Hear me out, this is a totally bizarre issue!

Just restored a $6MDM which included installing a new Alltek MPU. Game played fine for a couple of days, but then I noticed that the Shoot Again lamp had become stuck on. This is controlled by Q3 on the lamp driver board. This game has a Stern LDA100 lamp driver in it but the schematics are identical to the original Bally ones. Replaced Q3 to no effect.

So I swapped in another lamp driver board from my Stars. Same issue! Lamp is still lit. So, it must be a problem with the playfield or wiring, right? The lamp socket and wiring looks fine. It is connected to J2 pin 21 on the lamp driver. I repinned pin 21 to no effect. But while testing, I noticed something really weird. If I leave pin 21 unplugged, the lamp actually still works and lights like it normally should when the feature is lit.

How is this possible? If it's unplugged from J2, how can the lamp possibly be getting a signal to turn on? If I reinstall the pin, the lamp goes back to being stuck on.

If anyone can help me understand what the hell is going on here I'd appreciate it!

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Thanks for your replies, guys. I have done some more troubleshooting and made some progress.

The wire that connects to pin 21 is green/yellow, whereas the wire connected to the shoot again lamp is grey/red. Hmm! Looking at the connectors, there is another wire on J2 which is grey/red. And the manual lists it as SPSA - same player shoots again! So this is actually the wire that controls the shoot again lamp on the playfield. This lamp works just fine from this connector wire. So, I have to then assume that pin 21 of J2 is for the SPSA lamps on the backbox.

With pin 21 of J2 connected, the SPSA lamps on the backbox (2) work fine. However, the shoot again lamp on the playfield then locks on and gets twice as bright. Obviously, if I leave it unplugged, the backbox SPSA stop working. It's almost as if the green/yellow wire doubles the current sent to the playfield lamp.

Based on the schematics, they are all controlled by the same SCR at Q3 (same circuit). If that is working for the playfield lamp then I'm not sure what else to check on the board.

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I've also noticed that the backbox SPSA lamps stay dimly lit even when they should be off (pin 21 connected).
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Another oddity. With J2 pin 21 (backbox) connected, the voltage to J1 pin 26 (playfield) is about half the voltage that other lamps on J1 are getting. With J2 pin 21 disconnected, the voltage to J2 pin 21 is double what it is to other lamps on J2.

I also have <1 ohm resistance between the two pins, so continuity seems good.

Definitely not wired to the GI.

Correct.

Unfortunately I was never able to figure it out. I just left the backbox SPSA lamps unplugged. That way they wouldn't work but at least the playfield SPSA lamp worked properly.

Definitely a weird issue. Let me know if you find the cause!

Hmm, interesting! I repinned the connectors on the MPU but not the lamp driver. Maybe I should have done that, but I didn't see any issues with the terminals or header pins.

The header is on the bottom left hand side of the board. The MPU is above but to the right. I don't think it was affected by battery corrosion, if that's what you're thinking.

Damn. Well we will have to rely on Thorzhammer88 to figure it out and let us know!