[volt-nuts] Serious problems fluke 731A, 720 and solartron

Dave M dgminala at mediacombb.net
Thu Sep 29 14:37:58 UTC 2011


volt-nuts-request at febo.com wrote:
>
> After I mounted new batterys, did some repairs I calibrated it, I
> left it on for about a month. Today I checked it again and it was
> drifted about 40ppm. But the problem now was I can not get it right.
> It was allready a problem, allmost all adjustments influence each
> other. I do not think that is normal. The manual is not realy clear.
> They do not adjust the 10 V. I think it should be. They set it at 10
> V position, then use a null detector to set a calibrator at the same
> voltage as the 731A. But that is not 10 V in my case.
>
> I looked in the schematic and the cal pot in the front should be the
> one for that i think. They do not mention it only at the last line
> for setting the standardcell value. In my manual an employer of the
> former owner has placed a cross over that part of the procedure.
> But if I set 10V right and 1V correct the 1.018 and 1.019 are 1.020
> and 1.021 with the potentiometers at the extremes.
>
> Also not ood I think, I can not connect a 10 MOhm voltmeter to it,
> the voltage will drop then. I do not know if that is correct. But I
> am afraid there is something really wrong. But not what. It is just a
> Vref and a opamp. Not much other parts. It also reacts on a
> powersupply change, so slowly drops in value if the battery is on. I
> think the opamp is not good. But if so, it is strange the 10V can be
> adjusted.
>
> Also found out my solartron 7061 is not lineair. If I software
> calibrated the 1V and 10 V range against a 332 using a null detector,
> divider and a standardcell. I have a few dividers and a nulldetector,
> and more meters to do some cross checkes. But If I measure, after
> that,  for instance 7.5 V it is spot on but when I measure 1 V in the
> 10 V range it is 26 uV wrong.
>
> If I measure that value in the 1 V range it is OK but when I measure
> 2 V it is a bit off again. The main problem is it will not do the
> null calibrating in the calibration menu. it gives an error the
> difference is to big. I have the input of the rear connector standard
> shorted so I do not have to take it out the rack. I allso have
> replaced the battery, it now holds its values ( I think, at least the
> date does) i have no service manual ( they seems to be very rare) but
> I think it needs some hardware adjusting.
>
> Third problem: my 720 is perfect, lineair in every step, except for
> the B range. It is 100 ppm off in every position but this changes
> sometimes from  100 to 69 if I wiggle or turn  the knob. Allmost all
> the isolators of my switches were cracked so I made new ones from
> teflon. Cleaned everything and then it worked good. After a while the
> B switch sometimes gave a fault, but turning a fewtimes corrected it
> again, but now it won't. There are two switches are made complete
> closed. But I can not remember if this is A and B or the two last
> ones. If the problem switch turns out to be the closed one, can they
> be opened. Or am am I stuck with junk in that case ? I realy like
> that thing and they are hard to find and expensive. Fluke Holland
> does not respond on questions about parts.
>
> Fred PA4TIM
>

Fred, you can buy a manual for your Fluke 731A from 
https://www.ridgeequipment.com/store/manuals/detail.php?id=12771.

>From your description of all of your problems, I think it's time for some 
outside assistance.  You need to get one or more of your instruments checked 
out and repaired by someone who has the necessary equipment to verify 
operation.  You don't have any equipment that is in known good condition. 
Are you confident that the null meter is working properly?
I think I would start with the Fluke 332 and the 731A voltage standards. 
Once you know that they are working properly and calibrated, you can then 
use them to verify your 729 divider and null meter.

David
dgminala at mediacombb dot net






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