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

[David C. Partridge]

Still hoping someone can guide me in locating the offset in this chopper amp ...

Thanks
Dave

-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of David C. Partridge
Sent: 10 November 2017 15:09
To: hp_agilent_equipment at yahoogroups.com; 'Discussion of precise voltage measurement'
Subject: [volt-nuts] Unable to zero 3420B on 10**5 sensitivity

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 ...

_______________________________________________
volt-nuts mailing list -- volt-nuts at febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.