[volt-nuts] volt-nuts Digest, Vol 7, Issue 12
richiem at hughes.net
Fri Mar 19 21:47:22 UTC 2010
On Mar 19, 2010, at 5:00 AM, volt-nuts-request at febo.com wrote:
> Message: 1
> Date: Thu, 18 Mar 2010 15:35:33 -0400
> From: Stan Katz <stan.katz.hk at gmail.com>
> Subject: Re: [volt-nuts] HP 3420B calibration assitance
> Hi Dick,
> If you haven't posted to Yahoo Groups hp_agilent_equipment because you're
> exercising proper internet ettiquette, iwaiting for a first reply here,
> don't wait too long. The agilent group is terrific.
> I have no experience with the HP 3420B, although I have a much older 740B
> diff. meter. Fortunately, all its internal alignment could be accomplished
> (excluding ref. voltage) with onboard resistor networks.
> Good Luck,
Thanks for the tip on the Agilent group Stan. I'll visit.
> Message: 3
> Date: Fri, 19 Mar 2010 01:04:25 +0000
> From: Mark Sims <holrum at hotmail.com>
> Subject: [volt-nuts] Fluke 883AB differential voltmeter
> I just finished restoring a Fluke 883AB differential voltmeter (and a 845AB null voltmeter).
> The 883 AB needed new battery packs (I used 4 x nine AA cell nicad packs and a 4 subC cell pack)... remarkably cheap from a Ebay vendor. The original 9.6V packs used an unobtainium coin cell and would have cost around $200 to replace). Also the power supply for the AC input converter was fried (transistor, zener, cap).
> Final problem was it would not zero properly. The zero circuit uses two diodes to make a +/- 0.6V supply driving the offset pot. The unit was offset so that you could not reach zero and the offset pot had fairly little range. Adjusting the chopper didn't help. I replaced the diodes with three 1.2V reference chips (making a -1.2 to +2.4V supply). This centered the zero adjustment and gave better range.
Mark, I've rebuilt/repaired a number of Fluke analog meters. Very often, the electrolytics in the chopper amp system are bad/leaky, and the zeroing goes nuts. I've used 1.25V band-gap devices from National to replace the parts (or redesign the circuit) in the zero circuits on several types of these meters 885, 887, 845, 335 DC Standard, etc, and as you found in your mod, that works well. It's amazing to me how well these old instruments work when given a little TLC.
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