Ok got a decent DMM at my disposal. Scope due to arrive tomorrow.
I applied the following test cases:
1. From step 5-6. Short V/ohms and COM terminals, set DCV and 10m range. Unstable reading at 0.3-0.4 on the display. ZERO ADJ has no visible function.
2. From step 5-31. Again ZERO ADJ has no visible function.
3. From step 5-36. Shorted Q16 to ground, adjusted R50 to print DCTP in spec to 0.1mV. It was at 93mV which suggests someone was already playing with it :(
Now ZERO ADJ now has 2-3mV swing in either direction but the 10m range it's showing 0.214 still and drifting when inputs shorted and Q16 no longer shorted.
One thing I did try, which I am hoping is not what I think it is, was to check the input impedance of the meter with the 34401A. On the 20,200,2000 ranges it's 10M as expected but on 20m, 200m, 2 ranges it's a suspiciously low 210K.
Any ideas of next steps. The diagnostics at this level are somewhat scarce. I'm slowly tracing out the front end switching now.
