On Mar 21, 2010, at 5:00 AM, volt-nuts-request@febo.com wrote: > Message: 4 > Date: Sun, 21 Mar 2010 10:27:07 +0100 > From: "Dr. Frank Stellmach" <drfrank.stellmach@freenet.de> > Subject: [volt-nuts] Replacement of Mechanical choppers for precision > instruments > To: volt-nuts@febo.com > Message-ID: <4BA5E66B.8050003@freenet.de> > Content-Type: text/plain; charset=ISO-8859-15; format=flowed > > If I would get a defective unit with coil or opto chopper, I would > simply replace the complete DC-path / chopper circuitry by a simple 7650 > chopper IC. > > Just add an appropriate/simple +/- 5V supply stabilization; existing > overvolatge protection and null compensation should be sufficient. > > Frank Good idea Frank, and I've done it several ways depending on the instrument. In an 887A, I replaced the mechanical chopper and amplifier(s) with an OPA111 fet input op-amp (way too expensive and now unobtanium, but I just happened to have a couple) that has extremely low drift, and it worked very well without a chopper -- this was possible because the whole meter circuit had no low-z ground reference and the DC output was not transformer coupled to the demodulator. An IC chopper op-amp or an OPA27 non-chopper could be used for many old Fluke boxes depending on circuitry. I've also replaced the neon lamp-CdS chopper and meter amplifier in an 895A with an LT1150 and disabled the transformer coupled DC output, so that the chopper op-amp only provided signal to the meter circuit as it does in the 885/887 etc. I've left the neon lamp-CdS chopper in my 845AB and it's working quite well -- no need to fix what ain't broke...... Best, Dick Moore

Dick I have a faulty HP 419 Null Meter with this problem. Have you encountered these ? regards Roy Phillips. -------------------------------------------------- From: "Dick Moore" <richiem@hughes.net> Sent: Sunday, March 21, 2010 9:54 PM To: <volt-nuts@febo.com> Subject: Re: [volt-nuts] Mechanical choppers > > On Mar 21, 2010, at 5:00 AM, volt-nuts-request@febo.com wrote: > >> Message: 4 >> Date: Sun, 21 Mar 2010 10:27:07 +0100 >> From: "Dr. Frank Stellmach" <drfrank.stellmach@freenet.de> >> Subject: [volt-nuts] Replacement of Mechanical choppers for precision >> instruments >> To: volt-nuts@febo.com >> Message-ID: <4BA5E66B.8050003@freenet.de> >> Content-Type: text/plain; charset=ISO-8859-15; format=flowed >> >> If I would get a defective unit with coil or opto chopper, I would >> simply replace the complete DC-path / chopper circuitry by a simple 7650 >> chopper IC. >> >> Just add an appropriate/simple +/- 5V supply stabilization; existing >> overvolatge protection and null compensation should be sufficient. >> >> Frank > > > Good idea Frank, and I've done it several ways depending on the > instrument. In an 887A, I replaced the mechanical chopper and amplifier(s) > with an OPA111 fet input op-amp (way too expensive and now unobtanium, but > I just happened to have a couple) that has extremely low drift, and it > worked very well without a chopper -- this was possible because the whole > meter circuit had no low-z ground reference and the DC output was not > transformer coupled to the demodulator. An IC chopper op-amp or an OPA27 > non-chopper could be used for many old Fluke boxes depending on circuitry. > > I've also replaced the neon lamp-CdS chopper and meter amplifier in an > 895A with an LT1150 and disabled the transformer coupled DC output, so > that the chopper op-amp only provided signal to the meter circuit as it > does in the 885/887 etc. I've left the neon lamp-CdS chopper in my 845AB > and it's working quite well -- no need to fix what ain't broke...... > > Best, > Dick Moore > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. >