[Test-Equipment] Troubleshooting a HP 34401A DMM

[DaveC]

No response to DC or AC voltage inputs. AC function displays OVerLoad 
on some intermediate ranges, 0v on high and low ranges (plus a few 
random LSDs). DC shows zero (plus a few random LSDs).

Get a good display and all button functions. Self-diag routine 604 
fails. This is defined as "A/D slope convergence failed".

All supplies (+/- 18v, +5v, reference voltages: +/-10, +5, +7) are 
within tolerances.

I presume the ADC is whacked. How does one go about troubleshooting 
the analog section? Put in a test voltage of, say, 7.5 v and see 
where it goes?

Lots of good schematics in the service manual, so I'm set for documentation.

Suggestions welcome.

Thanks,
Dave