[volt-nuts] Problems with 720A

Tue Jul 11 09:00:07 EDT 2017

A short follow up.  I disconnected both suspect resistors from the rest of the unit.  Sad to relate they both measure the same as my post below.

I admit to being very puzzled how these can have shifted down in value so dramatically.   I shudder to think what Fluke would charge to fix it.

Dave
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of David C. Partridge
Sent: 10 July 2017 16:21
To: 'Discussion of precise voltage measurement'
Subject: Re: [volt-nuts] Problems with 720A

Further study of the schematics leads me to think that one should check the oil bath resistors by setting decade A to CAL and the main function switch to CK B (to isolate R313 and R314).

If I do this most of them read as expected around 9.89k but R302 reads 9.711k and R311 reads 9.793k.

Dave

-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of David C. Partridge
Sent: 10 July 2017 14:28
To: 'Discussion of precise voltage measurement'
Subject: Re: [volt-nuts] Problems with 720A

I just realised that with the dials set for zero that R311 will be shunted by the following decades.  I think that with the A dial set for 0.8 that a measurement of R311 should be valid.

If that's right, then I get 9K79 which still seems low.

Dave

-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of David C. Partridge
Sent: 10 July 2017 13:13
To: 'Discussion of precise voltage measurement'
Subject: Re: [volt-nuts] Problems with 720A

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?

Thanks
Dave

-----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?

Thanks
Dave

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