[volt-nuts] Help with 7081 AC buffer circuit

[David C. Partridge]

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/Solartron>

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