In regard to serial numbers and the quite possibly factory mod to the 2520 Buffer board, the working unit that does
not have the mod was built in March of '76, and the rebuilt unit was built in August '75. The parts carcass I have did not include a back panel, so no serial number. So maybe they were altering the early units before they shipped them out?
In regard to the problem child, it powered up today when I re-installed the driver boards from my working unit. Of course, these are the same boards that I tried immediately after this phenomenon of pulsing dbt and refusing to power up occurred with the rebuilt set of boards.
I had re-intalled the working unit boards yesterday as a last ditch effort before I started tearing things apart again, and a 100W bulb would not dim out. I walked away from it, and tried it again today with a 500W bulb and it powered right up.
That in itself is a relief, but I still have to try to track down the issue with the rebuilt boards, and will start by installing the NOS 984 bias transistors on the working boards and see what happens...
More sleuthing in my future, but at least I can rest easy knowing that I'm not likely to have to start tearing apart the heatsink and connector boards.