Wading through just about anything to get just a little bit of something hidden therein ain't relaxing. I like to leisurely watch your videos, maybe with a cold glass of lemonade in hand. They're much appreciated by all here I think and thanks much!
Mystery solved! Found an old post that mentioned the 280c logic board has 4 x 35v 100uf (check), 1 x 35v 47uf (check), 1 x 25v 33uf (check), and 1 x 25v 100uf (eureka!). I had all of the above in my parts bins, so recap complete.
My new friend gave me a bit of a scare today after buttoning it up though: Hung on the initial gray screen, even after numerous power-cycles. From my research trying to track down that cap value, I read numerous horror stories about leaking caps causing exactly that behavior on Duos
Gravely concerned at this point, I opened it back up and started poking around with the DMM and scope. I couldn't find a schematic anywhere and the board is a forest of vias (who knows how many layers...), but while probing around in the DC-DC circuitry, I noticed that "DZ6" was open! D = diode, Z = Zener I assumed. The little SOT-23 SMD had "8Y" silked across the top, and a bit of googling suggested an SOT-23 packaged Zener diode thusly marked was perhaps an MMBZ5248? That I certainly didn't have, but I contacted a tech I'm friendly with at a local electronics repair shop and he did Booted right up after replacement. A $0.13 part almost ruined my day.
This particular failure mode was probably specific to my board, but if any of you are hanging on to failed Duo boards, replacing DZ6 might be worth a shot...
I do this with my Macs - it prevents it coming loose when you transit the Mac, the video board socket doesn't hold onto the CRT's neck too well on its own from my experience.
Have you checked continuity from the brightness knob on the analog board?