[volt-nuts] Mechanical choppers

Roy Phillips phill.r1 at btinternet.com
Mon Mar 22 10:07:13 UTC 2010

I have a faulty HP 419 Null Meter with this problem.  Have you encountered 
these ?
Roy Phillips.

From: "Dick Moore" <richiem at hughes.net>
Sent: Sunday, March 21, 2010 9:54 PM
To: <volt-nuts at febo.com>
Subject: Re: [volt-nuts] Mechanical choppers

> 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......
> Best,
> Dick Moore
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