Hi Frank, Do you have any additional information about this? I have a Keithley 148 nanovoltmeter with a now-defunct chopper, and I had considered trying to either rebuild it, or replace it with a low thermal-EMF reed relay. I hadn't thought of using a FET. Do you happen to know which Fluke designs implemented this? The thermal EMF can probably be dealt with by appropriate packaging and mounting. The clock feedthrough / charge injection may require a little more thought. The existing chopper in the 148 has a bucking coil that can be adjusted to null out the coupling to the input line from the relay coil. -Randall > Subject: > [volt-nuts] Fluke 883AB differential voltmeter > From: > "Dr. Frank Stellmach" <drfrank.stellmach@freenet.de> > Date: > Fri, 19 Mar 2010 22:57:12 +0100 > To: > volt-nuts@febo.com > > To: > volt-nuts@febo.com > > >>> ..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... > > > Hi Mark, > > this problem seems to be related to the mechanical, reed contact chopper. > > This device had to be replaced very often, when I worked in a cal lab > about 30 years ago.. > > I wonder if the unit realy works oK after your modification. > > Try to replace the mechanical contact by a FET., like in later Fluke > designs.. > > Frank > > > > ------------------------------------------------------------------------ > > _______________________________________________ > volt-nuts mailing list > volt-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts