[volt-nuts] Fluke 720a self calibration repair

[Bill Gold]

David:

    I have a 720A that I purchase around 1995 from a local surplus dealer.
The price was right at around $250.  When I did the "Self Calibration
Procedure" from 2-24 I found that several of the "A" decade pots could not
be adjusted within 0.5 microvolts, and I think at least one of the "B"
decade pots also.  After trying all of the other "Calibration" adjustments
from 4-19 and seeing that nothing had improved I decided that the Precision
Resistors in the "oil tank" had probably drifted over time.

    After looking at the schematic I saw that there were "trim" resistors
( i.e R1008, R1012 and so on ) that were "factory selected".  I didn't have
the equipment to measure the resistors in the "oil tank" to sufficent
accuracy to determine exactly what there values were and if the match was
still working.  So I decided to change or "trim" the "factory selected"
resistor on those settings which wouldn't adjust to the +/- 0.5 uv spec.  I
think I started at the lower end of the Ratio, ".0", and then worked my way
up to the top, checking the adjustment range of each adjustable multi-turn
pot ( I think they are 20 turns ).

    I added or subtracted a little resistance, as necessary, to the "factory
selected" resistors.  If they needed more resistance then I added in series
whatever was necessary to bring the multi-turn pot to the approximate center
of it's range.  If they needed less resistance then I added in parallel
whatever was necessary to bring the multi-turn pot to the approximate center
of it's range.  This required several passes in the procedure as each decade
depends upon the total resistance of all of the lower resistors.  I used the
best tolerance resistors in the series/parallel adjustments that I had on
hand or could find in the local surplus stores ( usually at least .01% ),
assuming that they would have the best temperature stability also.  When I
got done I had the whole thing working correctly and it has been easily
"Self Calibrated" for ratio for all of these years.  The overall temperature
stability seems excellent for room temperature changes.

    Of course you can send the 720A back to FLUKE and get charged an "arm
and a leg as well as your first born" for this repair.

    Just one suggestion, good luck!

Bill

----- Original Message ----- 
From: "David Garrido" <d.garrido at me.com>
To: <volt-nuts at febo.com>
Sent: Thursday, September 24, 2015 9:38 AM
Subject: [volt-nuts] Fluke 720a self calibration repair


> Hello All,
>
> I am trying to track down what I think is a small issue in the calibration
of my new to me 720a.  I am dreadfully paranoid of jacking something up in
the process and need a little hand holding.
>
> I was going through the Self Calibration steps and every thing was going
well up to the adjustment of the "A 1.0" decade switch position.  All of the
variable resistors up to that point needed to be turned CW in order to null
the appropriate switch position.  The “A 1.0” adjustment pot does not null
the switch position and is not getting any closer than about 2.5uV.  I
verified the meter balance between every other step.  I have checked, in
circuit, R302 and R303 and I am getting a VERY SOLID reading of 9.896 on all
of those.  Of course, checking R1006 and R1010 give me significantly
different readings than the stipulated 8.45k.
>
> I cannot seem to be able to find a CCW or CW limit to this resistor and I
think I read somewhere that there are supposed to be run from end to end
several times if an issue arises.  Do these var resistors have limits?  I
need a little help moving forward.
>
> Cheers,
>
> David
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.