[volt-nuts] Problems with 720A

David C. Partridge david.partridge at perdrix.co.uk
Mon Jul 10 08:12:32 EDT 2017

If I have read that right I'm getting what I think are "odd" readings across R302 (9K710) (pins 21-22)  and R311 (7K395) (pins 3-4) when the dials are set to all zero.  I think both should read about 9K9.

Could someone who has a 720A check the resistance across pins 21-22 and 3-4 on the oil bath and let me know the numbers?


-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of David C. Partridge
Sent: 10 July 2017 12:20
To: 'Discussion of precise voltage measurement'
Subject: [volt-nuts] Problems with 720A

You knew this was coming I'm sure:  It was almost inevitable that there would be problems with the 720A!

While trying to self-calibrate the 720A, (CAL A) the at the 0.1 and 0.10 settings the null meter was way out with around 100mV out of balance, and the A.1 and A.10 pots seemed not to do anything.

I'm trying to work out from the schematic what's likely to be wrong, but so far am not clear where the fault lies.

I *think* that when I have the main switch set to CAL A and the A decade set to 0.1 that I'm balancing R311, R1044, R1042 and R1041  against the leg of the bridge comprising R1, R206, R207, R208 and R209

Similarly for the 1.0 setting of the A decade, I think the resistors are R302, R1008, R1006, and R1005.

Have I read this correctly?


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