[volt-nuts] Help with 7081 AC buffer circuit

[David C. Partridge]

Changing the potentiometer was indeed the solution - thanks to Ed and to John (whose post never made it here, though I found it on the site). 

The DC level at TP705 is now relatively stable and close to 0uV (jumping around a bit but typically well less than 100uV).

Now that I get a stable reading near 0uV here, I see another problem :(

If I then switch from the VAC to the VAC plus VDC setting, the DC output level at TP705 jumps positive by about 600uV, which results in a difference in reading of around 60uV which I don't believe should happen.   Strangely, if I leave the input open circuit, I can switch between VAC and VAC+VDC modes without this shift occurring.

I re-wired the circuitry around the relays using PTFE feed-through insulators and direct point to point wiring to eliminate leakage problems between the relay power lines (-10V, -15V) and the signals before I baked the PCB.

Regards,
David Partridge
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On Behalf Of ed breya
Sent: 09 June 2012 07:39
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Help with 7081 AC buffer circuit

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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