any good substitution for 845AB?

[Ex-Navy]

Thanks for the replies.   
Some additional information:   
   The meter wanders when the input switch is in zero position as well as in operate with a copper wire across the input terminals.  No external thermals or cables are involved.   
   A HP 419A null meter does not wander like the 845AR does.   
   I have read the article by Martin Kidd about thermal voltages.   

I suspect the problem is in the 845A input chopper or amplifier, but I have no experience repairing these types of circuits.  The Fluke manual is for an early version of the 845AR; mine appears to be an exceptionally clean, late version.

The best of season's greetings to all,
Joe