[volt-nuts] Help with 7081 AC buffer circuit

[ed breya]

Does that matter on the AC buffer in this case? Maybe that's the 
normal 1/f noise of the circuit down near DC.

>If it is problematic, then you may want to try reconditioning the 
>low level signal relay contacts. You can rig up a circuit with a 9V 
>battery and a 1 k or so ohm resistor, temporarily hooked up across 
>each contact, then run the relay through a few dozen cycles. If you 
>do that with each contact in the signal path, it may clean it up. 
>You may have to disconnect some of them to avoid damage to the 
>circuits - especially at the OP-16 input - it depends on the 
>overload rating. You may be able to first see if it's worth doing 
>this laborious process by very gently tapping each relay body to see 
>if there's any response.

Also, if you are working on the circuit while it's out in the open, 
you could have light generating currents in some of the devices, so 
make sure it's protected.

Lastly, the zero adjust pot itself could be the culprit - try running 
it to each extreme a few dozen times to clean the wiper and element.

Ed

>----- Original Message ----- From: "David C. Partridge" 
><david.partridge at perdrix.co.uk>
>To: "'Discussion of precise voltage measurement'" <volt-nuts at febo.com>
>Sent: Friday, June 08, 2012 12:46 PM
>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/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
>>
>>
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