[Test-Equipment] DATA PRECISION 3600 DMM

[J.Byers]

DATA PRECISION  3600 DMM

Does anyone have any service information on this model ? (A set of schematics would be nice)... It has started to drift up with inputs shorted until an 'Error' is annunciated.
I have removed the two relays from the board and cleaned and reset the contacts, but it still does the same.  The dc reference output is stable and am beginning to suspect that an  integrator , part of the conversion circuitry, has an open in the feedback loop somewhere, causing  it to slowly drift upwards.

I would be glad to offset, or pay for the costs of copying and sending a schematic or any other data, such as the w/shop manual (wish, wish)
I have not been able to track a manual for the 3600 at any i/net site so far, despite some weeks searching
The 3600 is a nice DMM and one of my favourite bench instruments.
I'de hate to lose it.
Does anyone have any better ideas as to what the fault is ?
Thank you 
JBEE