[volt-nuts] Help with 7081 AC buffer circuit

[J. L. Trantham]

David,

I would approach it from this perspective (and I suspect you have already
done this as well).

1.  Is the problem on this PCB?  Do you have another board to swap with?  If
not, where else could the problem be?  Obviously, all the connections and a
'soaked' PCB would be problematic.  Time for another 'bake'?

2.  If it is on that PCB, can you replace or clean the pot?  Can you replace
the Op Amp?

Hope this helps but suspect you have already done these.

Joe

-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On
Behalf Of David C. Partridge
Sent: Friday, June 08, 2012 11:46 AM
To: 'Discussion of precise voltage measurement'
Subject: [volt-nuts] Help with 7081 AC buffer circuit

Guys 'n gals,

I'm looking for some assistance with this circuit.  The OP16 op-amp has an
offset trim pot which I can use to adjust the output DC offset with the
input to the meter shorted - so far so good.  The catch is that it's really
*very* noisy, wandering by as much as 20-100 uV in less than half a second,
and wandering around the zero set point by as much as +/- 800uV over a
period of time (say an hour).

This is much better than it was before I cooked the PCB for 6 hours at 120
celsius, as before I did that there was a huge monotonic drift as the PCB
warmed up with the result that after an hour or so the DC output of the
buffer circuit was many millivolts off.

The schematic is sheet 70817506 Sheet 4.  In the soft copy of the service
manual which can be found here:

<http://www.ko4bb.com/manuals/index.php?dir=09%29_Misc_Test_Equipment/Solart
ron>

it is on pages 117 and 118.

I'm looking for any suggestions as to what component in this circuit is
likely to causing this problem.

Thanks
David Partridge


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