Hi, with several rms-dc converters inside dvm's leads me to find that there is a tradeoff between low level linearity and setting to the lowest levels. check the output at 1% of scale vs the zero offset.
M K > From: [email protected] > To: [email protected] > Date: Fri, 14 Dec 2012 06:09:32 +0000 > Subject: Re: [volt-nuts] Solartron 7150plus - Chasing AC gremlins > > Adjustment of the offset trim should be trivial - short inputs on an AC > range, and adjust offset trim to give lowest possible uV DC reading at TP3. > > I just checked the Service Manual and that's exactly what page 3.6 tells you > to do (Test 7) - it says to use 20V AC range. > > Where did you find LH0062CH at a civilized price!??? > > Dave > -----Original Message----- > From: [email protected] [mailto:[email protected]] On > Behalf Of Philip Pemberton > Sent: 14 December 2012 01:44 > To: Discussion of precise voltage measurement > Subject: Re: [volt-nuts] Solartron 7150plus - Chasing AC gremlins > > On 13/12/12 17:32, Rob Klein wrote: > > Phil, > > > > However contradictory it may sound, that is actually a good thing. > > Continuous > > failures are always easier to chase down than your gremlins. > > > > Tracing front to back should bring you to Mount Doom relatively > > quickly ;-) > > Indeed! > > Well, here's the sum total of this evening's testing: > > * Disconnecting LK2 is not a good idea. It appears to connect the A/D > converter up to the input loop. Disconnecting it kills all the measurement > ranges (they'll read a constant low voltage). > I thought it was for disconnecting the drift-correction circuitry from the > input loop... c'est la vie. > > * Lifting one leg of R82 (the 75R input resistor for the RMS-DC > converter) and injecting 2V pk-pk 400Hz from a sine generator (Zo=50R) > results in a stable reading of ~0.65V RMS on the 7150 display, the same as my > Tek DSO reports. The reading is still drifting around a bit. > > * All the resistors in the AC input attenuator and opamp feedback loop are > fine. > > * At the junction of RLE, C2 and R10 I'm seeing the input signal with a > negative-6V DC offset. The output from the opamp is stuck at negative 15V. > > This leaves the following suspects: > Relay RLF (an SDS-Relais RS12 metal can relay) > IC18 (DG211 analog switch) > IC15 (LH0062C opamp) > TR12 (WN1001 FET) > TR1 and TR2 (3N163 FET) > > I have some LH0062H opamps on order -- the tin can variety, not the DIL part. > Incidentally the 7150Plus PCB is padded out for the DIL part but a tin-can > part has been installed by Solartron and had its leads bent to fit the DIL > footprint. Whatever works, I guess. > > In summary: I'm getting closer but I'm not quite there yet. > > Adjusting the opamp "offset trim" may be quite interesting -- I don't think > the adjustment procedure is in the service manual! > > > I'm wondering if it's possible to swap the 3N163s for something else if they > prove difficult to locate. A 3-pin FET wouldn't work (it wouldn't allow > negative Vds like the 3N163 does)... that means it'd probably have to be an > analog switch and some form of level translator. > > > As for the WN1001 - I can't even find a datasheet for it! > For that matter, the WD460 dual matched FET in the DC loop is also proving > difficult to find even a spec sheet. > I think it's safe to say I'd be pretty well stuffed if the '460 in the DC amp > ever failed! > > > I did notice a mildly amusing easter-egg on the schematic: the star-ground on > the first floating board schematic is labelled "ROME". As in "All roads lead > to Rome"... Reminds me of the "washer lady" and "wizard" who were often found > in Tektronix service manuals, back when the schematics were lovingly (?) hand > drawn. > > Cheers, > -- > 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.
