[volt-nuts] Mechanical choppers

Dick Moore richiem at hughes.net
Sun Mar 21 21:54:10 UTC 2010

On Mar 21, 2010, at 5:00 AM, volt-nuts-request at febo.com wrote:

> Message: 4
> Date: Sun, 21 Mar 2010 10:27:07 +0100
> From: "Dr. Frank Stellmach" <drfrank.stellmach at freenet.de>
> Subject: [volt-nuts] Replacement of Mechanical choppers for precision
> 	instruments
> To: volt-nuts at febo.com
> Message-ID: <4BA5E66B.8050003 at 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......

Dick Moore

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