the supplies take feedback from the rest of the scope to make sure the current is okay and such. So you can't just pull the supply out onto the bench and fire it up in isolation.
I don't think this will be a problem for me as I don't think the PSU on the 7904 non-A version uses such a feedback circuit. The service manual from Tekwiki mentions isolating the PSU from the actual mainframe.
Looking at the block diagram for the PSU, the only feedback I see is from the CRT circuit, presumably for safety reasons.
But this is just at a quick glance though. I may be missing something.
(You do have all the docs and info from TekWiki, yes?)
Yes, I do. I'm actually the one who added the service manual to the tek wiki.
The other bit is that the power supply module seemed to me to be put together like a 3D puzzle, with interlocking bits and a general sense of structural complexity.
...I can see what you mean... Looking at images on tekwiki, the PSU just looks like a brick full of PCBs, capacitors, coils, heat sinks, and transformers.
Luckily, Tekwiki has shared a document explaining how to service the power supply in the 7904. I predict this will be very useful as the service manual doesn't provide many details about the power supply.