[volt-nuts] Unable to zero 3420B on 10**5 sensitivity

[Dr. Frank]

David,

you should have checked the different parts of the chopped amplifier 
according to the manual, so to make sure that the DC amplifier works 
correctly, and that the AC amplifier is also ok, regarding the amplified 
neon waveforms.

But at first you could make another quick check:

As you have reworked the zeroing network, make sure by measuring at the 
wiper of zero pot R1, that you can adjust this voltage between -0.6V and 
+0.2V. Make sure, that the 1.8M resistor (A1S1R5) has about the correct 
value. If it driftet upwards excessively, that would reduce the zero 
trim range greatly.

Comparing this schematic with the 845A, which is really very similar, 
this big un-symmetry of -0.6 vs. +0.2V caught my eye.. in the 845A, it's 
more like -0.46V vs. +0.56V.

As this voltage acts upon the negative input of the chopper 
(photoelement V2), so indicating that your positive offset is caused by 
a too negative zero voltage, you could try to short R49, this 47k 
resistor, to have a symmetric +/- 0.6V. Maybe then you are able to 
achieve zero at the 10^5 gain.

If this is the case, please note the wiper voltage. This is attenuated 
by 1.8M over 100 Ohm , nominally covering +/- 33µV, which can be checked 
in the 100µV range. Also note, how the wiper voltage matches with the 
offset deflection.

If that wiper voltage is too high for zeroing, or if it does not match 
so well with the deflection, there may be something wrong in one of the 
amplifiers, or there is excessive leakage current, (by contamination by 
fingers of pre-owner), or else...
That is to be analaysed later on.

Frank