Keithley 195 Multimeter repair -  - Page 1
Forum section: electronics / repair

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 16, 2018, 02:00:12 pm
content: I bought a broken Keithley 195 on ebay last week hoping to keep myself busy over the weekend. So far, it's been a succes (the being busy part, the repair part not so much). I won't be posting much pictures of the actual unit, since as far as I can tell, there is no visual damage.ManualJust to be clear: I've already spent most of my weekend working on this!What does the meter do?Digital/interface wise, the meter seems to operate fine. It does not complain about anything, really, and all self-tests pass. It can still store settings I change, so that also seems to work.When I apply no voltage to the input, the following values are displayed:RangeValue1 kV87.7 V200 V1.50 V20 V-OFLO2 V- 1.19 V200 mV-OFLO20 mV-OFLO20 MOhm18.47 MOhm2 MOhmOFLO200 kOhm91.654 kOhm20 kOhmOFLO2 kOhm0.916 kOhm200 OhmOFLO20 OhmOFLOWhen I zero the meters on the ranges where they display something that isn't OFLO, I get half-usefull measurements. (like, to a few percent - also very poor linearity).TroubleshootingI checked all the voltage rails - they are fine. Reference voltage is also fine.From the fact that some ranges show some sign of life suggests to me that this problem must happen before the ADC.The digital section also seems to work. All the control signals go through one single pair of optocouplers, so those must be fine too.So that makes me consider two main areas: The input multiplexer and the VGA buffer. When considering the switching of the input mux and buffer stages in the different ranges we find the following (Every range also tends to use Q103 and Q114 in each measurement stage)RangeOFLO?Input MUX switchesGain switches1 kVNoQ111Q117200 VNoQ113Q11720 VYesQ113Q1182 VNoQ109Q117200 mVYesQ109Q11820 mVYesQ109Q11920 MOhmNoQ111Q117 and Q1182 MOhmYesQ111Q118200 kOhmNoQ111Q117 and Q11820 kOhmYesQ111Q1182 kOhmNoQ111Q117 and Q118200 OhmYesQ111Q11820 OhmYesQ111Q119So this shows that only when Q117 is on something usefull comes out (Or is it perhaps that when Q118 or Q119 are on, something goes wrong?).In order to make my life easier, I tried to find the locations of most components on the board (as there is no silkscreen):

author: coromonadalix
author_rank: Super Contributor
timestamp: December 16, 2018, 02:22:28 pm
content: You may have some resistors network bad, or some of the input mosfets switching busted ??Check all the voltages +15 -15 +5 -5 vdc and the unreg voltage tooFigure 8-7 is the input schematic, check everything related to the manually choosen range up to the input buffer ic U131 and the -2vdc referenceTable 7-17, 7-18, 7-19 explains lots of checks

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 16, 2018, 02:32:25 pm
content: Quote from: coromonadalix on December 16, 2018, 02:22:28 pmThanks for the reply, but I was still adding my progressI have found all the scehmatics and the manual, and checked all the rails. The steps for troubleshooting were of little use to me, as I don't always understand what is going on. I will re-do these checks later now that I better understand the meters inner workings.I'm gonna get my matlab started so I can plot som graphs of the input voltages VS time to show some interesting things going on.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 16, 2018, 02:55:39 pm
content: the section table 7-12 explain all the paths taken on any range ... this is your holy grail loll

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 16, 2018, 03:00:33 pm
content: Quote from: coromonadalix on December 16, 2018, 02:55:39 pmYes, I am aware... How do you think I got to the table where I look at the relation between the ranges where there is OFLO and the ranges where certain switches are active?

author: coromonadalix
author_rank: Super Contributor
timestamp: December 16, 2018, 03:13:47 pm
content: No problemo

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 16, 2018, 03:32:04 pm
content: Quote from: coromonadalix on December 16, 2018, 03:13:47 pmSorry, my reply was a bit harsh, I was frustrated (about something unrelated) and it got to me. Thanks for your input and for trying to help!

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2018, 04:36:35 pm
content: A not working JFET is very real possibility. Quite often a broken JFET causes problems when off, by having extra gate leakage or not really turning off all the way. The on case is sometimes still working.Just to avoid a common mistake: in the high input impedance modes (e.g. 0.2, 2 and maybe 20 V) the voltage with an open input is not well defined. So the point to test would be with shorted inputs.With the odd R120 as a bridge, a good test might be to measure the voltage at R120 with a scope. In AZ mode for DC volts there should be a sequence with 0 V, a 7 V reference and the selected input voltage. The instructions should tell the sequence.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 16, 2018, 04:45:28 pm
content: Yes, I should have added the fact that those voltage and current measurements were all with input shorted.I am working on getting some pictures of the waveforms with my scope but wanted to try using MATLAB to do so and then that didn't work so I had to troubleshoot that and lost track of time.... Will post some screenshots of the scope in an hour or so. I found some interesting results (buffer output shoots to -10V in the broken modes, when q118 switches).So a damaged and leaky q118 might explain some stuff, though I'm still confused (but will explain when I get home)

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 16, 2018, 06:29:14 pm
content: So here is an update, with plots!First off: I removed R120 and input some DC voltages externally, and measured the output voltage. (I used the test programs built in to the meter to stop it from switching gain every x ms). First glance some wierdness was going on - the x100 range started at -0.5 V when DC input. I realized I'm not sure of my measurements now, so I will repeat these later today or tomorrow.When I place R120 back, and measure the output at TP5 (the output of the input buffer), I get the results below. I think what is most interesting about this is the fact that there is a clear relationship between the ranges that output nonsense and the ranges that output some data. What I think is even more interesting, is that the ranges that do spit out data, still have a reference voltage of just -1.5 V and not, as one would expect, -2 V.Note that I made an error on the time-axis of these graphs, and they should go from 0-100 ms instead. So multiply everything on the x-axis by 230 V DC in, 1 kV range30 V DC in, 200 V range:15 V DC in, 20 V range:1 V DC in, 2 V range:0.1 V DC in, 200 mV range:0.01 V DC in, 20 mV range:

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2018, 08:23:23 pm
content: There seem to be several things wrong. One is that the zero level seems to be more near -1 V. So there might be something really bad with the amplifier !The -10 V level seen in some cases could be because of Q118 not working, but it could also be a problem with the driver. So it may be worth to check the gate driver level first. If the amplifier has so much offset (e.g. - 1 V) this alone could cause trouble for the modes with gain.The offset of the amplifier could be due to U138 (excessive bias current) or U131.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 17, 2018, 05:31:57 am
content: I'm going to have a look at the amplifier tomorrow. I imagine what I could do is simply remove R120 again, and then really characterize the amplifier for the various gain settings. During my break at work today I'm also going to write down what I expect in terms of levels for a given DC input around the feedback resistors - hopefully by comparing this to what I measure I can then try and figure out what is going on.The -10 V on the output of the amplifier also corresponds perfectly to the signal applied to Q118 I think, but more proper measurements should be done to make sure this is always the case.Also, is there a way to scale my images on here?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2018, 08:33:43 am
content: With the amplifier output at below -10 V the JFETs may no longer turn off reliably. This could cause some confusion.It might help to also look at the voltage at R120 - the wrong voltage could also happen if the -2 V and GND channel are on together. One could also force the R120 node to ground, as the sources are high impedance.The x axis scale is still odd: I would expect the times for each voltage step to be at least 1 power line cycle. So the total time should be a little longer.For the images, one could scale the graphics before hand. For such graphs the .PNG format would be better than JPEG.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 17, 2018, 08:38:35 am
content: Quote from: Kleinstein on December 17, 2018, 08:33:43 amYes, I think I will have to scale them more in the future. I'm used to keepng everything in SVG/PDF formats for all that vector-graphics goodness, hence I had no idea of scale.That aside: the more I think about it, the more I suspect it is not just a single JFET not working. I will try and do input/output sweeps of the amplifier on all the range settings later this week. Hopefully I can figure out what is wrong before Thursday so I can get components before the weekend - otherwise it will have to wait till 2019.

author: MadTux
author_rank: Frequent Contributor
timestamp: December 17, 2018, 02:04:19 pm
content: Have you tried to "recalibrate" (like apply 10V from PSU, if you have nothing better) a range and see if that makes the readings more plausible?I had a similar problem with weird readings on a K196 once and all that was wrong with it, was that the NVRAM with calibration data was erased/corrupted and thereby the data on the display was calculated with rubbish in NVRAM. After a basic recalibration, all readings were much more close to the real input.

author: wn1fju
author_rank: Frequent Contributor
timestamp: December 17, 2018, 04:16:36 pm
content: I concur with the previous post. I had a K195 with similar symptoms - the cal memory was full of nonsense, hence the readings on thedisplay were being scaled and offset with nonsense too.Also, check to make sure if there is something like a relay to switch in the ohms function, that it is not faulty. My meter's relay was closedpermanently and the previous owner, not realizing anything was wrong, calibrated the unit. That's how the cal memory got corrupted.A simple check is to measure the input resistance of the meter in the DCV ranges. If something in the ohms line is affecting it, the resistancewill be lower than it should be.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2018, 05:39:56 pm
content: The measured curve do no look like a calibration problem, as the output voltage of the amplifier is already way off in essentially all cases.It is more like a problem switching the right input (e.g. one or more if the JFETs or maybe controlling chips - likely LM339). A bad amplifier (e.g. U131 or U138 ) could be another possible cause. Gate leakage from Q117-Q119 might also cause a similar problem.The most likely cause is trouble switching before R120, as the voltage change is about 1/4 of what it should be. So a little like there are 3 bad channels always on, with a negative offset, e.g. from the ohms source. As in in the cases without gain there still is a reasonable sequence, it is likely that the JFETs for Zero, the -2 V reference and the input for 2 V and 200 V and 1 kV (Q114,Q103,Q109,Q113,Q111) are still working. So not that many left.

author: rastro
author_rank: Frequent Contributor
timestamp: December 18, 2018, 01:35:46 pm
content: I suspect that Q117 is bad. If it is shorted or partially shorted it will stay "on" when it should turn "off" on other settings and causing them to fail (overload). It may also impact the settings that it should be "on" if there is leakage through the gate also causing non-overload but erroneous readings.rastro

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: December 18, 2018, 06:45:41 pm
content: On a different note, did you get it with the AC measurement board or it's just DC and Ohms? Option 1950 I believe.I've had one of those not taking measurements at all and it was problem with the j-fet transistors. Changing them solved the problem.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 19, 2018, 09:48:09 am
content: Quote from: wn1fju on December 17, 2018, 04:16:36 pmI do agree with Kleinstein that the calibration seems unlikely to be the culprit. Later today or tomorrow, I'm going to run a program that will sweep all the ranges of the input buffer and plot gain. This will hopefully give a better indication if the amplifier is to blame or not. If these results come back good, I think it is safe to assume Q117, Q118 and Q119 are good - after all if they were not, their errors should show up in gain issues.I'm also thinking of just ordering a bunch of LM399's ahead of time, just so I can replace them right away if they are indeed the broken parts here (my local farnell dealer closes down for the holiday period so..)I will have to also locate a handfull of replacement JFETs, but am unsure which one I should get. The one used is a keithley part. Ideally, just one JFET will be broken and I can salvage the one of the option input?Quote from: Ordinaryman1971 on December 18, 2018, 06:45:41 pmNope, just got the basic one. Building my own option 1950 might be a fun project though, so I will see what I do once the meter is fixed.

author: rastro
author_rank: Frequent Contributor
timestamp: December 19, 2018, 02:07:13 pm
content: You could also just un-solder Q117 and see if the range/functions that don't use this transistor (20 V, 200 mV, 20 mV, 2 MOhm, 20 kOhm, 200 Ohm, and 20 Ohm) go back to non-overflow condition.rastro

author: Kleinstein
author_rank: Super Contributor
timestamp: December 19, 2018, 02:26:49 pm
content: The usual switching JFET would be 2N4393/4392 or J112. J202 could also work. For high ohms (and low leakage) parts 2N4118 would be a good choice. However it gets increasingly difficult to get JFETs in TO92 or TO18 case.

author: rastro
author_rank: Frequent Contributor
timestamp: December 19, 2018, 03:27:15 pm
content: Quote from: Kleinstein on December 19, 2018, 02:26:49 pmIt looks like Q117 has only a keithley designation. Unless there is a marking on one of the 'TG-128' JFETs you'll have to make a best guess. Obviously an N-Channel JFET but you also want to check location of Gate,Source, and Drain pins on this unknown part. On the K197's some of the switching JFET's have oddball pin placements. In those cases I used a readily available N-Channel JFET but had to cross 2 of the legs. It's been a few years and I can't remember specific part numbers for a replacement - but it was easy sourced.To determine pin outs you need to look at the PCB and schematics to find other components that connect to the gate for example. Just don't assume pinout compatibility with a replacement JFET.I recently repaired a K485 that also had a bad range JFET (switch). It actually used aJ201J210 N-Channel JFET. I used a J201 in it's place without a problem. So you may want to consider a J201 but you may need to account for different pin outs if you use it in place of Q117/TG-128. J201 should be easy sourced.https://www.mouser.com/ProductDetail/ON-Semiconductor-Fairchild/KSD5041QTA?qs=ljbEvF4DwOOPfenfyneIug%3D%3D&gclid=CjwKCAiA9efgBRAYEiwAUT-jtCP2RC4zxu9kIwT1TR2kEsuem2GYHaeJeAigEqs4EP31QAA_MXoqcBoCfN0QAvD_BwErastro

author: Kleinstein
author_rank: Super Contributor
timestamp: December 19, 2018, 04:44:42 pm
content: Those switching JFETs are usually symmetric, so drain ans source are interchangeable. This simplifies the pinout question to having the gate right.At least for the first test one could use a more standard JFET and if need look for a low leakage FET later. The low leakage type may be only more stringently tested to ensure better worst case ratings.

author: rastro
author_rank: Frequent Contributor
timestamp: December 19, 2018, 07:48:21 pm
content: If they have dots of paint on them they are probably hand selected out of a batch of JFETs. I suspect they are also looking for good frequency response for any AC signals.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 20, 2018, 10:16:57 am
content: Quote from: rastro on December 19, 2018, 02:07:13 pmUnfortunatly this did not help. However, now, it seems to sometimes give a 0 for one reading and then go back to OFLO - more testing has to be done once I have time. I will order some generic N-channel JFETs today, just so I can get something working over the Christmas break.What I am going to do first as a next step is to sweep all ranges to look at gain of the preamp. If that gives no results, I can try looking at letting the meter run (so don't force the amplifier on one range) but still inject my own signal. This way I can look at the performance under switching, in case something goes wrong there?I would like to thank everyone already for their tips! I hope I can get it fixed soon, and it will be a kind of new-years gift to myself

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 27, 2018, 11:53:59 am
content: So after a brief break with the christmas happenings, here is an update: The amplifier seems to be just fine. Below are sweeps of the gain (I messed up the math, and it should be inverted, so a gain of .1 is actually 10). Note also that for the 100x gain stage, I screwed up the graph and it is off by an order of magnitude (and I only realized it when I started uploading so I am not going back to redo it).The setup used: I removed the DIP short-circuit block that is represented by R120. I then used my arb to inject a ramp voltage, sweeping from +VMAX to -VMAX, vmax being the maximum input allowed, knowing the output range of the amplifier is +-2V. So for the 1x range, that would be a sweep from +2 V to -2 V. For the 2x range, it goes from 1 V to -1 V, etc.This signal is then fed to CH1 of my scope, where it is also terminated into 50 ohm. This is done so I don't have to rely on the output accuracy of the ARB. What I might do as next step is repeat the experiment, but place back R120, and inject the signal into the front.CH2 of the scope is then connected (using a x10 probe) to the top leg of C130, which is connected to the output of the amplifier. Using matlab I then sweep the voltage, and simply divide the two channels to get the gain. I also have vin vs vout and vin vs t and vout vs t plots, in case someone is interested.1x:2x10x100x, again, note that this is off by 1 order of magnitude, and you should divide the tick marks on the y-axis by 10.Oh, and for those wondering: the peaks near the middle is just numerical error - dividing something small by something small makes the errors blow up. I could do much better by sweeping bits with different ranges, but that requires too much effort - esp. since it is quite clear that this thing seems to work just fine.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 27, 2018, 01:37:54 pm
content: So some more measurements, and things are starting to make some sense.Connected the same test signal to input, and swept through the different settings of each input multiplexer (as shown in table 7-12 in the manual). Values here are measured at the output of the buffer (Test point 5, pin 6 of U137).RangeInput selectedZeroReference1 kV-1 V-1 V-1.5 V200 V-1 V-1 V-1.5 V20 V-10.2 V-10.2 V-1.5 V2 V-1 V- 1 V-1.5 V200 mV-10 V-10 V-1.5 VRange-11.9 V-11.9 V-1.5 VI will now start thinking what this could be caused by, but wanted to share the results asap.EDIT: Fixed table - been working in latex all morning and accidentally used \ instead of / for all my \td's , and the forum parser /really/ didn't like that

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 29, 2018, 12:17:04 pm
content: Sigh, I was a bit silly. I thought I was on to something when I found out some of the JFETs have gate voltages of -23 V, even though the lowest regulated supply was -15 V . However, it turns out that one of the LM399 drivers is simply connected to a un-regulated supply rail, that happens to be at about -23 V. More troubleshooting needed, however currently my bench is occupied by something more urgent so I'll give an update when I had time to work on this.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 01, 2019, 11:15:05 am
content: It is finished (well sorta...).Turns out it was a combination of two faults - U141 and Q115.I was starting to get desparate, and simply started taking out the JFETs one by one (keeping track of which one went where). I then measured the outputs, and found the one at which point the offsets went away. This was with Q108 - Ohm Ref Sense Hi. I replaced U141, and then started putting stuff back togehter, and found that the ranges were no longer all reading OVLD. I started putting back all the devices, only to find that while the ranges of 20 mV, 200 mV and 2 V were working again, there were still odd errors on the 20 V and 200 V range. This led me to suspect Q113, which I removed. The problem went away, I took one of the JFETS of the Ohms ranges which I knew worked, and placed that. And the problem was back.This led me to suspect Q115, and indeed, after removing it, the meter worked again. Now I am thinking if I want to bother finding replacements, or just use it as a voltmeter only, which was my only real purpose of getting this meter in the first place.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 01, 2019, 12:01:51 pm
content: Glad it worked out, maybe one day you'll find another 195 to pick some parts in it

author: Kleinstein
author_rank: Super Contributor
timestamp: January 01, 2019, 01:09:22 pm
content: Most the JFETs are switching high impedance signals or have a 900 K range resistor in series. So low leakage, high R_on Fets (e.g. 2N4117/8) should work. Only for Q121 is might need to be slightly lower R_on.There are a few other similar types - as a last resort SOT 23 types (e.g. MMBF4117) with bodged on wires.

author: rastro
author_rank: Frequent Contributor
timestamp: January 05, 2019, 01:28:49 am
content: Good work! Your volts reading look good. Sounds like you are pretty close to finishing.rastro