[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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