Playing around with an old Keithley 236 Source Measure Unit - Page 1
Forum section: electronics / testgear

author: baltersice
author_rank: Regular Contributor
timestamp: February 11, 2017, 08:11:20 am
content: I didn't plan buying more of these old brown Keithleys, but the price was too good to let it goNow I am glad I did it, because it is fully functional, accurate and might even be upgradable.Does anyone know anything about the schematics-publication-policy at Keithley?Mine has a transparent PCB finish with good visibility (in contrast tothis one?) but schematics would be even better...

author: 2N3055
author_rank: Super Contributor
timestamp: February 11, 2017, 09:26:43 am
content: Quote from: baltersice on February 11, 2017, 08:11:20 amYou have service manual here:http://w.ko4bb.com/manuals/93.137.149.118/Keithley_236_237_Source_Measure_Unit_SMU_Service_Manual_and_schematics.pdf

author: baltersice
author_rank: Regular Contributor
timestamp: February 11, 2017, 10:50:29 am
content: Wow, yes that's it! Thank you so much!!

author: TiN
author_rank: Super Contributor
timestamp: February 11, 2017, 12:04:53 pm
content: Nice video, I loved the bear partI had no luck (yet?) to play with 23x series SMUs, but from what I've saw online, they are more capable on low currents than K2400. If you have any tests in mind to compare with 2400, just let me know, I'd be happy to test on my old 2400 and Agilent 4142B (41421B SMU module).Leaving reference to docs and pics I've collected so far:K236 folderK236-237 folderK236-237-238 folder

author: picburner
author_rank: Frequent Contributor
timestamp: February 12, 2017, 08:39:31 am
content: .... and here the A10 firmware version:

author: baltersice
author_rank: Regular Contributor
timestamp: February 12, 2017, 10:29:11 am
content: Quote from: picburner on February 12, 2017, 08:39:31 amCool! And a changelog?EDIT:Here is a diff for the 800 eprom:https://www.diffnow.com/?report=eygfoAnd the 801 eprom:https://www.diffnow.com/?report=u8dsl

author: picburner
author_rank: Frequent Contributor
timestamp: February 12, 2017, 11:26:09 am
content: I'm sorry but I can't help you because I don't have this instrument, I just did a internet search...

author: baltersice
author_rank: Regular Contributor
timestamp: February 15, 2017, 12:05:30 am
content: Does anybody happen to own a Keithley 237 and would take a photo for me?I am working on the upgrade of my 236 now and am not sure how to mount the resonant converter module, because the screw holes don't line up. I suspect that they put a full enclosure in the instrument, that acts as a screw-hole-adapter ... but can't be sure unless I see it. It would get a little bit tight because of those 2 kV caps ...Here is an old eBay listing for resonant converter module with photos:http://www.ebay.com/itm/351704372382

author: Monolith
author_rank: Contributor
timestamp: March 01, 2017, 04:33:05 pm
content: I am working currently on my Keithley 237. I'll shoot some pics and will post them hopefully tomorrow!

author: Monolith
author_rank: Contributor
timestamp: March 02, 2017, 01:16:42 pm
content: Quote from: baltersice on February 15, 2017, 12:05:30 amHere are some pics of the resonant converter and it's enclosure.

author: Monolith
author_rank: Contributor
timestamp: March 02, 2017, 01:18:01 pm
content: more..

author: Monolith
author_rank: Contributor
timestamp: March 02, 2017, 01:21:54 pm
content: resonant cover opened.

author: Monolith
author_rank: Contributor
timestamp: March 02, 2017, 01:23:39 pm
content: enclosure removed..

author: Monolith
author_rank: Contributor
timestamp: March 02, 2017, 01:25:28 pm
content: resonant board enclosure parts...

author: baltersice
author_rank: Regular Contributor
timestamp: March 02, 2017, 08:01:55 pm
content: Monolith, thank you so much! Tremendously helpfulI have just finished a quick video about my 236 to 237 upgrade plans, but with your photos I might revisit a few things ...https://youtu.be/DLp8jdxnypcIf it is not too much to ask, could you maybe check if the grey and blue input wires on the HV converter are shorted?Those are grounds, and in the schematic it looks like they planned them to be fully isolated, but then they step down the 15V and 5V from the 50V and isolation was no more.If you have put it all back together already, no matter, was just curious

author: guenthert
author_rank: Frequent Contributor
timestamp: March 03, 2017, 01:44:01 am
content: Quote from: baltersice on March 02, 2017, 08:01:55 pmbest electronics related video I've seen in a long while, if not ever. You really have a knack for this.

author: 2N3055
author_rank: Super Contributor
timestamp: March 03, 2017, 07:02:48 am
content: @baltersiceVideo is awesome! As Quenthert says you have a knack for it..And you are so kind to mention me, it was nothing, really.. I was glad to help...Take care!

author: Vgkid
author_rank: Super Contributor
timestamp: March 03, 2017, 07:20:45 am
content: That video was well done, thanks.

author: Monolith
author_rank: Contributor
timestamp: March 03, 2017, 07:35:34 am
content: Quote from: baltersice on March 02, 2017, 08:01:55 pmThat is really really a great video!Yes, i can confirm: Between grey and blue is a short! I made some more pics of the resonant board and will post it soon.Btw, i was the lucky guy who bought the service manual; scanned it in hires ran it through OCR and uploaded the whole stuff to KO4BB. But i was not mentioned in the video, so sad... and great that it is used immediatley. That's why i like this community so much!Please continue your enternaining and educational videos! If you need more infos about the Keithley 237 i am glad to help!

author: Monolith
author_rank: Contributor
timestamp: March 03, 2017, 08:32:09 am
content: Resonator Board..

author: Monolith
author_rank: Contributor
timestamp: March 03, 2017, 08:33:45 am
content: Resonator continued..

author: albert22
author_rank: Regular Contributor
timestamp: March 07, 2017, 04:38:29 am
content: Somebody already mentioned awesome so:Both for the work and the videoThanks

author: MadTux
author_rank: Frequent Contributor
timestamp: April 18, 2017, 08:58:32 pm
content: Just fixed my cheap broken K237 (at least the low voltage section).As expected, a transistor, specifically Q19 on output board was bad. As a result, it could only source voltages between -160V to -110V before the overcurrent protection for positive rail (Q21 and MOSFETs) kicked in (Q1 and Q26 which sense current over R26).Locating the fault was a bit tricky, because junction voltage drops of Q19 appeared to be fine. But exchange of K237 output board with known good K236 output board confirmed that everything else on K237 was fine and output board had broken parts. All MOSFETs had nicely distributed voltage drop between source/drain, so I assumed that these are fine. Initially unsoldered Q21, because I thought it might have bad gain or blown BE junction. But tested fine in my Tek 7CT1N, so I put it back in.Next I looked at funny Q1/Q2,Q26/Q27 circuitry. After a while I figured out that it is probably some kind of overcurrent protection, Q1/Q2 off, it triggers at around 20mA, Q1/Q2 on => trigger at around 100mA. Now the "100mA BIAS" label in the schematic made some more sense to me. Rest was quite easy, measured voltage drops on R24 and R26 and watched how the voltage across R26 increased as I increased the output voltage on K237 from -150V until it stopped at -110V. Voltage across R26 was around 0.68-0.75V by then and triggered overvoltage protection. (BTW,voltage across R24/R26 is around 450mV on good board with no load applied) Since I had tested Q21 before and all MOSFETs had a good voltage drops, the only remaining suspect was a degraded Q19.Inverter board had blown IRF530 MOSFET, easy to find and fix. Already machined a heat sink for these MOSFETs, as they had been replaced quite a few times before (there was a lot of flux residue on the MOSFET pins)Unfortunately I shorted the output of inverter board with chassis during testing and blew some transistors on that weird circuitry on the output of the inverter board. (Q3 and Q4 and VR7, 24V zener are dead, Q3/Q4 are 2N5089, VR7=1N970B). Anyone an idea what that output circuit (Q3/4/5/6) does, my guess is some kind of voltage regulator, but I don't really understand the magic behind it. Transistors are 2N5089 on positive side and 2N5087 on negative side, both have rather high gain and are in darlington configuration. Unfortunately, Keithley doesn't describe their circuit tricks a well as HP or Tektronix did in their manuals.