[volt-nuts] Update on 720A

[Bill Gold]

David:

    Ignore my previous post.  In thinking about this while out shopping this
morning I know the answer.

    The output resistance of the 720A is about 66k ohms.  Your voltmeter has
an input resistance of 10 megohms.  You are simply loading the output of the
720A.  The reason that the 0.1 ratio has about the expected voltage is
because the 10 Volt range your voltmeter probably has a very high input
resistance.  You are on "autorange" and when the output is set for 0.2 ratio
the voltmeter is now on the 20 volt range which is now 10 megohms.

    I get the same results as you do when measuring 0.9 ratio on my setup.
89.920 volts.  Check your 720A by applying 10 volts and then do your ratio
checks on the first decade.

    I just haven't used a 720A for a while and I didn't remember about the
output resistance.  When you get to become a "well aged citizen" it takes a
while to recall things.

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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