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 > it’s 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.
