(Topic ID: 317906)

-17 ERA Bally MPU Duplicate Display Data Issue

By Phoerber

2 years ago



Topic Stats

  • 9 posts
  • 5 Pinsiders participating
  • Latest reply 7 months ago by slghokie
  • Topic is favorited by 2 Pinsiders

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    #1 2 years ago

    I just resurrected a -17 MPU and all is working fine with the exception of the credit display data showing up identically on the Player 3 display. U20, pins 2,7,9, and 11 are the display latch strobes. The U20 chip is socketed and it looks like pin 2 (player 3 strobe) is crispy at the solder pad despite continuity checking out ok. Checked for short between pin 2 & 11 (credit display), none. There's nothing obvious like solder splash and unfortunately I can't read what type of chip this is, but manual says hex buffer 14502 (B) or (8)? Can't tell.

    I swapped the PIA's, no change. I have 2 other working -17 MPU's where this is not happening, so def on the board.

    Has anyone run into this issue before?

    #2 2 years ago

    I have something similar on a Flash Gordon where player 2 mirrors player one. I tried PIA's as well but no change. Got a new connector to repin and see if it is the crappy push in connector for displays causing it. Have not had a chance to try. Otherwise it has to be one of the display chips on the MPU or bad components in the path to display connector.

    #3 2 years ago
    Quoted from slghokie:

    I have something similar on a Flash Gordon where player 2 mirrors player one. I tried PIA's as well but no change. Got a new connector to repin and see if it is the crappy push in connector for displays causing it. Have not had a chance to try. Otherwise it has to be one of the display chips on the MPU or bad components in the path to display connector.

    Agree 100%, I will let you know if I solve - please do the same. My issue is definitely on the MPU.

    #4 2 years ago
    Quoted from Phoerber:

    I just resurrected a -17 MPU and all is working fine with the exception of the credit display data showing up identically on the Player 3 display. U20, pins 2,7,9, and 11 are the display latch strobes. The U20 chip is socketed and it looks like pin 2 (player 3 strobe) is crispy at the solder pad despite continuity checking out ok. Checked for short between pin 2 & 11 (credit display), none. There's nothing obvious like solder splash and unfortunately I can't read what type of chip this is, but manual says hex buffer 14502 (B) or (8)? Can't tell.
    I swapped the PIA's, no change. I have 2 other working -17 MPU's where this is not happening, so def on the board.
    Has anyone run into this issue before?

    Yes, that's a B (Motorola 14502B hex inverter/buffer).

    Edit: ignore the rest of this -- Quench's answer is better.

    What's the resistance between pins 13 & 1 on U20?
    Since your U20 is socketed, to prove that the problem is U20 (or before), you could pull U20, bend pin 2 a bit, and plug it back in. Then, display latch strobe #3 should never go high. If the player 3 display still mirrors credits, your short is between U20 and the connector. If display #3 turns off, then your problem is U20 or before.

    #5 2 years ago
    Quoted from Phoerber:

    I just resurrected a -17 MPU and all is working fine with the exception of the credit display data showing up identically on the Player 3 display.

    An open circuit on the player #3 latch signal causes it to float high at that displays 4543 decoder chip. It results in that display accepting all binary coded data written to any display. Since the Credit/Ball in Play display is written last in the refresh sequence, that's what you'll end up seeing on the player #3 display.

    If you're saying pin 2 (display latch #3) on U20 looks crispy, the outputs for that pin in the chip might be blown out. Any standard 4502 logic chip should be suitable.

    #6 2 years ago

    This will happen if the switch wires at the coin door are grounded. As a quick test, you can unplug the switches from the MPU to see if the problem persists.

    #7 2 years ago
    Quoted from Quench:

    An open circuit on the player #3 latch signal causes it to float high at that displays 4543 decoder chip. It results in that display accepting all binary coded data written to any display. Since the Credit/Ball in Play display is written last in the refresh sequence, that's what you'll end up seeing on the player #3 display.
    If you're saying pin 2 (display latch #3) on U20 looks crispy, the outputs for that pin in the chip might be blown out. Any standard 4502 logic chip should be suitable.

    Thanks Dick and Quench - I will have time this weekend to report back, sorry for the late reply, it's been a week.... More to come!

    #8 2 years ago

    Solved. Bad U20, thankfully it was socketed (that never happens!!) Appreciate the help Quench, Dick H, and all!

    1 year later
    #9 7 months ago

    On mine I replaced U20, U11 but in my case it was a bad U8 5101. Thread is here in case it can help others...

    https://pinside.com/pinball/forum/topic/bally-wrong-display-data-mpu-related

    Quench was instrumental in helping there too

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