Well, it's my own fault for feeling pleased with myself...
I repaired the PSU but the scope wouldn't come up.

Disconnected the acquisition board and it came up, diagnostics
complained about not having an acquisition board (surprise!).

Reconnected acq board and the 7 seg display stops at ".P",
disconnected acq board again and still get ".P"

Any ideas what .P means?

Hmm, I think I know now... it means you haven't connected the
analogue power up properly - duh.

Still no joy with the acq board - in the error log it
complains about failed communication with the analogue processor.

Anyone any idea if the HC705 is the analogue processor?

A set of schematics would be really useful about now...

---Paul