I'm back!
Got the replacement transistor for Q418, the MG silicone heat compound, and the mica sheets. I think the quality of mica sheets has severely declined since the early 2000's. I could only order them in a pack of 20, and out of all those only 1 looked somewhat better than what I already had.
Anyways, put all that stuff back on the board, put the board back in the cabinet, turned it on and....nothing. I get the *thump* sound when I flick on the power, and the neck tube is glowing, and I can hear a relay clicking 3 separate times, but the screen is black and it doesn't do that frightening electrostatic sizzle a tube typically makes when it energizes.
Seems like I have a power problem now. I don't think I hooked up any wires incorrectly, but I'll go double check. If not that, what else? The flyback? Q401, Q425, IC101, or IC403?
One step forward, two steps back...