I accidentally trapped a gray insulated wire under a screw head that runs the 11VAC lead on the bottom of my playfield. The torque on the screw was enough to fray the insulation and short the wire. The gray wire is multiplexed to two flashing bulbs that stopped working after the accident I eliminated the short, and the bulbs still work when I shunt the wiring connector to a "sister" pin on the control board. One of the lamps is on the A phase and the other on B phase. Both of those pins no longer connect to ground when tested.

I suspect that I fried the SCR at Q36 (MCR106-1). It is not visibly blown. Is this the most likely thing to blow if I short it out? The SCR (and the rest of the circuit for that matter) is accustomed to having some resistance from the bulbs and diodes on the lamp assemblies (and effectively 6VAC due to multiplexing). There is also a resistor in the circuit before going to the 6821 PIA. My machine has two 6821 chips, so I swapped them and the same bulbs are out, so that proves the 6821 chips are good.

All of the other lights work, so at least I didn't do more serious damage but I wasn't lucky enough to have a fuse to protect my mistake.

I haven't had much luck testing SCRs in circuit, so I will probably just replace it. The only thing left on the circuit would be traces and a resistor. I should be able to get a good reading with my DMM on the resistor in circuit, correct? If a resistor blows, the circuit would be open if I am not mistaken.

For good measure, I checked continuity from pin 13 on the 4514 chip in slot U15 to both sides of R106 and it is good. I also get continuity to an outer leg of Q36. I think this means that either Q36 or the 4514 chip is bad. I was mistaken earlier when I was looking at the 6821 chips.

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I replaced Q36 and the lights work again.

I made a pretty darn good mushroom goat cheese pappardelle if you are interested. No low fat/gluten free here!