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

David C. Partridge david.partridge at perdrix.co.uk
Fri Nov 10 10:08:43 EST 2017


The first step of the calibration instructions 5-38 a) ask me to select the
1V range, and x10**5 sensitivity and to zero the meter using the front panel
zero pot.  I couldn't get that to zero.

The next step which was to zero at x1 sensitivity using A4R16 works just
fine.

I checked and replaced the fixed resistors R49, R14 and R15 on the A4 as
they were well out of tolerance (particularly R14 and R15 at 60K and 62K).
This improved things somewhat but the best I can do with the front panel
zero control on the 10**5 setting is a reading of about +0.8 on the meter
which is almost full scale.

There were no obviously out of line DC voltages in the DC portion of the
amp, save that the output at the emitters of Q12/Q13 in this condition was
about 0.78V

Chopper amps are not exactly something I've messed with before now except in
the form of IC opamps and I'm struggling again, so any help will be much
appreciated.

The schematic for the 3420B A4 amplifier board is Figure 7-8 (pp. 7-17/7-18)
which is on page 39 of the Keysight pdf file.

I worked through the troubleshooting tree but that just led me to believe
that the unit is mostly working ...



More information about the volt-nuts mailing list