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

Fred pa4tim at gmail.com
Sat Oct 1 18:18:48 UTC 2011

Thanks, I will look to that. But after it nulls again (earth problem
fixed and put the solartron on the same group as the calibrators the
problem is less. So I am going to make more work from the shielding and

A replaced the opamp in the 731A. I did not believe it was the problem
but it was not much work so worth to try.
But it did not matter. No difference. I simululated the circuit in spice
and when I short out the compensating transistor I get the same results
as in real live. I think my Vref is now just the voltage divider made
from R5 ( paralleled by R8A) and R8B so that is why the other dividers
have influence. 

So first I will test the behavour of Vref (Most times I try to figure it
out on paper and simulation after first global measurement results, and
if I find the problem like that, I start to measure the specific things
I need to know if I'm right. Maybe a bit an unusual methode but it works
for me ;-) )

I bought a BBC Servogor writer. i connected my 845 between the 332 and
my homebuild reference. Put it on the 30uV scale, the writer on the
output. In the 10mV/cm position. But I am terrible at math. So 30uV is
100 cm, 1 cm is 300nV. I placed a 10uF cap over the writer output to
suppress spikes and short time effects, ect and now it is writing at
3cm/hour. I turned it on when still cool but it got hot this afternoon
In about an hour the line moved a few mm to the right (also still a bit
warming up of the instruments I think) but after that the writer writes
a straight line. I leave it on this night. Nice experiment. After that I
want to look at short time noise but then I have to limmit the

What is the best way to get a good measurement ?


Will schreef op za 01-10-2011 om 11:43 [+0300]:
> > If i do a calibration on 10 V. It showes 10.000,000 on the display, but if I
> > switch the 720 to 1.000,000 Volt the display shows 0.999,980 Volt. If I
> > switch the solartron to the 1 V range it reads 1.000,000.x Volt. So the 10 V
> > range is not lineair. The stupid thing is, I can get it right. If I use the
> > filter while meauring the 1 V in th 10 V range it is correct but then it
> > will not read the 10 V correct.  So it looks like there is some noise of
> > common mode problem. I have not checked the 1 V range the same way. I will
> > do that too.
> >
> > Does that ring a bell to someone ?
> >
> > Fred PA4TIM
> The 7061 input resistance loads the 720A output. 10 Gohm should result
> approximately 4ppm error.
> IF the 720A is working perfectly, the input resistance of your 7061
> (including connection cables) is lower than specified. Could be dust,
> fingerprints...
> Will
> _______________________________________________
> 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.

More information about the volt-nuts mailing list