[volt-nuts] 7081 calibration "Zero too large"

[David C. Partridge]

It looks like the fault is in the input amplifier which is logically part of the DC ranging circuit.  The error seems to appear at the output of IC402.  Did anyone find the patent for this circuit - it is actually quite a complex circuit surrounding an LM343 with a discrete chopping arrangement using FETs an OP05 and LH0052 all of which I think is there to cancel the offset of IC402.

Dave
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On Behalf Of David C. Partridge
Sent: 02 May 2012 18:04
To: 'Discussion of precise voltage measurement'
Subject: [volt-nuts] 7081 calibration "Zero too large"

Thanks to a very kind gentleman, I received in the post today an Itron FG209C2 VFD and an ER-3400 NVM for my ailing 7081.

I issued a calibrate,zener,103 and received in return a "Cal incomplete" message.  Power off followed by initialise also gets me a "Cal incomplete" message to it looks like the new NVM is OK - yippee!

When I then try 

calibrate,vdc=0.1,zero

I get a "Zero too large" error message so there's clearly still a fault on the floating boards. Mrs. Murphy said (*) that it would still have problems even after replacing the display and NVM, and boy was she right!  Clearly it will be a labour of love to get this working again!

If anyone has any suggestions as to what is likely to be causing the "Zero too large" error, I'm all ears!

(*) Mrs Murphy's law: Murphy is an optimist.

Thanks
Dave


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