Sick Keithley 197A -- 30 uV instability - Page 1
Forum section: electronics / testgear

author: tooki
author_rank: Super Contributor
timestamp: February 18, 2017, 04:10:56 pm
content: So even though I already have a Keithley 197, I picked up a cheap used 197A because I wanted the backlight*. In doing basic performance testing of the two, it looks like the 197A has troubles, namely instability at the low end. It looks be jumping around about 30-ish microvolts, while the 197 is stable on the same test voltage. On higher ranges this isn't noticeable, on the lowest ones it is, since this is a unit with 1 microvolt resolution.In basic testing, it looks like the 2V reference voltage is jumping around by the same 30 microvolts. The voltage across the reference zener (VR103) looks stable (but since at that range, I can't see the microvolts, it could be unstable without me being able to measure it), but if I measure the voltage differential between the ground side of the zener and a ground elsewhere (e.g. at pin 4 of U116, where all the grounds come together), I'm seeing around 100 microvolts, bouncing around from about 80-110 microvolts -- the same 30 microvolt bounce in the reference and in the measured voltage.Unfortunately, with this, we've more or less arrived at the limits of my troubleshooting ability. Can someone guide me to the next step? (Test equipment available: the working 197, a Fluke 87V and some cheapie meters, and a DS1054Z scope.)Thanks for any guidance!!Here's theservice manual for the 197 (with schematic). (Theone for the 197Alacks the schematic and PCB layout, but they are identical other than the backlight circuit and AC input jack.)*Looks like the EL backlight on this unit is nearly worn out, but that's neither here nor there.

author: rastro
author_rank: Frequent Contributor
timestamp: February 18, 2017, 05:45:44 pm
content: What are you doing with the inputs of the DMM? Have you tried shorting the inputs? Is the noise both AC/DC Volts? How about AMP and OHM's stability?Why don't you try switching the display PCB's (front panel) between the 197 and the 197A. The display on the 197A has a supply for the back light that may cause interference. It sends an AC voltage to the back light strip. I had a 197A and you could hear the back light when it was on.Keep in mind that your probing and test equipment may induce noise into the circuit you are testing especially at the u-volt level.Also I was never impressed with the back light level on the 197A. Now I just have the 197's.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2017, 06:11:09 pm
content: If the problem only appears in the lowest range, it is likely not the reference that is causing the trouble. If it was the ref one would expect a similar relative change in all ranges and a very poor performance when measuring something different from 0 (e.g. a 1.5 V battery). When measuring a short, there is usually very little influence of the reference.A first point would be to check supply voltages and ripple. There is a chance filter caps in the supply are bad and this way cause ripply and possible ground problems.Another point to check are switch contacts: if not used for a long time they can oxidize. So it might be a good idea to operate them for maybe 10 -20 times, especially the AC/DC one.

author: rastro
author_rank: Frequent Contributor
timestamp: February 18, 2017, 06:16:58 pm
content: Another thought.The 197 series has a battery pack option. If you wanted to eliminate noise source from "AC power" you could temporarily run it off battery - but make sure its unplugged!!!.You don't need the option. Just hook a battery to the appropriate connectors on the mother board. Refer to the schematics for the option which is included in the 197 manual. I'm pretty sure the option it uses 12V NI-CAD. But since you're not charging the unit you could use standard cells.-rastro

author: rastro
author_rank: Frequent Contributor
timestamp: February 18, 2017, 06:22:39 pm
content: Quote from: Kleinstein on February 18, 2017, 06:11:09 pmKleinstein has a very good point. I've owned a number of the 197's and a lot of flakeness can be cleared up by spraying and exercising the push button contacts with a good quality contact cleaner.-rastro

author: tooki
author_rank: Super Contributor
timestamp: February 18, 2017, 07:07:44 pm
content: Thanks for all the replies!Quote from: rastro on February 18, 2017, 05:45:44 pmI did a number of tests. Used a bench PSU set to 0.1V, batteries, a cap charged to 0.1V, etc.Turning off the backlight makes no difference (I had that thought too). I don't think there's any point in swapping the boards since the backlight is switchable with a mechanical switch around the back.Shorted out in DCV, I get a slight offset (around +7 uV) from zero, but no jumpiness.I haven't tested ACV or amps since I don't have any source for a suitable test signals.In ohms it's unstable. E.g. with a 100R resistor, I get jumpiness on the last two digits.My test setup isn't the problem, since I've repeated each test on the 197 using identical everything. In every test, the 197 is rock solid, the 197A is jumpy.Quote from: Kleinstein on February 18, 2017, 06:11:09 pmAs best I can tell, the change seems to be absolute, such that going down 1 range (= 1 decade), the jumpiness goes down by 1 order of magnitude as well. In higher ranges it then vanishes into the LSB.I don't know how to properly measure PSU ripple. The voltages seem to be fine. (Forgot to say that I checked them.)As for switch contacts: could they produce the kinda cyclic, repetitive jumpiness I'm seeing? (It doesn't look like the kind of flakiness I'd intuitively attribute to contact dirt.)Is the exactly matching voltage I'm measuring between different ground points an indication? Like, I'm seeing what looks like those 30uV, but I don't actually knowwhatI'm measuring there.Quote from: rastro on February 18, 2017, 06:16:58 pmAs a matter of fact, the 197A came with the battery option! But I'm pretty sure the pack is dead. (I've gotta give it a full overnight charge with the meter off.) But at least the battery circuit is there, so I can definitely test it on DC somehow. (I think you DO need the battery option to test it with a battery, by the way. The battery board contains an ICL7661 to generate the negative supply rail from the battery.)

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2017, 07:53:23 pm
content: With these old meters there is a slight possibility they can react to RF signals, especially a mobile phone near by. This sometimes gives funny jumps.Bad contacts could give all kinds of strange behavior, as it might leave an input more or less open an thus able to pic up signal from around.Problems with the Ohms could also very well be due to problems with switches, as there are quite a lot of switches used in Ohms more. Also the ohms mode should not be very sensitive to reference noise.One can check ripple with the scope in AC mode. There is should be no visible ripple after the regulators and not too much (e.g. 0.5-2 V range) before.

author: rastro
author_rank: Frequent Contributor
timestamp: February 18, 2017, 09:12:25 pm
content: Quote from: tooki on February 18, 2017, 07:07:44 pmActually you COULD still run it on batteries without the option/PCB. I'm sure you can come up with a negative rail using common batteries - don't you agree? In fact running the battery option with a compromised rechargeable battery my not be a very sound test for tracking down noise. Maybe the pure ad hoc battery solution is a better baseline. No noise from a converter.You know it's also possible that the battery option is a potential source of instability did you run the 197A without this installed? I had a 197 fail to turn on until I removed the battey pack option. You may try plugging the option onto the stable 197 to see what happens.I would still clean the contact switches as suggested, but that's your decision.Anything else not mentioned like previous signs of rework? Other functions with issues? Overall baseline issues outside 30uV stability? Other differences between the 2 dmm's?

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 18, 2017, 09:48:41 pm
content: Despite the fact that I still have two (differently) broken 197 meters, I have become something of an expert on probing them and working on the schematics, parts list, etc.I haven't published any of it yet because I haven't managed to fix my two meters. But I have put a great deal of work into them, and I have schematics and layout pictures that have been cleaned up, labeled, corrected, and colored. I also have an Arduino code base (built against the Mega 2560, but it could be used on any) which completely decodes the data sent to the display -- it functions identically for all modes, including normal operation, diagnosis and calibration. I also started on a project to support the GPIB cards (I have both the 1972 and 1973 cards).I would love to get on a chat program with you so I can ask some questions about signal properties of your meter, so I can help diagnose my meter. Unfortunately, due to the JFET-heavy construction of these meters, it is difficult to diagnose problems that involve any of the JFETS in the range Q104 - Q116. Or at least it is for me. Perhaps you can PM me, or get on ##electronics on freenode?I can post more stuff tonight (in particular, the colored and cleaned schematics). For now, here is mycompleteparts list. This list is *exactly* the same list as in the manual (you can see the numbering scheme is identical, including gaps). However, this list containsactualpart numbers (which have been correlated using other Keithley products),alternatepart numbers (using different versions of 197s, other keithley products, and modern substitutions), and measurements of these things in my meters.Keep in mind they fixed things over time in these boards. In particular, revision M was a big change (I have a pre-M and post-M board). If you have two bodge capacitors onU116, then you have a post-revision M board.C148is also bodged on the same revision.If you haveU125on your board, you basically have the latest version and your board should match the schematic exactly.One other difficulty with this meter (especially when powering via AC mains, and this is the only way I have currently attempted to power mine) is that the power supply is asplit rail full-bridge-rectifiedconstruction. I am not actually sure how to best describe the effect this has, but basically the positive and negative rails are "perfectly" balanced against each other (taking into account the resistors R132 and R131). So anything that throws off this balance seems to cause ripple on either supply, and sometimes on the ground itself.Note:the schematic is actually drawn incorrectly here. There is a short between the hollow number 4 and the leftmost solid dot of CR107 that is not really there in the power supply.Anyway, for now,here is the parts list. I am not aware of any remaining mistakes in the parts list. The coloration for the JFETs is because those two JFETs are "anti-matched" (purposely different pinch-off voltages) so that a single FET driver can drive two JFETs off of a single opamp.If your unit had the battery option and/or there is any evidence of corrosion anywhere, make sure to DeoxIT the bolt that the shielding plate is attached with, along with the nut inside the PCB it attaches to. If the shield is not making a good connection to that nut, then the shield is useless.Lastly: make sure all of the connectors for the red, black, orange, grey, and white front panel connectors have heatshrink on them. This was supposed to be done on all of these meters, but was often not done.

author: tooki
author_rank: Super Contributor
timestamp: February 18, 2017, 09:54:41 pm
content: Quote from: Kleinstein on February 18, 2017, 07:53:23 pmI guess you missed the part about the essentially identical 197 (non-A), under identical setup in the same location (literally stacked on top of each other) not exhibiting any problems.Quote from: Kleinstein on February 18, 2017, 07:53:23 pmAn open input? How would that cause the last 2 digits of a 6-digit measurement (110.000mV) to be affected, but measure the first 4 digits correctly?Quote from: Kleinstein on February 18, 2017, 07:53:23 pmYes, but given that Dave did an entire episode on it, it seems there are "gotchas" that I probably don't know.Quote from: rastro on February 18, 2017, 09:12:25 pmI tried it with the battery option removed, no difference unfortunately. (Didn't bother trying it in the working 197 since the battery isn't charged yet, assuming it will take a charge...)Quote from: rastro on February 18, 2017, 09:12:25 pmI will try cleaning, since I have a can of Kontakt 60 laying around.Everything else seems fine so far, other than the 30uV instability, and that in general it seems to be in worse calibration than the older 197. (The old 197 and my basically new Fluke 87V always agree, as much as they can given their different counts. The new 197A measures a bit off. A bit low on DCV, a bit high on ohms.)

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 18, 2017, 09:56:27 pm
content: Quote from: tooki on February 18, 2017, 07:07:44 pmMake sure to test again and differentiateinstabilityfrom "very" long settling time. Again, I still don't fully understand everything about this multimeter, but I am pretty sure thatC148was added to improve the settling time (and this also is essentially attached directly to the 2V reference, so...).One of my two meters hasC148, the other does not. The meter which had C148 hadmuchfaster settling time than the meter which did not have C148 (and also did not have C145 and C146 bodges). This could very well be the problem you are having.

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 18, 2017, 10:01:58 pm
content: Quote from: tooki on February 18, 2017, 09:54:41 pmAs for cleaning the contacts:1. You don't ever have to clean the contacts for the 6 range switches, unless they are truly acting up or broken.Noneof them are in the signal path.2. Contrary to popular belief, it is possible to disassemble the switches and clean them manually. It involves removing components behind the switches, blocking them from sliding backwards. Then you have to put pressure on the switch shaft downward, so it can squeeze out the back side. Each switch has 4 metal springs in it, which can easily be cleaned using an ultrasonic cleaner and alcohol. The inside of the switch (which just consists of two arrays of vertical pins) can alsoeasilybe cleaned thanks to the fact that a Q-Tip cotton swab covered with alcohol isexactlythe same size as the channel, so it fits in there, cleans perfectly, and doesn't leave any cotton behind.3. I don't know if DeoxIT or a similar solution is a better idea or not than disassembling the switches (I didn't have any at the time). I actually think a manual cleaning of a definitely bad switch followed up by a DeoxIT (or similar) lubrication after reassembly would be the best route.

author: tooki
author_rank: Super Contributor
timestamp: February 18, 2017, 10:04:34 pm
content: Quote from: technogeeky on February 18, 2017, 09:48:41 pmWow, thanks! I'll PM you with some contact info.Battery: Luckily, not even a hint of corrosion. The battery pack could have been installed yesterday, it looks that clean.Heatshrink: You mean heatshrink over the crimped insulated spade connectors? Looks like the 197A has it on the 10A and Lo input, but not on the ohm sense or HI input jacks.In the 197A, there are no bodge caps on U116, but it does have two bodge caps in series across R104. They appear to be the same as two caps on the 197 mounted near R104.

author: tooki
author_rank: Super Contributor
timestamp: February 18, 2017, 10:07:03 pm
content: Quote from: technogeeky on February 18, 2017, 10:01:58 pmYeah, the range switches are innocent, as proven also by the fact that the problem occurs regardless of whether manual or autoranging is used!I'll wait on taking apart the mode switches for cleaning until I've ruled out PSU ripple and anything else we can come up with first.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2017, 11:19:16 pm
content: The caps at R104 are for the high frequency AC part - this is capacitance in the single digit pF range. So better not touch / bend those.There is no real trick in measuring ripple. The DMM is isolated from PE, so there is no problem measuring the DMM circuit with the scope.The bodges near U116 suggest that there might be some tendency of ringing / oscillation with the +5 V supply. So the capacitors at the 5 V could be important. This circuit part looks suspicious at least - I would have expected a base resistor to reduce the tendency for oscillations.The 200 mV range and the current ranges should use very much similar circuitry. Is the instability in the current ranges (e.g. measuring 0 current) also there.

author: rastro
author_rank: Frequent Contributor
timestamp: February 18, 2017, 11:32:58 pm
content: Quote from: technogeeky on February 18, 2017, 10:01:58 pmtechnogeeky, that's a good find. Do you have any pictures to post showing the disassembly process?

author: timb
author_rank: Super Contributor
timestamp: February 19, 2017, 12:48:52 am
content: Quote from: tooki on February 18, 2017, 07:07:44 pmYou don't need the option. The battery pack simply feeds 12VDC to the point just before the power switch. Basically the main transformer and batteries are hooked together. (This allows charging when plugged in.)The battery board also contains a positive to negative charge pump as you mentioned that feeds to the negative side of the power switch, however this could be bypassed for testing purposes.Essentially, for noise testing, you could use two sets of 15V batteries (one for positive and one for negative; that is 20 AA batteries in series, with the point between ten of the AA's as GND, the bottom of the stack as negative and the top as positive) hooked right into the header on the main board that the battery board hooks into.

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 19, 2017, 01:52:05 am
content: Quote from: timb on February 19, 2017, 12:48:52 amCould you not just use a pair of isolated bench power supplies in a split rail configuration (so long as they have diodes protecting the charging current)?

author: rastro
author_rank: Frequent Contributor
timestamp: February 19, 2017, 05:01:54 am
content: Quote from: technogeeky on February 19, 2017, 01:52:05 amI think the whole idea of using batteries is to get completely away from potential AC power supply noise. I can't think of a quieter power source than a battery. Well...as long as it not a lipo catching on fire.Also if you are using batteries you should completely remove the charged battery option (basically board and battery).

author: rastro
author_rank: Frequent Contributor
timestamp: February 19, 2017, 05:16:52 am
content: FWIW I have a couple of 197's purchased a few months prior that both have working battery options. I decided to test their DC u-volt stability. Unplugged and on battery they where both within 1 uV of each other and rock solid. Plugged in I also got similar results.

author: tooki
author_rank: Super Contributor
timestamp: February 19, 2017, 09:28:25 am
content: Quote from: technogeeky on February 18, 2017, 09:56:27 pmMissed part of this reply. Again, I have a working 197 so I know exactly what the correct behavior is!!! It's not a "long settling time" on the 197A, it's that itneversettles.

author: hankee
author_rank: Newbie
timestamp: February 19, 2017, 12:07:43 pm
content: Hi,I also had such a kind of problem on my 197A.After some searching it turned out te be caused by the FASTONs on the input J1001 etc.After disconnection/connection them once all was ok, maybe I will clean them sometime, maybe I will wait.grtz Hank

author: tooki
author_rank: Super Contributor
timestamp: February 19, 2017, 12:44:18 pm
content: Well, lovely... today I can't reproduce the jumpiness problem at all! :/I put together a little voltage divider to make test voltage for the 200mV range (bench PSU seems to be OK for this, with a little added capacitance on the output of the voltage divider). The Fluke 87V reads 199.94mV, the 197 reads 199.948mV, and the 197A 199.911mV, so I still think it's out of calibration.As an aside, the reason for getting the 197A was for the backlight, but its backlight sucks. There's a Keithley 2700 up for about $300 (no bids yet), would that be a worthy bench meter with an illuminated display?

author: tooki
author_rank: Super Contributor
timestamp: February 19, 2017, 01:05:37 pm
content: And on higher ranges (e.g. 2V and 20V, the highest full range I can produce), the 197A actually is closer to the Fluke 87V...

author: tooki
author_rank: Super Contributor
timestamp: February 19, 2017, 01:10:28 pm
content: And finally, the 197A doesn't zero on DCV with the inputs shorted. Cold it was measuring about 16uV, now that it's warmed up, around 7uV.

author: rastro
author_rank: Frequent Contributor
timestamp: February 19, 2017, 02:13:27 pm
content: It's time to reconsider using contact cleaner on all the switches.Hank's story as well as the apparent intermittent nature of the 30uV noise issue again points to poor contacts.I have fixed at least 6-8 of these series of meters just using contact cleaner on the switches - including the range switches. The only significant electronic issues I've encountered were on two 197's which had the same defective JFET controlling the high setting for AC volts thus creating problems for all other ranges and functions.You can argue that the signal does not go through the range switch's but that doesn't mean these can't cause trouble by leaving the system in an partially invalid functional state (and yes even when it's auto ranging).I don't understand why theresistancetowards using contact cleaner on the switches. It's seems like easy low-hanging fruit.

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 19, 2017, 02:15:39 pm
content: Quote from: rastro on February 19, 2017, 02:13:27 pmPlease be careful and slow and think about this. You meter may be calibrated with the contact resistance degradation in there!

author: rastro
author_rank: Frequent Contributor
timestamp: February 19, 2017, 02:22:24 pm
content: Quote from: technogeeky on February 19, 2017, 02:15:39 pmYou're joking - right??? ...lets not fix the instrument because it might NOT be calibrated any more...

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 19, 2017, 02:37:27 pm
content: Quote from: rastro on February 19, 2017, 02:22:24 pmI still haven't read every post in this thread carefully, but no, I'm not joking. If the instrument is truly broken, then by all means fix it. But if you are just worried about some very intermittent problem, know that by fixing the contacts you may go farther out of calibration.

author: rastro
author_rank: Frequent Contributor
timestamp: February 19, 2017, 03:06:38 pm
content: More to the point - if the instrument calibration relies on a baseline of poor connections then it's clearly a house built on shifting sand.Most of the systems on the secondary market have seen many years since their last calibration. That's not to say that the systems aren't stable or reading correct - most will probably still pass baseline performance. However it's more likely that the meter was last checked/calibrated with the contacts working properly and ifanythingcleaning would help bring the system back to a valid baseline state.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2017, 03:58:25 pm
content: Poor switch contacts should not have a reproducible influence of the readings. So even if the calibration was done with dirty contacts, cleaning the contacts would not make things worse. The main danger i using a contact cleaner is, that it could contaminate the board and this way increase surface leakage. So one might have to really clean the critical part of the board in case the input bias gets to high - in a not so clean environment, this might be the case anyway.One point to test is the input bias / input resistance on the 0.2 and 2 V ranges. This is done with measuring a high value resistor or a low leakage capacitor.One factor could already be running the instrument for some time. This helps to reduce surface humidity. So a meter like this that was in storage for a long time might need several hours, maybe days to get back to a more normal state.The specs call for a <=2 µV offset. So it could be something is not that good. This could be leakage on the board or of some of the FETs. It is still a question on what is good enough.

author: tggzzz
author_rank: Super Contributor
timestamp: February 19, 2017, 04:18:06 pm
content: Quote from: Kleinstein on February 19, 2017, 03:58:25 pmMy Solartron 7081 states has a procedure after being kept in low temperatures (e.g. 5C). It tells you how to pack it so that the temperature reaches "around 35C" when turned on, and you leave it there for 24 hours.

author: rastro
author_rank: Frequent Contributor
timestamp: February 19, 2017, 05:24:18 pm
content: Quote from: Kleinstein on February 19, 2017, 03:58:25 pmThis is a great point . Be careful when using contact cleaner as not to let it drip and run across the PCB. When I clean the switches on the 19X series I hold the switches/front panel facing the floor so that any dripping moves away from the mother board. Allow it to drain for some time in this position. Try to use only enough to cover the contact surfaces. You may want to swab the surrounding area with IPA.This kind of potential surface contamination could certainly impact high range ohm readings. However, if care is taken, cleaning the switches can clear up a lot erratic behavior on this series of meters.I would avoid disassembling the switch at this point and just try the spray cleaning first.

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 20, 2017, 04:09:45 am
content: Quote from: rastro on February 18, 2017, 11:32:58 pmNo, but I actually need to do one switch again: one of two of my meters came with a busted power on/off switch, solved (by them) by soldering pairs of pins together for that switch.I replaced the switch with one from a dirt cheap 179, but there is some subtle difference causing the switch to block the locking of [ volts , amps, ohms ]. So I'll document the process.As for the calibration thing, yeah. As someone here has said, there are two kinds of failures I'm aware of regarding the switches:1) Them being broken (like when the toggle metal pin digs out of whatever that thing that's in the top of the button shafts, like the power button; the mode select buttons). The springs being broken and jammed. Whatever. The important thing is, these can do anything from totally ruin the usability of the device all the way to having no effect at all. But I class these kind of switch breaking that will not effect calibration (e.g. they do not show up as a difference in resistance or if they do, it turns out not to matter).2) [Note: I am an amateur, but...] In nearly every case where a corrosion-induced change in resistance of a switch contactcanmake a difference, it seems like it actuallydoes make a difference. To see this, you can just consider all combinations of switches. But first, you simplify:You can ignore all of the range switches in the Volts and Ohms ranges; here they are just selectors fed to the mcu.The power switch either works, or it doesn't. In any case, resistances involved can get absorbed into the corrective abilities of the power supplies. So we can ignore these..However, in nearly every remaining case, the switch contact itself somewherein the signal path:In amps mode, the range switches will be combined into the shunt resistance itself, in series-parallel.The Amps switch contacts themselves are also in the signal path, for the same reason.Every two-position triplet of the AC/DC switch isinthe signal path (and even the guarded path)The Volts, Ohms, and Amps switches all have places where one of their contacts is involved in a crucial circuit: connecting the 2V ref to the circuit, connecting signal ground to the circuit, connecting in with the -6.4v ref usage on the 2nd page of the schematic (ohms mode), even the ohms switch triplet [7 8 9] which seems like it can't matter (just a 5v rail). But even that switch contact's resistance is thrown in among 1500 ohms to ground. And then it gets sent to every FET driver opamp on the negative rail.So unless all of this is totally a wrong way to look at this circuit, and for some reason I don't understand none of these things can matter... then as long as I had a meter which gave calibrated results when it was obviously working, but had intermittent errors due to switch problems of type one... I'll take the problems of type 1, and keep the (probably) calibrated meter. I can't afford calibration, and for the reasons that everyone has (very correctly) harped on, it is very risky working around the many high-impedance sections.So that is essentially the argument for living with some types of switch problems. It's not perfect, and if I could afford actual calibration, it would be invalid reasoning I suppose.There are also ton of selected parts in this meter, and while I have guessed at the selection reasons, I don't know any of them for sure. But it seems to me like there is some real FET greybeard work going on. And even some pretty clever selected BJT uses too.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 20, 2017, 11:22:27 am
content: Quote from: tooki on February 19, 2017, 01:10:28 pmHmm, I would start with the U102 IC and if it is not a problem, then with JFETs around it.CheersAlex

author: tooki
author_rank: Super Contributor
timestamp: February 20, 2017, 01:32:55 pm
content: Quote from: Alex Nikitin on February 20, 2017, 11:22:27 amWhat exactly would I be looking for?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 20, 2017, 02:32:11 pm
content: U102 is the input buffer/amplifier, I would just replace it and see if that helps. However, re-reading your description it almost feels like a bad earth, a dry joint or a bad switch contact somewhere. Do these voltage variations react on a mechanical stress (tapping/pushing the board)?CheersAlex

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2017, 04:42:33 pm
content: The meter seems to use a kind of automatic zero scheme. So measuring the input and zero and than display the difference. So a bad input amplifier is unlikely to cause an offset. A poor (e.g. damaged from ESD) U102 would be more something causing extra noise.I see 3 possible sources for an offset:1) thermal EMF due to temperature gradients at the FET switches or 330 K resistors used for protection - this is given by design, not much one can do about it.2) RF signals from outside get into the circuit and get demodulated different, depending on the FETs switching. One could test if additional shielding on the outside has an influence and maybe repeat the test in a very quiet environment.3) Leakage currents (e.g. from the JFETs, transistors used a zener or just dirt) cause a variable offset.For the last part, one could at least measure the input current in the 200 mV or 2 V range. Ideally it should be rather low (e.g. < 100 pA). If the input bias is high this would point to a leakage problem.

author: hendorog
author_rank: Super Contributor
timestamp: May 13, 2017, 04:10:18 am
content: Just got a 'parts' 197A and it was very intermittent on ohms and pretty sketchy on DCV too.Traced the ohms issue to the 7,8,9 connections on the ohms switch.Surprisingly working the switches (many) times made no difference. So I've sprayed IPA into the holes at back of the switches with the board tilted forward.That has fixed the reliability and resistance of the 8,9 connection, now waiting for the IPA to evaporate to see if the thing works properly now...

author: hendorog
author_rank: Super Contributor
timestamp: May 20, 2017, 02:59:54 am
content: Quote from: hendorog on May 13, 2017, 04:10:18 amUpdate on this. It worked and now the 197A seems to be fine. It matches my other meters very well on DCV at least.The backlight isn't working - of course I forgot to spray that switch so it could be the same issue.