[volt-nuts] volt-nuts Digest, Vol 69, Issue 19, Datron 1281 offset null, bias current adjustment

[Four Designs Company]

The offset adjustment R317, is an AC adjustment, not DC as one might 
surmise. As I recall I applied a 10X scope probe to the  O/P point at 
the junction of R329 and R330 to view the AC component. I probably used 
a 10Meg ohm with a cap in parallel, in a well shielded enclosure across 
the input jacks. I probably set the 1281 to the 10V range.

As for R222 the input bias, I think I just barely touched up the 
adjustment so that there was little drift with the input open. That is, 
the input would float around a low displayed value, and climb to a large 
number in either polarity.

I paraphrased the adjustment info from the 1081 and applied it as much 
as possible to the 1281.

Another thing I learned from owning two, and working on a few more 
1281's is that the first thing to do is replace all the aluminum 
electrolytic caps on the analog board as a minimum. Especially the small 
1uf caps on the 3-terminal regulators die and the noise increases. This 
will cause all kinds of problems. I change them all, on all the boards. 
Due to the low profile of the larger caps careful selection is necessary.

A few years ago I had email contact with one of the technicians at Fluke 
Farnborough, the home of the 1281. He wasn't much help, but
indicated that internal adjustment procedures were never released. He 
wouldn't provide any details either..

Sorry for the nebulous response, but it has been awhile since I adjusted 
mine.


Bob Koller

On 5/23/2015 9:00 AM, volt-nuts-request at febo.com wrote:
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>     1. Datron 1281 offset null, Bias current adjustments
>        (starbook at uplink.net)
>     2. Datron 1281 offset null, Bias current adjustments
>        (starbook at uplink.net)
>     3. Re: Datron 1281 offset null, Bias current adjustments
>        (Electronics and Books)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Sat, 23 May 2015 04:38:48 -0400
> From: <starbook at uplink.net>
> To: Discussion of precise voltage measurement <volt-nuts at febo.com>
> Subject: [volt-nuts] Datron 1281 offset null, Bias current adjustments
> Message-ID: <55603C98.10501 at uplink.net>
> Content-Type: text/plain; charset=utf-8; format=flowed
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>
> I have a Datron 1281 DMM.
> How to set the bias current adjustment (R222)?
> How to set the offset null adjustment (R317)?
> Thanks,
> John Staron
>
>
> ------------------------------
>
> Message: 2
> Date: Sat, 23 May 2015 04:39:52 -0400
> From: <starbook at uplink.net>
> To: Discussion of precise voltage measurement <volt-nuts at febo.com>
> Subject: [volt-nuts] Datron 1281 offset null, Bias current adjustments
> Message-ID: <55603CD8.9070009 at uplink.net>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
>
> I have a Datron 1281 DMM.
> How to set the bias current adjustment (R222)?
> How to set the offset null adjustment (R317)?
> Thanks,
> John Staron
>
>
> ------------------------------
>
> Message: 3
> Date: Sat, 23 May 2015 10:41:09 +0000 (UTC)
> From: Electronics and Books <electronicsandbooks at yahoo.com>
> To: Discussion of precise voltage measurement <volt-nuts at febo.com>
> Subject: Re: [volt-nuts] Datron 1281 offset null, Bias current
> 	adjustments
> Message-ID:
> 	<1942394052.99115.1432377669452.JavaMail.yahoo at mail.yahoo.com>
> Content-Type: text/plain; charset=UTF-8
>
> offset
> looking at the schematics on page 11.2-3 the bias should be adjusted by shorting the inputs of the amplifier and then a zero output.
> biasshorting the external input and measure the output of the amplifier
> i could not find the internal adjustments in the manual.
>   Met vriendelijke groeten
> Regards
>
> Frans
>
>
> http://ElectronicsAndBooks.com
> Netherlands
>
> Discere ne cesses
>        From: "starbook at uplink.net" <starbook at uplink.net>
>   To: Discussion of precise voltage measurement <volt-nuts at febo.com>
>   Sent: Saturday, May 23, 2015 10:39 AM
>   Subject: [volt-nuts] Datron 1281 offset null, Bias current adjustments
>     
>
> I have a Datron 1281 DMM.
> How to set the bias current adjustment (R222)?
> How to set the offset null adjustment (R317)?
> Thanks,
> John Staron
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> End of volt-nuts Digest, Vol 69, Issue 19
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