Well I can do the board - but i need one desoldered completely. it's possible to buzz via's etc for ground or +5v connections, so you dont need to sand a board, but using a few dead boards is a better plan than wrecking a working one. Plus i only need to salvage certain chips like the 26LS and anything Apple custom.
For all testers of this initial batch - i forgot the GND and +5v connections for the disk and serial - DOH! For some reason i skipped over them, it's fixed in the latest Gerbers, but the prototype batch will need these connecting - nothing major, just solder bodge wires and bridge pins for the SCSI - should see from these images where the GND and +5 connections are - light brown is +5v, dark brown is GND.
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...