Well, that nice digital oscilloscope I scored on eBay back in 2004 - a TDS644A - is now irreparably dead. Almost everything still works, but it no longer triggers, and the vintage-scope-repair guru informs me that this failure mode involves multiple things wrong on the acquisition board, at least some of them not repairable with available methods & parts.
So, barring a replacement acquisition board turning up - maybe from a unit with a busted display - that scope is now officially deceased. Not triggering makes it useless for most of my purposes.
The good news is that the market for used test equipment seems to be slow at the moment, and digital 'scopes are going relatively cheap on eBay. Between that and a certain lack of urgency at the moment (my little 2-channel, 100 MHz portable scope is still working fine), I can probably get a good replacement that fits my budget and schedule.