[volt-nuts] Update on 720A

Bill Gold wpgold3637 at att.net
Mon Aug 7 11:34:46 EDT 2017


David:

    First suggestion would be to see what the output measures, with the
input still set to 100.0014 volts (same as before), when setting the dials
to:

    0.99999X            (equivalent to setting the "A" decade to "0.1")
    1.99999X            (equivalent to setting the "A" decade to "0.2")
    2.99999X            (equivalent to setting the "A" decade to "0.3")
    3.99999X            (equivalent to setting the "A" decade to "0.4")
and so on up to
    8.99999X            (equivalent to setting the "A" decade to "0.9")

    What is the output when the dials are set to  9.99999X?  It should be
100.0014 (or very, very close) just like the input voltage.

    Also what do you mean by your statement "Sadly the beast isn't giving me
a nice linear voltage sequence when using
just the A decade."  Do you mean that other settings when decade B, C, D and
so on are not "0", produce ok results?

Bill


----- Original Message ----- 
From: "David C. Partridge" <david.partridge at perdrix.co.uk>
To: "'Discussion of precise voltage measurement'" <volt-nuts at febo.com>
Sent: Monday, August 07, 2017 1:49 AM
Subject: [volt-nuts] Update on 720A


> 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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