Keithley 238 SMU rebuild (?) - Page 1
Forum section: electronics / testgear

author: Nevada
author_rank: Contributor
timestamp: January 16, 2022, 12:43:05 pm
content: I have an analog board from a Keithley 238 SMU. I'm wondering if is possible to make a complete unit with it? What do you think? As far as I can see all the trick is done on this PCB, just need to build a power supply and a digital control.Unfortunately I have no schematic for the 238.I don't necessarily like to build an exact clone, I just need a basic SMU with 1uA-1A current range, and 1mV-100V voltage range.In the 236's schematic, there are two AD7538 and one MP7543 DACs. I don't see any dedicated ADC, it seems to be a custom built ADC with discreate components partly on the Digital Board PCB.

author: Hydron
author_rank: Super Contributor
timestamp: January 16, 2022, 07:17:13 pm
content: I'd start with posting high resolution pictures of the 238 analogue board, so that comparisons can be made with the 236/237 (which does have a full schematic available). You'd need to take the cans off it - be careful not to touch the high value resistors etc with oily fingers.Has it got the output board too? Would need pics of that too. If it's missing then unfortunately it makes things that much more difficult to DIY.As for the main ADC you are correct - it has a custom ADC built on the corner of the digital board. Should be enough info in the 236/237 service manual to replace it with something modern though if you're doing a complete DIY digital section anyway.Might be worth looking at the SMU threads in the projects section - there are a couple of (related) DIY efforts going on there using the 236/237 design as an inspiration, which might considerably reduce the amount of work necessary if they could be built upon.If there are any pics needed of a 236/237 for comparison (in addition to those found on xdevs etc) then I might be able to help - I am in the middle of the 236->237 upgrade process so I have my unit already partly disassembled.

author: alm
author_rank: Super Contributor
timestamp: January 17, 2022, 03:28:33 pm
content: The 1989 catalog contains only the Keithley 236/237, while the 1991 catalog contains the 236/237/238. I don't have access to the 1990 catalog. So the 238 was introduced 1-2 years after the 237. I would expect the differences to be minimal apart from the obvious differences in voltage and current ranges, and hence differences in current shunts, output transistors and analog power supply rails. But since it was introduced a bit later, it's plausible that they incorporated design improvements in other parts that made it more reliable or cheaper to produce. The block diagram in the 238 service manual looks identical to the 236. I would compare the theory of operations regarding the analog board and power supply between the 236 and 238 manuals. If you don't see any differences, I would think that with a functional equivalent of the 236 digital board and its very rudimentary power supply (adjusted for voltage and current), you might well be able to get this to work.The output board would be quite different regarding the type and number of transistors, and quite possibly the cooling.The are some pictures of the 238 in these two threads that might help:https://www.eevblog.com/forum/repair/should-i-return-a-keithley-238/https://www.eevblog.com/forum/testgear/keitley-238-restoration/

author: Nevada
author_rank: Contributor
timestamp: January 18, 2022, 10:57:14 pm
content: This is what I have exactly. I beleive it is a complete analog board with output stage attached.

author: alm
author_rank: Super Contributor
timestamp: January 18, 2022, 11:12:37 pm
content: That looks indeed like a complete analog and output board to me, but check the pictures in the threads I linked to. It should be fairly straight-forward to figure out what the cables are from the silk screen and 236 documentation. Power should be pretty straight forward. You could feed them with bench supplies if you have enough with the right voltage and current. Then there will be a number of digital signals and the signal to the ADC on the digital board. It would surprise me if the pin-out was very different, except that obviously the 238 will have a set of low voltage, high current rails.Keep in mind that this goes down to a 100 fA resolution, so there will be some very high-impedance nodes in there. So be very careful touching the analog board, especially anything under the shield. I think the original unit had another shield over the entire analog board.

author: alm
author_rank: Super Contributor
timestamp: February 18, 2022, 11:26:06 am
content: There's an eBay seller offering aPDF Keithley 238 service manual that they claim includes schematics. I would probably ask for a sample to make sure it doesn't just show schematics in the table of contents but actually includes the schematics, and $25 is steep in my opinion for a PDF service manual. But it could be worth considering if you get stuck.

author: Nevada
author_rank: Contributor
timestamp: February 18, 2022, 11:28:29 am
content: Thank you! I have suspended the project for a while, but I'll restart it when I have more spare time.