[volt-nuts] Update on 720A
David C. Partridge
david.partridge at perdrix.co.uk
Mon Aug 7 04:49:41 EDT 2017
I've added resistors in series with R1008 and R1044 to bring the resistance
for those parts of the A divider to a value that it would calibrate OK (two
of the resistor sets in the oil bath very low in value). I also had to
remove five turns (about 1.3 Ohms) from R1051 as the combined value of R313
and R314 was too high to allow me to calibrate S2. I also replaced R1041
as it was worn out (almost impossible to get a null).
I've successfully calibrated the C decade as per the maintenance part of the
manual.
Sadly the beast isn't giving me a nice linear voltage sequence when using
just the A decade.
Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to
zero except Decade A.
Decade A Output V
0.1 9.999,88
0.2 19.967,8
0.3 29.936,7
0.4 39.903,8
0.5 49.874,7
0.6 59.855,6
0.7 69.852,4
0.8 79.871,3
0.9 89.919,6
See also attached graph.
Any suggestions on where to look for trouble will be most welcome ...
All sections of the A decade attenuator now measure about 9.999,2 ohms.
It was suggested on eevblog that the next decade (or a subsequent one) was
loading it, but with all the other decades set to 0 I don't quite "get" how.
Thanks
Dave
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