Repair and calibration of Keithley 237 (236/238) - Page 1
Forum section: electronics / general

author: balage
author_rank: Regular Contributor
timestamp: May 06, 2019, 10:54:45 am
content: Hi Guys,I have bought a Keithely 237 high-voltage SMU. I turned out that it needs to be calibrated. I expected it to be still within specifications as a Keithley 614 electrometer I have bought few months ago was absolutely correct. Anyway, calibration of the 237 is not so an easy task. Or not for me as I've never done such thing.I can get a calibrated 2450 SMU for the role of calibrator, and I can do the whole procedure in an environment that has controlled temp and humidity. The hard part is still to make the corrent connections and to do the step-by-step calibration procedure through the GPIB interface.Has anybody here have experience with calibrating the 23x series SMUs? Can I do this sucessfully without the recommended 8006 shielded test box? I would use my homemade steelbox.The operator's manual and the service manual says everything about calibration.https://www.tek.com/manual/model-236-237-238-source-measure-units-operators-manual-rev-e-13-mbhttps://www.tek.com/manual/model-236-237-source-measure-units-service-manual-rev-d-9-mb-manualService manual has detailed procedure for power supply check, AD check, analog, switching and digital circuitry checks. I should do them prior to the calibration.Thanks for your comments.

author: VintageNut
author_rank: Frequent Contributor
timestamp: May 06, 2019, 11:44:36 am
content: If you have a 2450 that is in calibration, you should be good for all of the current sourcing and sinking.What I suggest is that you connect the 2450 directly to the 23X using triax cable(s). Then go through every range, from the front panel, of the 23X for sourcing and sinking and see how the 23X agrees with the 2450. You may only need to adjust the lowest current ranges. I have found that the lowest ranges drift the most. Probably there are large value resistors there and they drift the most.For voltage sourcing of the 23X, I used a KE 2750 to measure the 23X. You can use any calibrated DMM that is 6 1/2 digits and has better specs than the 23X. Even if the DMM does not have better specs, it may be good enough to see if the 23X is out of tolerance.For voltage measurement by the 23X, you can use the 2450 up to 200V. If you have a 237, you have to find an 1100v calibrated source.

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 06, 2019, 11:51:09 am
content: Any suggestions for cheap triax cables and/or connectors? I have few 238's I'm patching up and that's a recurring theme

author: balage
author_rank: Regular Contributor
timestamp: May 06, 2019, 12:41:46 pm
content: Quote from: VintageNut on May 06, 2019, 11:44:36 amThanks for your reply! It takes me a week to make the first steps on the cal. I will post the results here if you find it interesting.Have you connected to the 23x easily though its GPIB? The other problem regarding the calibration is that I cannot establish a stable connection to the 237. Once I have addressed the instrument to a listener, then everything works but most of the time I cannot address it. I am trying to program it with LabVIEW. Do you have any specific/practical info on programming? I am actually interested on the specific commands sent to the instrument to make it remote.

author: balage
author_rank: Regular Contributor
timestamp: May 06, 2019, 12:56:57 pm
content: Quote from: Ice-Tea on May 06, 2019, 11:51:09 amUnfortunatelly there is no cheap way.I am considering to buy the original Keithley SC-22 bare triax cable per feet. My problem is that the minimum amount is 240 feets (72 meters) and totally it costs around $240. I could ask somebody to bring it to Europe. But it is much for me so it would be nice to split this cable with somebody. I always wanted to the the original Keithley triax cable.If I could find somebody to share that cable with I would organize the buy. VintageNut, are you interested?

author: TiN
author_rank: Super Contributor
timestamp: May 07, 2019, 12:07:45 pm
content: You can get HP triax cable assembly from parametric analyzer, such asthisand re-purpose it. Triax connectors themself usually cost 40-80$ or more, so just a cable will not help you much on saving costs

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 07, 2019, 03:19:24 pm
content: That.. makes some pretty good sense! Thanks for the tip!

author: balage
author_rank: Regular Contributor
timestamp: May 09, 2019, 07:17:50 pm
content: Actually I use G_02330_HT triaxial cable and Trompeter PL75-33 triax (or TRB) connector. It is not the cheapest solution but the easiest.

author: VintageNut
author_rank: Frequent Contributor
timestamp: June 06, 2019, 05:26:29 am
content: Quote from: balage on May 06, 2019, 12:41:46 pmSorry for the delay responding. My work and other endeavors have supplanted my DIY metrology efforts.I use two methods for communicating with 236/237/238 (I own one of each). Method one is Keithley Communicator. This is part of the Keithley I/O Layer installation. I use this for sending commands one at a time. The Keithley I/O layer is a free download fromwww.tek.com.Method two is Python which uses PyVISA which uses NI VISA. I use this for running scripts and taking measurements. I have written a script that performs transistor curve tracer functionality with two of the 236/237/238.For communications hardware I use Keithley KUSB-488B. My laptop runs Windows 7.For calibration, I use Keithley Communicator and have a printed manual sitting on my desk.For cables I use genuine Keithley cables.

author: balage
author_rank: Regular Contributor
timestamp: June 07, 2019, 10:28:15 am
content: Great, thanks!So I think I will use the Keithely Communicator for establishing the connection and communication and LabVIEW for performing the calibration. However the communication is still cumbersome. Once I have established it is working correct, but I can put the instrument to remote and address it to a listener very rarely, something is wrong...Just for some fun: Also in the meantime it has turned out that the negative side of the HV supply is not working properly, Now it can source up to 1100 Volts, but with one of the output filtering capacitor removed. It seems like it cannot tolerate capacitive load somehow. The schematic shows the replaced components. When the removed capacitor was in, the converter started oscillating (I guess) when I set the voltage more than ~400Volts. When it was oscillating, the green parts became very hot soon. Finally I removed that, and the converter working fine...

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2019, 11:29:12 am
content: Removing C44 kind of negates the function of the transistor block. This cap is there to get some filtering in a kind of capacitance multiplier. It may be sufficient to add some series resistant (e.g. 50-500 ohms range) to C44 or the base of Q5. This would at least keep most of the filtering effect.

author: balage
author_rank: Regular Contributor
timestamp: June 15, 2019, 04:30:51 pm
content: Dear Kleinstein,Thanks for your help. I have put a 330 Ohm resistor to the base of Q5. The problem is the same - it is getting oscillating loudly below -400 V.Operation of that transistor block is also not clear for me. Is this block smoothing the output voltage by changing the voltage across Q6? Is the ripple voltage opening Q5 through C44, and Q5 controls Q6?I am planning to measure the ripple voltage somehow to see if I can remove C44 permanently.Cheers,

author: Kleinstein
author_rank: Super Contributor
timestamp: June 15, 2019, 08:54:19 pm
content: R22 and C44 form an RC filter. Q5 and Q6 from a emitter follower amplify the current and ideally form a filtered supply. This kind of circuit is not really new and sometimes call capacitance multiplier.It may be a little confusing, but an emitter-follower, especially as a Darlington can oscillate if used with a capacitive load and low impedance source. The usual countermeasures are resistors (sometime ferrite) at the emitter or base. My favorite position would be a resistor in series to the base pin of Q6. Maybe the 330 Ohms at C44 were not enough. Given that the current is relatively small the resistor could be relatively large (e.g. 1 K range).It could also help to have a RC series (e.g. some 100 ohms and 10 nF) element at the output (e.g. in parallel to R25).C44 and Q5,Q6 are there for a reason. So I don't think one can just remove them.

author: balage
author_rank: Regular Contributor
timestamp: June 26, 2019, 09:19:07 am
content: Thanks for your suggestions.I have tried the paralell RC (750R + 4,7nF), while the base resistors were on both transistors. No success. I am ordering from Mouser soon, and I will include transistors a zener diodes for this part of schematic.I was thinking if this oscillation can come from the capacitor-diode network of the converter. It is not likely I think. I have measured the diodes, they seem okay, and the caps were replaced by the previous owner I guess. So the problem must be somewhere around this darlington circuit.For those - like me - who have not heard about capacitance multiplier circuit:https://www.electronics-notes.com/articles/analogue_circuits/transistor/capacitance-multiplier-circuit.phpI will write here if the fully replaced filter has any results.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: June 26, 2019, 07:59:58 pm
content: I have some very nice triaxial cables with Thrompeter connectors... they are about 8 feet long... 2meters with no connectors on the other end which you would call.. bare I guess.Let me know if I can help.I also have some longer ones.. I think they are about double the length, about 16 feet or so.

author: balage
author_rank: Regular Contributor
timestamp: July 30, 2019, 02:49:19 pm
content: Hi Guys,I have replaced Q5, Q6 and VR5. I have removed the base resistors, so it is now the original circuit. Nothing has changed, the converter oscillates and gets loud below -370Volts. That part also gets hot.C44 was broken when I first realized this error. I was wondering if this can mean something to us but I cannot judge if the breaking capacitor caused the problem or the capacitor was killed by the problem.I always start such repairs with great pleasure, and it is always resulting in failure.Update: I have not mentioned earlier, but when it is oscillating, the 237 shows around 4uA while nothing is on the output. I have mentioned that when C44 is missing, everything is good. Can the output stage cause this problem? Maybe the output is loading the converter heavily. I cannot judge.Update2: There are the same big ceramic caps on the input of the output stage. I will remove that capacitor to see if it is resulting in a working converter. Maybe it is overloading the converter.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 30, 2019, 03:18:03 pm
content: Looking for the actual problem at the output stage, is a good idea. The power supply part could have a current dependent behavior, but should only indirectly via the current depend on the set voltage.So much Oscillation may be a reason for C44 to age faster - either directly or just indirect from the surrounding parts getting hotter.So a look at the output stage is a good idea. For a test also just loading the output could also give a clue, if the onset of oscillation depends on the voltage or current. If current depending more load should shift the critical voltage to a lower values.

author: balage
author_rank: Regular Contributor
timestamp: August 01, 2019, 02:00:06 pm
content: Update here.I have focused now on the output stage. As I have mentioned when the output capacitor (10nF) was removed in the converter, it was not oscillating. The output stage has the same cap on its input, so I have removed that one. See the picture below. With this cap removed the symptoms were the same. This two capacitors are almost on the same galvanic points there is only a 5,1V zener between them. As a conclusion I think the problem can be localised to the output of the high voltage converter. Do you think it is all right?Can a faulty electrolytic cap in the converter itself can cause this? I think all of them were replaced, but not for sure.Almost forgotten: the diodes, transistors and the high-voltage FETs seems okay in the output stage. For the FETs I've only measures the drain-source resistances with both polarities.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2019, 04:16:45 pm
content: C44 and C16 are only separated by a zener diode. However this zener is normally non conduction. So the function / effect of the 2 caps is different. Zener diode should be there more for protection purpose. The more important link is the R5+R6 (of the inverter) Darlington transistor circuit.The output stage is tricky. Because of the high voltage and so many transistors involved it is really difficult (and dangerous)) to measure anything on the life circuit. Chances are if one of the FETs is broken it could cause a cascade of faults damaging many more.

author: balage
author_rank: Regular Contributor
timestamp: January 05, 2020, 02:06:45 pm
content: I am still trying - with no success.I have found a high-voltage converter on ebay:https://www.ebay.com/itm/Keithley-237-HV-Netzteil-Resonanz-Konverter-1200V-Source-Power-Supply-237-172-02/113879830894?hash=item1a83c4516e:g:z9gAAOSw-1ZcxaaEIf I would be sure, the converter itself is bad, I would buy this one.

author: MiDi
author_rank: Frequent Contributor
timestamp: January 05, 2020, 05:28:56 pm
content: If it is worth to you to pay that much for the swap...If you still want to try repair I can offer you measurements to compare

author: balage
author_rank: Regular Contributor
timestamp: January 05, 2020, 06:18:59 pm
content: But I am not sure the converter is wrong.How do you mean that? You have a working one and you could check the specific measurements on the board I ask for? As a reference.

author: MiDi
author_rank: Frequent Contributor
timestamp: January 05, 2020, 06:53:48 pm
content: More or less working:Keithley 237 repairWould appreciate some measurements on transformer, so we could both benefit

author: balage
author_rank: Regular Contributor
timestamp: January 05, 2020, 08:16:43 pm
content: Okay, I'm in. But lets start with the things you need to see first!In my lab things are going slow nowadays.

author: rolkinas
author_rank: Contributor
timestamp: January 10, 2020, 11:14:50 pm
content: The easiest way for me to adjust K237 was using Agilent USB-GPIB interface and communication expert software. It is not difficult to enter commands from service manual. I managed to perform voltage calibration in about 20 min.But first of all you have to make unit fully functional before calibration. Make sure that sourcing and measuring is somewhat within specs by checking with external equipment

author: balage
author_rank: Regular Contributor
timestamp: January 12, 2020, 12:39:53 pm
content: I have tried the same way with Agilent Ebay GPIB converter. I have tried LabVIEW and the communication expert software. Only once I've succeeded to put the instrument into remote mode and was able to read the measurement data. But since that after several attempts I was not able to put the instrument to remote.Possibly can you please tell/send me the exact method/procedure/message with which you did it?I am waiting for a National Instruments GPIB USB converter, just to avoid the Agilent one.

author: rolkinas
author_rank: Contributor
timestamp: January 12, 2020, 08:35:13 pm
content: I also have bad luck with Ebay Agilent interface + Labview combo.On only one of many computer it works more or less reliably , but all settings are the same in other maschines.If you have Agilent USB-GPIB adapter it come with driver+software. It can be downloaded from keysight also.In Agilent instrument detection software is option to send individual commands to instruments.I used that way, simply inputing commands from service manual calibration chapter.C0X {enter}C1X {enter}and so on

author: balage
author_rank: Regular Contributor
timestamp: January 19, 2020, 09:54:24 pm
content: F...in' freakin' bastard 82357B... I've spent so much time with the Agilent one with no success.It took two clicks to communicate with the Keithley using the NI GPIB converter.Anyway... I have a 82357B converter for sale. Anybody?I decided to calibrate the 237 regardless the faulty resonant converter. I want to feel success after all this trouble.

author: MiDi
author_rank: Frequent Contributor
timestamp: January 19, 2020, 10:56:44 pm
content: Have not reread all answers, so forgive me if this is already known:There is a topic about similar problems:High voltage troubleshooting of Keithley 237The issue with limited voltage output with installed short between low to gnd:Keitley 236 Source Meter - Teardown and Review

author: balage
author_rank: Regular Contributor
timestamp: January 23, 2020, 07:54:34 pm
content: Finally the problem is identified and solved! Here is the procedure I've been through.So the symptom was that sourcing below -350 Volts the resonant high-voltage converter became loud and the output voltage dropped. I have changed the output filter capacitors (orange ones) and the dummy load at the output. It has not solved anything. In fact one of the ceramic caps was blown. After many hours spent on an incorrect direction, I have put a 120kohm dummy load on both rails of the converter. It has sourced the 10mAmps perfectly. Converter was locked out as a problem-source.Output stage was the next step. The voltage dropped on the Q3 & Q4 seemed good with an output voltage of -50 V. Each of them dropped abound 400 V. When the output was raised lower than -170 V the voltages dropped on Q3 & 4 were fluctuating from 250 to 350 V. Below -350 V the converter started the loud oscillation and the voltages dropped on the FETs were zero. I have changed Q3, Q4, Q10 and Q19 but with no success.After all this I have stared monitoring the -150V rail on C19. It was -170 volts actually. Going down vith the source voltage this rail was decreasing down to around -400 volts!! CR4 is a high voltage diode with the purpose to separate the high voltage rail and the -150 V rail. It seemed the diode is not acting this way. I have changed it to a big ugly one and everything is fine.AFAIK there were not any 237 related problems here like this one. I hope it will help somebody one day.A fellow member help me a lot solving this issue. Thanks z01z!If anybody needs an MTP2P50 FET or MJE350 or MJE340: it is available here at the local dealer just PM me. They are geniue.Next step is calibration. Also I would like to pimp my 237, with changing the high voltage wires to nice, flexible silicone ones, replace the 150kohm resistors near the supply to higher power rated ones, etc. Thanks for all your help!

author: balage
author_rank: Regular Contributor
timestamp: January 24, 2020, 05:08:35 pm
content: I have just made a huge step to have a calibrated 237 that is working perfectly I am now stopped again.Something is wrong with the calibration. When I send the command C0X, the instrument enters calibration mode. I know this because in case I put the 237 back to local and I push any of the buttons it is written on the display that "in cal".But from this point the instrument is not responsing the commands. It is not doing anything (at least I cannot see) for C1X command. Also when I send C4X it is not outputting +1V. It is not sending any characters back at all.Can anybody say if their instrument is acting the same way? Should the instrument source +1V by the command or should I set it manually? It seemed me it should do itself. Also, Does it sending back any acknowledgement for the cal commands?

author: rolkinas
author_rank: Contributor
timestamp: January 26, 2020, 03:06:46 pm
content: instrument should output corresponding calibration voltages itself. Is unit in "operate" mode during calibration?

author: balage
author_rank: Regular Contributor
timestamp: January 26, 2020, 04:55:28 pm
content: Yes, it is in operation...Should it send back any acknowledge message?

author: balage
author_rank: Regular Contributor
timestamp: January 29, 2020, 08:30:36 pm
content: I tried to calibrate the low current measurements.To test if it is doing anything during calibration, I have applied 0,6nA to the 237 instead of 0,9nA. When I sent the cal command it hasn't done anything, as the applied 0,6nA remained on the screen. In this case the 237 should have tought the 0,6nA is the 0,9nA.Isn't it?

author: MiDi
author_rank: Frequent Contributor
timestamp: February 03, 2020, 09:20:16 am
content: I have similar problems in calibration mode.At least when C1X does not take ~30s there is an error.The errors can be read by SPOLL command followed by evaluation of the status from UnX command.A good source how the cal procedure should be done is the example calibration program in QBasic at end of cal chapter.It is rather complex, so it will take quite a while to implement this in python.Updates will follow...

author: balage
author_rank: Regular Contributor
timestamp: February 03, 2020, 02:52:28 pm
content: And how can you see if it has executed the C1X command? I cannot see anything changed on the display.

author: MiDi
author_rank: Frequent Contributor
timestamp: February 03, 2020, 04:00:17 pm
content: The display seems rather unhelpful for cal procedure.The time it takes for C1X will delay following readings over GPIB e.g. SPOLL, read UnX.E.g. if there is no short installed, there is an error and no delay.

author: balage
author_rank: Regular Contributor
timestamp: February 03, 2020, 05:53:21 pm
content: In the meantime it seems I was able to calibrate the low current measure range. During calibration I was checking the U7 and U1 status words. U7 was always updated with the current step of calibration. Also when the last step was executed, the instrument displayed "temporary cal" as it is supposed.I have not saved the cal data tough. But after a restart it showed a bit different measured values than with the temporary calibration.So I have gained of lot of skills about the 237 in the recent few weeks.

author: MiDi
author_rank: Frequent Contributor
timestamp: March 07, 2020, 07:06:35 pm
content: Now i have three good reasons to develop (semi) automatic cal program for whole SMU family

author: balage
author_rank: Regular Contributor
timestamp: March 07, 2020, 08:21:55 pm
content: That is really nice!Are all of them working well?I was thinking how such a semi automated program should look like. I would program it in LabVIEW, and pop-up windows would show the instructions what voltages/currents should be applied to the 236/7/8.

author: MiDi
author_rank: Frequent Contributor
timestamp: March 08, 2020, 08:29:31 am
content: It seems I had much luck with the purchase of 236 & 238Just needed to replace both blown fuses and after thorough inspection, they turned out just fineQuick check of the ranges revealed no surprises, maybe it just is out of cal on a couple of ranges.Now they are burning in and hopefully I have some time for cal program.It will be python based for linux-gpib and more or less a port from the example qbasic program in the manuals.Edit: I plan to implement full performance verification, cal procedure (adjustment), reports and interface for extension of instruments e.g. different DMMs and calibrators.

author: garrettm
author_rank: Frequent Contributor
timestamp: April 17, 2020, 02:47:25 am
content: @balageWould you happen to know what the part number of CR1, CR2, CR4 and CR6 is? Keithley says they are "HV-15" but I haven't had any luck finding a datasheet for this part.I recently picked up a 237 parts unit that turns on and noticed that the output board had already been "repaired". (Oh great...) The previous owner replaced the afore mentioned diodes with DG1-827 (mfd by General Semiconductor), but it actually looks like Q19 and maybe a few more P-chan / PNP transistors are the real fault. (The output is stuck at around -90 V to -140 V, depending on range / programmed value.) I've got the new output transistors ready to be installed, but before I turn the unit on again, I'd like to make sure the diodes that were swapped out will actually work and not cause the output board to fail after I go through the trouble of repairing it.

author: MiDi
author_rank: Frequent Contributor
timestamp: April 17, 2020, 09:07:32 am
content: Seems the diodes are unmarked and special selected with red dots.Output Mosfets are marked Motorola R251 and R315.

author: garrettm
author_rank: Frequent Contributor
timestamp: April 17, 2020, 09:11:43 am
content: Well, I looked into the DG1 diodes, and they are rated for 1.5kV peak repetitive. So I figure they are actually okay. That said, you always need to check if the guy before you actually knew what they were doing when swapping out parts. I've seen some shenanigans that still to this day make me question every little detail when something has been worked on previously. But enough of my ranting.The fix turned out to almost be too easy. It really was just Q19, a lowly MJE350 PNP that shorted out collector to emitter. The 26.1 ohm resistor in series with the emitter got pretty toasty, but still measures spot on. So I’m going to leave it alone for now. The two 2.2 megohm 1/2 watt resistors in the resonant converter also look like they got pretty hot (enough to discolor the PCB and turn the blue resistor bodies green-brown). I haven’t tested outputting over 190 V yet, since I’ve been using the ole Keithley 617 electrometer as an initial sanity test before I delve into recalibrating the 237.Over all the 237 looks to be a nice upgrade over my 220 current source. Better programming resolution and accuracy when sourcing current and infinitely better voltage compliance. Admittedly, the 220 does have larger output ranges below 100mA, 19995 digits vs 10000. So it’s not really a direct comparison. But the internal picoammeter/electrometer more than makes up for that.All-in-all, a good buy at 250 USD.

author: balage
author_rank: Regular Contributor
timestamp: April 19, 2020, 07:18:45 pm
content: Hi garrettm,Sorry for my late answer. I have removed one of the diodes you are asking and it was labeled PD76. Nothing can be found on the web.I have temporary replaced it with a huge 6kV diode, but I have bought GP10Y-E3/54 from Farnell. This will be the replacement for me.That was a really nice bought for you!

author: garrettm
author_rank: Frequent Contributor
timestamp: April 20, 2020, 08:11:41 am
content: Thanks balage.It's odd Keithley used such obscure diodes, but at least Vishay (who bought out General Semi) carries 1500 / 1600 V diodes that work as replacements.

author: balage
author_rank: Regular Contributor
timestamp: April 20, 2020, 08:21:08 am
content: But most of the parts are available nowadays as well. Like the big orange ceramic caps.Quote from: balage on January 23, 2020, 07:54:34 pmIf you happen to wish stock replacement FETs, I can buy and send them to you. Just sayin'.