[volt-nuts] Fluke 720a self calibration repair

[Bill Gold]

David:

    Run the numbers.  Assuming that the 5K-20 turn pot R1005 is set in the
middle of it's travel (2.5K), that all of the other resistors, in series and
parallel ( R1006,R1007,R1008 and R302), that make up one step of the "A"
decade have a "0" TC, only R1005 has a maximum of +/- 100 ppm per degree C,
then the maximum change on the total resistance between step 0.9 and 1.0 of
the "A" decade would only be 0.000000410 % change over the entire +/- TC of
R1005.  That is 0.00410 ppm.  The "Absolute Linearity" of the "A" decade is
specified at 0.1 ppm at +/- 1 degree C, without a temp correction chart.  Of
course all of the other resistors mentioned have a TC also, which could be +
or - to the total nominal of 10,000.  Therefore, unless I am mistaken, the
less expensive +/- 100ppm pots would work just fine and you could not
observe or measure the changes.  This is also why I didn't worry about my
"trimming" R1008, the "factory selected" resistor, slightly to get the 20
turn pots to adjust to the middle of their ranges, when necessary.

    IMHO

Bill

----- Original Message ----- 
From: "David Garrido" <d.garrido at me.com>
To: <volt-nuts at febo.com>
Sent: Saturday, September 26, 2015 5:05 AM
Subject: Re: [volt-nuts] Fluke 720a self calibration repair


> Thank you for all of the input on this folks.
>
> Does anyone have a copy of the datasheet for the original Ramo-Eltra P/N
3800P-502?
>
> The reason I ask is that the price of these parts is driven in part by the
tempco and I found info that indicated 20ppm/C on one website, but that was
not a DS, just a listing of the info.  If they can be 50ppm/C or even
100ppm/C, then the price drops exponentially.
>
> I will do whatever is necessary to maintain or (LOL) improve the 720’s
legendary performance.
>
> Cheers,
>
> David
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