Does the scoring stop if you unplug the playfield switch connector?

What if you leave it on for a while before starting a game?

We recently had the same problem with Gold Ball, turned out it was a wire actuator assembly that the pball would roll over when going through between the guide rails for the upper lanes. The leafs on the assembly needed a bit more distance from each other.

FWIW

Zac is spot on. Pull the playfield switch connector from the mpu. If scoring stops it's a playfield problem. Check the sling shots, over time the assembly gets loose and they fall unto the wires underneath causing a short.

Maybe one of the two 6821 chips is going wacky.

Maybe one of the caps on the switch matrix lines is bad? Do a diode test from each pin on the pia related to the switch matrix to ground and see if any read different

Try cold booting the game without J3 connected. If no points racking up then definitely a playfield problem. If it's racking up a lot of switch closures it would take a few seconds for the game to catch up. How many points are being added? That could narrow it down to the bad switch. I'm with ZAC on a bad cap.

If they are AMI chips, it's even more likely since that brand is more failure prone than other brands.

Just to clarify... but I think I am understanding the the game is perfectly functional when he uses either of two other boards. It’s not a playfield issue. Unless this needs to be corrected....

So- it sure sounds like something that is warming up and works better hot. That not a resistor... a cap maybe, as suggested, but I don’t know for sure that really makes sense. Someone has to be able to list a couple components that can break when cold but function when warm or at least energized for a while. Can this happen to a chip? I am now curious. Interesting problem....

It could be a component, chip, cracked solder joint, bad socket, a cut trace--any of those things will expand or contract due to temperature.

However, the nature of the symptom can help narrow down where the problem might be.

If you can get into diagnostics and into the switch test to see which switch is being detected, that may determine what pathway to follow on the board, if it's simply not a PIA chip malfunctioning.

Well, those all seem to match up. Darn.

First thing you need to do is get the game straight into switch test mode when cold and watch what switch numbers the MPU board *thinks* are closed until it reports "0". These switch numbers will help pinpoint which switch strobe/return lines on the MPU board you need to concentrate diagnosis on.
Just make sure J2 is disconnected from the MPU board before you power on so no actual closed playfield switches invalidate this cold test.

BTW, is this a battery corroded MPU board? Can you post a picture of it?

Thanks. Some corrosion around the edge but otherwise clean.
The important part of the equation is the switch mode test result. What did you get?

Be surprised if it even worked otherwise, but I'd double check that you have continuity all the way around the ground plane.

In the Playboy manual, switch 27 is the miss January Target. I know it works with other boards but try unsoldering the disc cap from across the switch.

If you put your multimeter on high resistance measuring scale, do you measure anything across that suspect Miss January switch capacitor (out of circuit)?

This suspect MPU board could have some slight resistive short (commonly caused by corrosion) between switch return lines.

Grab your multimeter and set it to read resistance. If your meter isn't auto-ranging set it to 200k ohms scale.

With the MPU board cold, machine OFF and J2 and J3 disconnected, measure the resistance across the following pins - the readings should all be around 114k ohms:

J2 pin 1 to pin 2 =
J2 pin 2 to pin 3 =
J2 pin 3 to pin 4 =
J2 pin 4 to pin 5 =

J2 pin 8 to pin 9 =
J2 pin 9 to pin 10 =
J2 pin 10 to pin 11 =
J2 pin 11 to pin 12 =
J2 pin 12 to pin 13 =
J2 pin 13 to pin 14 =
J2 pin 14 to pin 15 =

Well, the definitive test would be to install a new cap and see if the problem returns. If it does, it is unrelated to the old cap. If it doesn't, then it makes me believe it is either a connectivity issue with the strobe and return lines associated with switch 27. Or, as others have stated, a failing component connected to one or the other (resistors, capacitors etc.).

In the strobe (output) changing state from low to high, there is a slight charge time associated with the 0.047/0.05ufd disc cap across the switch. A good cap won't effect the switch read. But as the disc caps fail, they have a tendency to effect the switch matrix in weird ways. Like with many "bullet proofing" techniques, changing all those disc caps on Target, star roll over, and thumper bumper switches is a good idea.

Those readings all looks fine to me.
Install a new capacitor on that Miss January standup target like CactusJack said and see what happens.

I had a capacitor on a pop bumper on my Mata Hari that caused similar issues. Game worked perfectly with new alltek board but had issues with original board. I simply cut the capacitor and it fixed it.