Yes the level here does matter, any output (DC or AC) at the test point in question is DC coupled into the RMS converter which responds to both AC and DC.
The calibration specification says: "Monitor the DC level at TP705 "Buffer O/P" on the 100mV AC range. Adjust RV701 for a minimum value between +/- 100uV." Yes, suggesting the erratic behaviour could be the potentiometer makes good sense. I don't think I fancy running a 25 turn pot back and forth through its entire adjustment range a few dozen times though - I'll borrow one from another meter and report back. I've swapped the OP16 with another meter already. I'm working on it with the lid in place so light sensitivity shouldn't be a problem. Regards, David Partridge -----Original Message----- From: [email protected] [mailto:[email protected]] 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" ><[email protected]> >To: "'Discussion of precise voltage measurement'" <[email protected]> >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 >> >> >>_______________________________________________ >>volt-nuts mailing list -- [email protected] To unsubscribe, go to >>https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >>and follow the instructions there. > > >_______________________________________________ >volt-nuts mailing list -- [email protected] To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >and follow the instructions there. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
