The adjustment to which Phil was referring is to set the DC level at the output of the opamp which feeds the RMS converter. IMHO this should be set to zero volts.
I would agree that the "Low" value for calibrating an AC range should be somewhere around 10% of range as this tends to give better linearity than trying to set a Zero, while in general the "High" valsue should be close to 100% of range. Dave -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of m k Sent: 24 December 2012 14:28 To: [email protected] Subject: Re: [volt-nuts] Solartron 7150plus - Chasing AC gremlins I have had to calibrate quite a few DVM's that use RMS to DC converter chips and all of them give a lightly better overall accuracy if you do not trim to a true zero at zero input, it makes the low level linearity a LOT worse. the self noise of the gilbert cell needs to be partly left there as an output. M K > From: [email protected] > To: [email protected] > Date: Mon, 24 Dec 2012 14:19:43 +0000 > Subject: Re: [volt-nuts] Solartron 7150plus - Chasing AC gremlins > > Service manual (page 3.6 item 7) says to use 20VAC range, shorted > inputs, adjust trim pot for zero volts > > Re: gluing front panel - VERY tricky to remove without putting creases in - > been there done that :(. Better to just *sparingly* inject contact adhesive > in strategic areas, wait until mostly dry, and press together. > > As the only part replaced was IC15, the calibration should not be upset, as > its the resistors that determine the gain, not the op-amp. > > HtH > Dave > -----Original Message----- > From: [email protected] [mailto:[email protected]] > On Behalf Of Philip Pemberton > Sent: 24 December 2012 14:07 > To: [email protected] > Subject: Re: [volt-nuts] Solartron 7150plus - Chasing AC gremlins > > Right then, time for a writeup :) > > Long story short - the 7150Plus is just about fixed. Parts were sourced as > follows: > * Three 3N163 "analog switch" FETs at £11 each from Littlediode on Ebay. > These turned out not to be required... > > * Analog Devices AD637 (SOIC16) RMS-to-DC converted with SOIC breakout > board. About £30 of silicon and plastic... also not used (though may be > needed later if the 637 in the meter doesn't stop drifting). > > * Three LH0062H opamps. $45 US plus shipping for all three, or £27.81 in > the Queen's currency - also obtained from ebay > (<http://myworld.ebay.co.uk/techno*>). > > > To recap, the problem was that the AC ranges were drifting badly and > calibration reported an error condition after warm-up. After applying freeze > spray to the opamp/FET section of the AC stage, the whole AC stage failed and > the meter reported an input overload. > > Initial investigations suggested that IC15 (LH0062C, National > Semiconductor) was running open loop (output was clipped to within a few > volts of the negative-15V rail). No reason for this could be readily > ascertained and the feedback loop was producing erroneous results when probed > with a DMM (due to the opamp feeding back into the circuit). > > Initially I suspected IC18 (DG211, Siliconix) and replaced it. This had no > effect on the failure symptoms. > > This morning I desoldered the LH0062C and replaced it with an 8-pin IC socket > (machined-pin type of course). Probing the opamp support circuitry with the > opamp out of circuit suggested that the attenuator and feedback loop were now > operating as per the service manual description. > > For "Gain of 10" ranges, resistance between IC15 pin 6 and IC15 pin 2 was > measured at 1M-ohm after a settling delay of several tens of seconds. The > settling delay can be attributed to the charging of C29 (3u3); the resistance > is from R72 (1Meg). TR12 (WN1001) appears to prevent the multimeter from > measuring the R21a/R21b divider chain. > > For "Gain of 1" (unity) ranges, IC18 SW3 (pins 9, 10, 11) shorts over > C24 and puts the opamp into a unity gain state. Resistance between IC15 pins > 6 and 2 is approximately 30 Ohms in this mode. > > The function of TR12 and the circuitry around it still elude me. I've never > been much good with JFET circuits! > > After replacing the opamp, the AC range was selected with a 2V range. > This sets the attenuator to 1:1 mode and IC15's gain to unity. The whole AC > chain now acts as a buffer. Applying a 2V pk-pk (~0.7V RMS) 400Hz sine to the > input with a signal generator and probing the opamp output at TP3 suggested > that the opamp was now functioning to spec. Testing the AC ranges suggests > that aside from some expected calibration issues, the meter is functioning > normally. > > I'm hoping that trimming the offset is as simple as grounding the input, > selecting the lowest AC voltage range and adjusting for zero volts at TP3. > > After that, I need to figure out how to calibrate the AC voltage and > current ranges! I'm hoping the signal generator will work for the > lowest ranges, but the 20V, 200V and 750V ranges (not to mention the > AC current > range) may prove somewhat tricky... > > Then finally I need to remove the front panel and re-glue it to the plastic > base. Unfortunately the old glue (actually double-sided tape) has perished > and the front panel is now looking a little sorry for itself... > > Thanks, > -- > Phil. > [email protected] > http://www.philpem.me.uk/ > > _______________________________________________ > 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.
