[volt-nuts] Solartron 7081 noise

[m k]

Hi Mickle,
As it seems it was stored in poor conditions it  would need to have all the PCB's and wires washed carefully first.Try and get hold of some pure isopropanol, be careful as many of the easily obtained samples of this often contain some methanol, which WILL craze any polystyrene capacitors. The best is HPLC grade Isopropanol, but usually much too pricey. So first test your IPA for compatibility with some spare polystyrene capacitors.Then wash all PCB's carefully using a stiff brush, then use a hairdrier to dry out the boards, leave at least overnight before powering up again, Also check for IPA getting into the connectors, use an air line if possible to get them dry. I used to use that technique to clean PCB's that had a 30-100G ohm feedback resistor around an opamp.Then once you have it safely powered up again it will possibly need a month to stabilise again?
Good luck, and if any of my instructions are unclear please ask for clarification, as it is late and I am in a hurry for bed.
Mike

> From: gbusg at comcast.net
> To: volt-nuts at febo.com
> Date: Sun, 11 Sep 2011 12:40:31 -0600
> Subject: Re: [volt-nuts] Solartron 7081 noise
> 
> Excellent work, Mickle!
> 
> Nice touch adding the PTFE standoff insulators. How did you make connections 
> on the trace-side to the PTFE stand-offs? (To reduce leakage and dielectric 
> absorption, I assume you ran hard-wires instead of using existing PCB 
> traces?)
> 
> Your PCB leakage problem reminds me of a similar problem Sony had with a 
> high-impedance circuit back in the seventies. The problem began showing-up 
> in older units of that model after years of use. Technicians were able to 
> temporarily eliminate the electrical leakage currents by spraying the area 
> with PCB cleaner solutions. But the problem would return in a few months. 
> Finally Sony determined the problem was caused by bacterial growth in higher 
> humidity environments due to a contamination problem at the factory when the 
> products were made. Interestingly, Sony then released a Service Note, 
> recommending that you spray the area with super-cold Freon to kill the 
> bacteria by freezing it. ...I tried that approach and it worked. I've never 
> forgotten that situation, because it was so counterintuitive and strange!
> 
> Apparently according to the 7081 manual, its integration time is determined 
> by the Scale Length you select, hence number of digits displayed? So 
> apparently your 7 1/2 digits resolution corresponds to 3.2 seconds 
> integration time. Do I have that right?
> 
> It might be interesting if some other 7081 users will duplicate your tests 
> for comparison.
> 
> Cheers,
> Greg
> 
> 
> 
> ----- Original Message ----- 
> From: "Михаил" <timka2k at yandex.ru>
> To: <volt-nuts at febo.com>
> Sent: Sunday, September 11, 2011 6:47 AM
> Subject: [volt-nuts] Solartron 7081 noise
> 
> 
> Hi everyone,
> 
> I recently purchased a Solartron 7081 DMM. It▓s one of the first model, 
> dated Apr. 1984.
> I suspect that it is not as close to accurate as it could be due to aging 
> and
> storage in a humid environment. My test showed good accuracy, but the huge 
> input
> bias current and suspiciously large noise. After 8 hours of heating at 25
> degrees Celsius input current was 330 pA causing a zero shift in the
> short-circuited input √ 12 uV (internal resistance at the Hi input is 36000
> Ohms). Standard warm-up procedure after prolonged low-temperature storage 
> did
> not help. One of the problem was found in the input amplifier and DC 
> selector:
> surface and volumetric leakage in the PCB from the relays power supply 
> channel
> (- 10 V), especially in places that are not protected by guard traces. After
> checking each critical item in the input circuits (attenuator, DC ranging, 
> input
> amplifier with MDM channel) and installing the PTFE bushings and insulators
> (http://img14.imageshack.us/img14/3432/pcb6top.jpg 
> ,http://img825.imageshack.us/img825/3896/pcb6bottom.jpg )
> input bias has been reduced down to 1-2 pA!
> After all, S7081 was warmed up again for more than 12 hours and has been 
> tested
> by drifts through the RS232 interface and a terminal on a PC:
> 1) Short inputs voltage drift while 6 hours preheating 
> (http://img109.imageshack.us/img109/2384/7081zeropreheating.jpg);
> 2) Short inputs voltage drift in DC 10V range after preheating 
> (http://img198.imageshack.us/img198/4846/708110vzerofront.jpg);
> 3) ADC zero test result, used by S7081 in the selftest procedure, periodical
> (every 15 minutes) drift correction and while 6..8-1/2 resolution selection
> (http://img155.imageshack.us/img155/2254/7081test0.jpg);
> 4) Internal reference test result, used by S7081 in the selftest procedure 
> (http://img694.imageshack.us/img694/3379/7081test10.jpg).
> After analyzing of the results it seems that the next big problem is the ADC
> noise, since its amplitude does not decrease even with a short ADC inputs 
> during
> the test of zero (TEST0). Maybe I should change the integrators op-amp 
> ICL7650S.
> 
> Mickle Timofeyev (Russia)
> 
> 
> 
> 
> 
> 
> 
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