HP 35660A DSA Repair

[geo_leeman]

I had asked this on the test equipment section, but have decided it probably belongs here instead.   I was able to score a HP35660A recently, which is great considering I work with seismic equipment! I haven't ever owned/used one of these before, but couldn't pass the opportunity.  I bought it "As-Is" and have been perusing the manuals. 

The unit powers up fine, passes all self tests, the fault log is empty, but the "Calibration in Progress" message stays on the screen indefinitely. After poking at it for a minute or so the keypad stops responding and I get "Key buffer full".  I took it apart and don't see anything obviously wrong when I pulled the boards (processor, ch1, ch2) out.  Any ideas on what steps to take? It's hard to probe the boards in the chassis, but if a rail was dead it shouldn't pass self tests.