Watched MPU and Squawk-and-Talk diagnosic LEDs at boot. Neither lit up. Checked input voltages at test points; 5V line seemed dead.

Power board looked pretty rough, so I dismounted it to check components. The 5V fuse was bad, so I replaced it. Other components seemed fine, possibly excepting BR2, which had different diode measurements on one side. One concern is that the BR1 mount looks kinda cooked. That bridge rectifier was clearly replaced, so it may be that the cooking is old. Current one is shallower, and has a nut between the board and the rectifier to make sure it stays in contact with the board underneath, which apparently acts as a heat sink.

Cleaned and reinstalled it; game plays as normal.