display but you might want to check all of them anyway. I suspect you won't see the enables for the non-working digits strobing. If not then do this:

I would use Leon's test ROM and check the PIA outputs at the header pins (not just at the PIA pins). That will be the true test of the PIA outputs working and making it out to the headers. It's definitely possible that the MPU will still boot with a bad PIA. Sounds like some of the digit enables are not working. Could be the PIA, socket, headers, etc...

You said you swapped the PIAs so that makes me think it could be a bad socket.

Once that's confirmed I would double check continuity of the digit enable wires right from the PIA pin to the solder side of the header on the display. You know the strobes, blanking and BCD data pins are working because some of the digits are working.

If all of the digit enable lines check out then the problem is with the displays themselves but I kind of doubt that will be the case.

It's either the digit enables or all of those digits are fried on all displays. Stranger things have happened but I wouldn't suspect that as my first guess. I would suspect a problem with the enable lines for those digits.

If you're interested in more detail on how the displays actually work I recommend reading Bally's Theory Of Operation manual:

http://www.pinball4you.ch/okaegi/Bally Theory of Operation.pdf

Something like this could also be caused by bad RAM or ROM in theory but I would only start thinking about something like that if the digit enables appear to be working.
 
Last edited:
Just like the real bible those are just guidelines ;)


You said you swapped the PIAs so that makes me think it could be a bad socket.

It was the U11 socket. Replaced it & installed a new 6821 works fine. All of the displays are nice & vivid, & a couple of solenoids started working that were dead. Hooray!
Now to move on to the playfield......
 
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