[volt-nuts] Update on 720A

[wb6bnq]

Hi Dave,

I think you need to review how to calibrate the 720A and Kelvin Varley 
dividers in general.  The loading you need to worry about is the load 
you put on the output terminals, irrespective of the particular decade.

I do not know how you are measuring the output, but it would appear, 
from your list of values, that your method of measuring the output is 
loading the divider, particularly as the value of error increases as you 
step up the "A" range.  The divider was designed to look into an 
infinite impedance.  I.E. a null detector driven from an equally stable 
and known DC source.  Like, for example a 332 or 335 Fluke DC  voltage 
standard.  Any loading is going to drag down the output value and it 
will get worse as you go from the bottom of the range to the top.

The calibration process assures that the ratio from one step to the next 
is equal.  Your values indicate that there may be other issues you did 
not properly account for.

Good luck with getting it straightened out.

Bill....WB6BNQ


David C. Partridge wrote:

>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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