Help me repair a Keithley 2000 - Page 1
Forum section: electronics / testgear

author: mimmus78
author_rank: Unknown
timestamp: May 31, 2013, 02:35:09 pm
content: I scored a very good shape Keithley 2000 for 89€ (last calibrated in 2009).The seller stated that everything was working except ohm in 2 wire mode.I checked repair manual and this convinced me to bet that was K102 that has gone "marenghe".Today the meter arrived and after a fast check it seems everything was as described.With meter disconnected by mains, I opened it and checked K102 with its neighbour K101. The relay states was different confirming me that K102 was gone.How much replacing K102 will effect calibration?Another problem is front panel buttons are very stiff, do you think it worth to clean the membrane with some isopropyl alcohol, or just using it will improve?

author: plesa
author_rank: Frequent Contributor
timestamp: May 31, 2013, 02:57:46 pm
content: There is threadhttps://www.eevblog.com/forum/testgear/keithley-20002015-thd/The replacing the relay should not affect the calibration, but on repaired instruments you cannot be really sure what has been also affected.So for serios measurement ask someone to check the accuracy or buy at least few precision resistors to check it.Do not forget to check all capacitors on the power supply.

author: ddavidebor
author_rank: Super Contributor
timestamp: May 31, 2013, 03:18:05 pm
content: You must ceck the schematics to be sure, but substitution usually don't affect calibration.Maiby some milliohm in 2ohm mode, but who cares.

author: mimmus78
author_rank: Unknown
timestamp: May 31, 2013, 04:18:08 pm
content: Davide,it seams schematics does not (officially) exists for this multimeter.I read of your hakerspace ... which city in Italy?Domenico

author: ddavidebor
author_rank: Super Contributor
timestamp: May 31, 2013, 04:44:11 pm
content: padua

author: mimmus78
author_rank: Unknown
timestamp: May 31, 2013, 07:01:29 pm
content: Bel posto ma un po' lontano da roma :-)

author: ddavidebor
author_rank: Super Contributor
timestamp: May 31, 2013, 07:20:07 pm
content: eh si.roma è proprio una bellissima città, anzi LA città, il problema è che fare la spesa richiede 3 giorni e un elicottero.just some random chatting about our country guys.

author: mimmus78
author_rank: Unknown
timestamp: June 08, 2013, 01:06:58 am
content: Ohh this puppy is driving me crazy!!!When the meter arrived in my hands it showed the following faults: 302.1 302.2 600.1 600.2 601.2 (the latter maybe because I does not short the amp probe).I decided to change the relay k102 (because? read first post) but after this the problem was not solved. So I put back original relay in place.At this point I decided to follow what was stated in manual for fault 302.1 but some shit happens. After a while the 4 wire ohm started to read higher.I run autotest again and now I have also error 100.2 (and many others related to this).After this "shit" the 4W ohm is reading higher:100 and 1k ohm: +5%1M ohm: +30%other range: no consistent readsStrange is that DC is still OK.So I decided to go ahead and try to solve the 2wire ohm problem when at a certain point it started to measure similar values to the 4W ohm.I just had time to run the 300 series tests with no fault and probe some voltage before the miracle to vanish.During this time I notice that I have expected voltage (~ 0.490V for ~500 ohm resitor) to the input/output of U113. This is almost the same value I have with 4 wire and same resistor now.Don't know ... any idea?It's very hard without schematics to figure what's happening.Nobody has any sketch of analogic section of this meter?

author: mimmus78
author_rank: Unknown
timestamp: June 10, 2013, 09:13:53 am
content: I finally found what was originally causing 30X faults (a cold solder joint on resistor R108). I was thinking this resistor series was used also by DC pat so I initially did not checked it.I should be happy of this if it was not that I screwed everything else!!!So now I still have this 100.2 fault, high and noiser readings on ohm range.DC is still spot on (with others meters I have) so I do not lose the hope to repair it. I notice that if I set "fast" or "med" mode, with a 100 ohm resistor, switch to ACI and than back to 2 o 4 wires ohm it takes more than a minute to recover. During this time it starts reading something like 1M or 200k and counting down until 109 ohm (according to the other meter, resistor is actually 100.6 ohm).I now this is more harder problem than the original. I would appreciate if someone have any idea?

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: June 10, 2013, 03:49:53 pm
content: I had a problem with a 34401a where DCV was working, but only on the lower ranges. This turned out to be an open resistor in the input pathway. The lower ranges still worked because what's another GOhm when you've got GOhm input impedance anyway!Have you tried higher DCV ranges on your 2000? Running through the whole "performance test" section of the service manual may show some other failures, and they might be easier to troubleshoot.

author: mimmus78
author_rank: Unknown
timestamp: June 11, 2013, 10:50:07 am
content: Thanks Galaxyrise,I'm also thinking to another open resistor (or better FET) because of the slower recover on ohm.DCV is noisier but linear on all the ranges, so if some open path is the cause of both problems (noise and high/non linear measurement on ohm) has to be something on the ohm path ...I used a "spring loaded" probe to check reference and reference opamp buffer. I measured a strange noise after opamp, something like 10mV (it has a 100 Hz component, but a lot noisier).The reference too have some noise but so lower that I don't know if trust readings of 600uV with my Rigol 1052.Don't know if OP is the cause or if the noise is injected by some other component on his input/output.Any idea how to check from origin of this noise?PS: if I set meter to 100M ohm (withour probes) it reads -15M

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: June 11, 2013, 06:02:07 pm
content: Tests 200.1 and 200.2 are for the voltage reference. If you set it the manually run 200 and step, then at 200.1 you should measure 1V between the shield and TP105. (I measure .983 on mine right now, and the manual says +-0.1) At step 200.2, you should measure 10V (I get 9.73 and the manual says +-1V is acceptable.) Both look "clean" on the oscilloscope.If I run test 600.1 with an ammeter between HI and AMPS, the ammeter reads 0.9993ma (ideal=1ma), and I get 12mV at TP105 (ideal=10mV.) Again, both look "clean" on the oscilloscope. (I mean, I can zoom way in and get MHz noise, but they're basically flat at 10mV/div.)Incidentally, since you have your case open too, I have a question: Does your power supply make noise? Mine makes some white noise, as if I'm hearing the switching frequency.

author: mimmus78
author_rank: Unknown
timestamp: June 11, 2013, 11:45:47 pm
content: So on 200.1 and 200.2 it's OK (0.990V and 9,990V) and checking with scope both are clean (1mV and 4mV rms noise/ripple).I run this tests many times and it's strange that sometimes both test did not failed.Test 600.1 fails every time even if I see 1.063mA on the fluke 287 (maybe too much?) and a clean 11mV on TP105.600.2 seems not to fail.Relay works because I measured 10 and 0.1 ohm during tests. Anyway the meter measure current "correctly" on all its ranges.Where did you get this manual? Mine does not explain what, and how to use this test points (or I'm missing it?).Thanks,DomenicoPS: yes also may power supply make a very high frequency noice ... it's not normal?

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: June 12, 2013, 12:48:10 am
content: Quote from: mimmus78 on June 11, 2013, 11:45:47 pmThe manual I'm usingdoesn't mention the test points, it just says "at AD_IN". I found that TP105 corresponds to AD_IN by experimentation.QuoteThe manual says 9.5mV +- 1, but mine passes even though I read 12mV. So your still failing self tests? Which ones?QuoteI didn't know if it was normal or not, that's why I askedSince both our units do it, I'm guessing it's normal. (phew!)

author: mimmus78
author_rank: Unknown
timestamp: June 12, 2013, 08:47:29 am
content: Quote- 100.1 (50% fails - 50% is ok)- 100.2 fails 100%- 201.1 and 201.2 (80% fails - 20% is ok)- 401.2 402.2 403.2 fails- 500.1 500.2 fails 100%- 600.1 601.2 fails 100%I'm still thinking its only problem is noise ... so the 100.2 failure.QuoteYes I think almost every transformer make some noise.

author: mimmus78
author_rank: Unknown
timestamp: June 13, 2013, 09:41:48 pm
content: I add some oscilloscope waveform if someone wants to help or in any case for future reference.All capture made with spring on the probe to avoid common mode noise and with following settings:Analog Ch State Scale Position Coupling BW Limit InvertCH1 On 2.00mV/ 0.00uV AC On OffAnalog Ch Impedance ProbeCH1 1M Ohm 1XTime Time Ref Main Scale DelayMain Center 2.000ms/ -327.5600nsTrigger Source Slope Mode Coupling Level HoldoffEdge CH1 Rising Auto DC 12.0mV 500nsAcquisition Sampling Memory Depth Sample RateNormal Realtime Normal 100.0kSaSo this if first buffer (U139 LT1124) after the reference (LM399) and I'm trying to measure noise on the input of the opamp (pins: 2 and 6):Don't know but it seems to me too much (my "cheap" non reference bench power supply can do it better).Here instead is the reference out (pins: 1 and 3 of the U139):Here still some noise, if I remember datasheet set 40uV as max. But considering 2mV is the limit of my scope don't know if this has any meaning (DCV is 6.969V).Here is the most strange capture:This is buffered reference at pins 6 and 3 of U133 during test 301.1. The buffered output from U139 go to U133 that is switched around the analog part of the meter and finally arrive to AD-in.Don't know but this looks quite strange to me ... there is also common mode 100Hz noise.I checked how signal arrive to to the AD-in and it's something similar to this: filtered but with the same (rms) of the original 100Hz common noise.

author: Unknown
author_rank: Unknown
timestamp: June 14, 2013, 03:29:33 am
content: I am sure you have already tried this, but what the hell, did you try freeze spraying those areas of the board while trying the self test? worth a shot anyway, as for the noise it does look odd I wouldn't let the 100Hz distract you too much, even though its your first error i would seriously look for simple things... if you found one cold solder joint, possibly more opposite side of board may be?

author: mimmus78
author_rank: Unknown
timestamp: June 14, 2013, 12:02:31 pm
content: Yes I "freeze" both side ... and it's this way I found first fault on ohm range.Just to make sense of word noise: it means DCV reads jump "randomly" from +0.1XXX mV to -0.1XXX mV (100mV scale) when in fast mode, and +0.0020 to +0.0040 in slow mode (with input shorted).I should be something trivial, I'm sure, because during some tests noise went away for 2 minutes, and even fast mode was very stable (up to last digit). I've tried to replicate it without success.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: June 16, 2013, 12:39:40 am
content: 100.1 fails! I can see why you're looking for noise, then. The repair manual says that 100.1 sets the ADC to measure 0V.QuoteThat does seem like a problem. When running 100.1, I don't see any low-frequency noise at TP105. Just 4mV of high frequency stuff.Let me know if you want me to do any other comparisons for you, I still haven't calibrated mine and I'm happy to pop the cover and jam a probe in there from time to time.

author: mimmus78
author_rank: Unknown
timestamp: June 16, 2013, 07:23:34 pm
content: @galaxyrise100.1 fails only 50% times. 100.2 fails always.Anyway this is waveform during 100.1 and 100.2 tests at TP102 (shield) and TP105:and it seems less noiser than yours (less than 1mV).The common mode noise at ADC happens on test 301.1 ... if you can check during this test.Thanks,Domenico

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: June 19, 2013, 04:50:57 am
content: On 301.1, I see mains frequency (not 100Hz) at around 3mVpp, along with a LOT of noise. Given the high impedance pathway the 7V takes in that test, noise is not really surprising.If 100.1 and 100.2 are failing, then every test after them is near useless, so you're probably back to figuring out what's upsetting 100.1 and 100.2.

author: mimmus78
author_rank: Unknown
timestamp: June 20, 2013, 10:18:12 pm
content: So some updates guys!Yesterday I brought another K2K for parts/not working and it seems I managed to repair it.I found the usual cold joint on C150 or R182 that are on the path from ADC to pin 1 of U130. This pin is the strobe signal of the shift register connected to the A/D MUX. The missing signal was causing the A/D MUX (before ADC) to never switch the various signals (reference, input, etc) needed during ADC measurements.So now I have a working 6.5 digit multimeter (finger crossed) ... and something to compare to for not working one.Let's back to the not working one ... 100 Hz is double of 50Hz that is our mains frequency in Italy. Don't know why I have double frequency ... there should be some explanation.I compared output of the reference with the working K2K and noise seems to be pretty the same, except that on the working one I don't have the glitches you can see in the waveform of the not working one. I don't think this glitches can cause all the noise I have so I will ignore it for now.Something interesting happens when measuring a 120 ohm resistor with meter set to 1Kohm scale. In this configuration I expect a very clean DC signal (coming form some buffer of the reference) on the resistor. And this actually happens on the working K2K, I have almost flat trace with less than 1 mV high frequency noise (almost like 100.1 test). On the broken one I have instead a huge 15mV common noise. I found also that the common noise gets smaller as I increase ohm range, and disappear completely when meter switch to 10M or 100M ohm.I will think tomorrow if this make any sense to me ...

author: PedroDaGr8
author_rank: Super Contributor
timestamp: June 20, 2013, 10:25:34 pm
content: Quote from: mimmus78 on June 20, 2013, 10:18:12 pmFull wave rectification results in a 100Hz signal (because both peaks end up on the same side).

author: mimmus78
author_rank: Unknown
timestamp: June 21, 2013, 07:35:33 pm
content: Tada!!!I found cause of the 100Hz common noise (and maybe noise in general)! VR109 is gone.Without this zener (voltage regulator) U134 is directly connected to the capacitor after the bridge rectifier ... here where this noise is coming from.So now problem is to find this part without spending 20€ in shipping consts ... or maybe find similar part.

author: ddavidebor
author_rank: Super Contributor
timestamp: June 22, 2013, 04:38:33 pm
content: What part it is?

author: carpelux
author_rank: Regular Contributor
timestamp: June 22, 2013, 04:51:44 pm
content: According to theService manualit's a MMBZ5247BL 17V sot23 Zener diode.

author: ddavidebor
author_rank: Super Contributor
timestamp: June 22, 2013, 07:35:22 pm
content: put a damn 17v zener of the same wattage and live happy

author: mimmus78
author_rank: Unknown
timestamp: June 22, 2013, 11:22:08 pm
content: I found same part series but 18V at it.rs-online.com (only 6€ shipping), it should be ok just for checking if it solve this noise problem.I checked opamp and 18V is max rating, so it should be OK also as long term fix, but I need to check specs better.

author: mimmus78
author_rank: Unknown
timestamp: June 27, 2013, 08:40:53 am
content: Just some updates.After changing the "zener voltage regulator" the voltage is now under control but noise was still too much.I decided to check if opamp was burned by this 24V, so I removed it and:I cannot believe this puppy after all my abuses is still spot-on :-)I'm going to spend a lot of money to repair this meter, the opamp is 7€ plus shipping, and I already spent 30€ for changing relay and the zener ...

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: June 27, 2013, 05:55:38 pm
content: Very nice

author: mimmus78
author_rank: Unknown
timestamp: July 05, 2013, 10:30:37 pm
content: Some updates.I removed all components relative to current source U123 and now I have only errors in tests where U123 is engaged (starting from 300).I found also that by connecting the shield of both meter (TP102) I can use the voltage source of the working meter to measure Ohm with the other ... both meter read almost the same value on all the ohm ranges (and I like it).I ordered a bunch of components including this U123 ... we will see what happens when all the stuff arrives.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: September 24, 2013, 11:24:02 pm
content: Since this isn't the only bad VR109 thread, I figured I should check the ones in mine. Of the two K2k on my bench right now, one had a good VR109, the other seemed bad. So I replaced it with whatseemed to be the same thing(and then promptly lost the original, doh!) But now the meter is "noisy" (fails 100.2), and indeed, compared to the working k2k, the voltage has some AC coming through it. (In both cases, there's a strong sine wave component coming into it.) Since it wasn't failing that test before, it seems likely that I used the wrong part, but, what was the correct part, then?(The repair manual calls it a "DIODE, ZENER 17V, MMBZ5247BL (SOT-23)")

author: mimmus78
author_rank: Unknown
timestamp: October 02, 2013, 09:49:57 pm
content: Sorry but I really don't know ... when the meter was sitting down to the bench it build up another problem (ADC always overload) so I decided to not continue anymore with it.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: October 02, 2013, 10:14:16 pm
content: Thanks for the reply!I've actually made some progress on this; I'm waiting on some parts to arrive before I'll know more. But what I know is that the zener I put in was running way too hot. The resistor in series with it (R168 iirc) is only 270 ohms, and that put my zener at its max wattage. Maybe you should check yours? Curiously, my other k2k has a 1k for R168, even though the repair manual specifies 270.

author: mimmus78
author_rank: Unknown
timestamp: October 02, 2013, 11:29:35 pm
content: On the K2K that I repaired this zener is just worm.This part of the circuit around R168 is the current source for ohm measurements.Removing Q119 fixed noise on my meter but it will "disable" your ability to measure ohm (you will get also related errors).I tried to understand how this current source works but my knowledge and time where not sufficient for it.

author: flolic
author_rank: Frequent Contributor
timestamp: October 04, 2013, 06:43:00 am
content: On my K2k voltage measured across the VR109 was 18.6V, so I replaced it with regular small glass 16V zener diode. Now I am measuring little less than 17V across it. Diode is getting quite warm, and I will probably replace it with bigger 1W one.I have some 0.01% resistors for testing, and my meter is spot on on low ranges. But on higher ranges it reads little higher than it should, 100.001k resistor reads as 100.015k. It can be simple calibration issue, or maybe there is fault somewhere... I must do some more tests.Then there is also noise issue visible if I disable filtering. Last digit or two randomly flickering up or down. I can't find some pattern in that behavior, sometimes it is more and sometimes it is less noticeable...There are some more issues, I will document them after more testing. But I think they are related with calibration.I've dumped original content of calibration eeprom so I can safely play with calibration.My K2k passes self test without error.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: October 04, 2013, 06:54:15 am
content: What's the voltage from R168 to ground? Mine was around 23.2. If that's high, it could be the reason my zener died in the first place (and 270 Ohms would make a lot more sense.)

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: October 04, 2013, 06:57:51 am
content: Quote from: flolic on October 04, 2013, 06:43:00 amYeah, that's normal. But if you have the meter sample the same voltage 1000 times at the 10V range, you should get a std deviation of like 3uV and 20-30uV between the min and max reading. My noisy meter gets a std dev of 12-14 uV and max-min will be 70-100uV.

author: flolic
author_rank: Frequent Contributor
timestamp: October 04, 2013, 07:30:52 am
content: I will check and report those values later when I get home

author: macboy
author_rank: Super Contributor
timestamp: October 04, 2013, 01:01:10 pm
content: Quote from: flolic on October 04, 2013, 06:43:00 amIf you are testing the 100.001k? with the 1 M? range, then it is measuring within spec at 100.015k?. If you are using the 100k? range, then it is slightly out of spec (spec limit would be 100.011k?). This would likely be calibration drift, not a fault. If it was measuring say 130k, then you would suspect a fault. The resistance measurements are dependent on internal resistance references, and these do drift over time as they age. Aging of other components like the FETs, analog switches, and relays in the signal path also affect accuracy over time.As for noise, this is normal, depending on the settings. You can to use the 10 PLC integration time to reduce noise, and averaging helps too. You need to decide on measurement speed vs. noise. In my experience, at 1 PLC, I see 5.5 digits essentially noise-free (not with Keithley 2000 but with Keithley 2001 and Keithley 199). If there is excessive noise, that may be a fault.Making any repair can throw out the calibration. Even new fingerprints on the PCB can have an impact (no joke).

author: flolic
author_rank: Frequent Contributor
timestamp: October 04, 2013, 08:45:36 pm
content: Yes, you are right, those values fall within the specsIn fact, I must correct myself. Actual resistor value is 100.06k and I am measuring 100.174k on a 1M range and same on 100k range. That's even better.But 10.002k resistor reads 10.0133k on 100k and upper ranges, and 10.00311k on 10k range. The similar happens with 1.0001k resistor. I don't like discrepancy in reading between ranges, and that can probably be fixed with calibration.Another thing, on DCI mode i have cca. 5uA reading on all ranges, with test leads disconnected.

author: flolic
author_rank: Frequent Contributor
timestamp: October 04, 2013, 08:46:52 pm
content: Quote from: Galaxyrise on October 04, 2013, 06:54:15 amI read exact that value, so it looks ok

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: October 04, 2013, 09:51:00 pm
content: Quote from: flolic on October 04, 2013, 08:46:52 pmThanks!I don't understand how it was supposed to work originally, then... I don't see anything around that voltage bigger than 350mw in a SOT-23 on digikey or mouser. What kind of zener did you use, mimmus78?

author: flolic
author_rank: Frequent Contributor
timestamp: October 04, 2013, 10:23:12 pm
content: Yes, that poor little diode is pushed beyond maximum allowable disipation (377mW if I calculate correctly).I think all sot-23 zeners are limited to 350mW dissipation, that housing is simply too small to efficiently sink more heat.First next more powerful devices are 500mW diodes in sod-123 housing.

author: mimmus78
author_rank: Unknown
timestamp: October 06, 2013, 07:58:27 pm
content: I remember was not able to find an smd with same specs in Italy so I a bought a tht zener to check if replacing it would fix the problem, but it does not worked.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: October 07, 2013, 05:53:45 pm
content: I put a5ppm 1kresistor with alow Iz 16V zener, and that seemed to settle the voltage across VR109 a lot. Both parts run about 40C with the case open. But there's still ADC noise. I replaced U123 just in case it was causing some kind of supply noise, but the ADC noise remains unchanged.It's probably something I did bodging R271, but I'm conceding defeat; I found someone who would be happy with it as a 5 digit meter.

author: kubatyszko
author_rank: Unknown
timestamp: August 10, 2015, 02:52:08 am
content: //A bit of post necromancy but better to keep it relevant.//Here's a short blog post on repair of my Keithley 2000, it was initially failing with 600.1, 600.2, 601.2 errors.http://kubatyszko.com/2015/08/10/keithley-2000-bench-multimeter-repair-part-2/Cheers

author: TiN
author_rank: Super Contributor
timestamp: August 10, 2015, 04:10:52 am
content: Thanks for post.Seems lot of relay problems people discovering lately