[volt-nuts] Serious problems fluke 731A, 720 and solartron 7061

[Fred Schneider]

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