This one has me stumped.

For some reason my Bally Star Trek will only display odd numbers. This behavior is persistent across all five displays and it's not an individual segment issue. When running the self-test, the displays will light 1,1,3,3,5,5,7,7,9,9 - no even numbers. On start up, the scores all read "11" instead of "00".

I've swapped out U10 and U11 with new 6821's. I've inspected all the solder joints on the ic sockets, inspected all the solder joints on the connector pins, inspected the pins inside the J1 connector and each display connector. I have no idea where to go next...

Any ideas?

Thank you! Here's where we're at right now.

- With only Player 1 display plugged in I can see even numbers for a second or so upon startup before they flicker back to odd numbers. Cycling through the others still show odd numbers with some occasional flickering.
- Unplugging the lamp board has no effect.
- Unplugging MPU J2 and J3 has no effect.
- R108, C45, and C51 look normal on inspection and have good solder joints. I'll need to source some parts to replace them.

Just to be sure:
R108 is a 56k 1/4W
C45 and C51 are 470pf +/-5%

Correct?

This helps... a lot. I was able to trace out the BCD pins on the schematics last night and this helps confirm my work. Helps narrow it down to the exact pin.

I checked the J1-25 and display pin connections and reflowed the solder joints on all the display pin 19's.

Here's what's strange.

I'm still unable to isolate the issue to a display, connector, or combination of displays and/or connectors.

However, if I pull up on the offending wire (white/orange) as if I were checking the integrity of the pin insertion into the connector at J1-25 OR at ANY of the display pin 19's, ALL displays will work correctly for a while -- until they eventually flicker back to only odd numbers a half second later. It doesn't matter which connector I pull up on. Just pulling up on the white/orange wire on any random connector will make it work.

Next, I put a meter on each of the display pin 19s and as long as the meter is connected, the displays read correctly. When I remove it, they flicker back to odd numbers.

I inserted a toothpick into the connector to mimic the probe insertion and that has no effect. If I bend the display board a little from the bottom to mimic pulling up on the pin, nothing changes. If I disconnect the displays individually our connect only one display at a time, nothing changes.

I'm confident to say it is not a physical connection issue. It's electrical.

What does the (assumed) added capacitance from my fingers on the insulated wire do - or the connection of the multimeter probe to the pin do to make it work correctly? I'd assume a loose ground somewhere, but not sure where to check since I've isolated each of the displays and all the other MPU functions are good.

At this point, I'm guessing it must the capacitor associated with J1-25 on the MPU board??? (Just for the record: I've looked over the J1-25 capacitor and resistor closely and the connections are shinny and bright and they look good externally.)

Finally got a minute to work on game today. I think we're getting somewhere!

Just to make sure: I numbered the pins with pin 1 on the bottom left and counting up by moving pin by pin to the right...

MPU, U10, Pin 6 to Display 4, U1, Pin 5 = 69.6K (Out of range)
MPU, U10, Pin 7 to Display 4, U1, Pin 3 = 20.9K (In range +/- 5%)

J1, Pin 25 to ground = 3.2K
J1, Pin 26 to ground = 3.2K
J1, Pin 27 to ground = 3.2K
J1, Pin 28 to ground = 3.2K

With Lamp Driver Board J4 and MPU J2 and J3 disconnected

J1, Pin 25 to ground = 3.3K
J1, Pin 26 to ground = 3.3K
J1, Pin 27 to ground = 3.3K
J1, Pin 28 to ground = 3.3K

Looks like we need to replace MPU R80?

[ts4z: A bad display was my first hunch. I had previously disconnected one display at a time, then disconnected all displays and rotated connecting displays 1 through 5 one at a time. The problem persisted in all states of disconnect.)

Alrighty! It was J1 Pin 25 after all. It looked good with a solid crimp, was nice and shiny, but it was the problem. I pulled it and crimped a new pin in and all is well. Ohms are reading as they should and the game is scoring as it should. Thank you!!