Tech: Bally 6803 flash lamp "Phase D" problem

LyonsArcade

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Tech: Bally 6803 flash lamp "Phase D" problem

I've got one here that's got me scratching my head, thought i"d run it by and see if anybody had seen it before.

I'm working on a Bally Hardbody, it's a 6803 game. Bally did this really pretty brilliant design where they use SCR's to operate two lamps at a time on the playfield. They have it seperated into 4 'phases', so there's Phase A & B, and Phase C & D.

The issue I have is that none of the 8 flash lamps on phase D work. All of Phase A, B, and C work perfectly. You would think this would be as easy as the fuse is blown for Phase D.... but it's not. The fuse is fine, and I get 26volts on each socket as I should (and also on all the Phase C sockets of course)...

I looked at the schematics, and all 8 of the SCR's are the bigger T106's... there are two more T106's on the mpu, but guess what, they only have 1 phase (A, which is working fine).

The 8 SCR's that are only working on phase C are controlled by three different IC chips... so it seems like the only thing common to them all, is that they're all T106's, and the Phase D lamps aren't working. Since they're all getting power but get grounded from 3 different chips I'm kind of at a loss. The T106's all just work as a switch basically too, so since they're working on phase C they should be able to work exactly the same on phase D.

Any ideas of other things I should look at, I think it works by sensing the zero crossing somehow on the AC?
 
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