Keithley 195A Multimeter Teardown, Refurbishment, and Upgrades - Page 1
Forum section: electronics / testgear

author: figurativelythedevil
author_rank: Contributor
timestamp: August 28, 2021, 10:14:51 pm
content: Hey y'all! Long time lurker, first time poster. I've recently got into test gear, and I thought I'd finally try my hand at restoring an old multimeter. I found a cheap Keithley 195A on eBay, and decided to pull the trigger, as I didn't currently have a benchtop multimeter, and this wouldn't be too much of a loss if I ended up destroying something beyond my ability to repair while working on it. The unit was sold as-is for parts, but turned on just fine. I suspect the seller just wasn't sure what the state of it was when they sold it to me. The meter has a pretty simple construction, just 3 boards: the main motherboard, a display board, and an optional board providing the AC Amps capability, which this has equipped. Judging by the date codes on the IC's, I believe this was built sometime in 1984. Huge shout out to xdevs for hosting the service manual that includes the board overlays, as the revision hosted by Tektronix conveniently omitted those pages, and there's no silkscreen anywhere.After I got the meter in, I ripped it apart for a visual inspection before attempting to turn it on, just in case there was any magic smoke itching to escape. Everything looked fine, so I went for it. No sparks, no smoke, display looked good, no error messages. Went through the basic RAM & ROM self checks for the self test program, everything passed. All the controls seemed to be working, I could hear relays clicking when changing ranges and functions, so all seemed well. I'm fairly new to this and my homelab is pretty sparse on test equipment, so I didn't do any more in-depth testing before I started replacing components.Of course, the first order of business when going through something this old was to replace all the electrolytics, which I did, replacing everyone with low ESR 105℃ equivalents from Nichicon & Nippon Chemi-con, all with higher voltage ratings; hopefully this will keep things smooth for another 30-something years. Most of the caps I removed tested okay, but there were a few that had started bulging out from the bottom.And here's some of the replacements in their new homes.There was only one tag tantalum capacitor in the entire instrument, which lives on the display board. It seemed in fine condition, but I replaced it anyways, just for good measure.Reading through the service manual's replaceable parts list, there were numerous 5 & 10% specced composition resistors that I knew I wanted to replace. Since I was going that far, I also ordered replacements for basically every resistor I could find a tighter tolerance, lower tempco version of. Most notably, the ohms reference area I replaced the .1% 1K & 100K reference resistors with some (luckily in-stock) .01% .2ppm/℃ foil resistors from Vishay. This was probably overkill, but they weren't insanely expensive, and I plan on using this meter for a long time. I also paid particular attention to the 2V reference circuit, replacing all those resistors with as low of tempco that resistors I could find in-stock.I re-assembled the meter and luckily didn't get any smoke, and everything seems to be working. I did some basic checks (9V battery, resistance measurements, DC current through a resistor) and everything still reads okay, albeit in need of a cal from all my fiddlings. I still have a few things to replace, I decided to find some lower tempco resistors for R158-161 which are for the 4-wire resistance measurements, and I want to replace the binding posts (the current ones don't completely fit my retractable safety bananas from probemaster, and pomona 3770s fit perfectly and would be glorious overkill). Is there anything else y'all think is worth a preemptive replacement? I'm also planning on backing up the eeprom, but that's something I've never done before (I did pick up a generic programmer for this though). Let me know what you think of my progress so far, and I'll be posting some more pics in following posts as I hit my 10 attachment limit, also let me know if there's anything you want to see that I haven't pictured!edit: Here's a link to an imgur album with higher-res shots of everything here!https://imgur.com/gallery/EIEn2nP

author: figurativelythedevil
author_rank: Contributor
timestamp: August 28, 2021, 10:30:19 pm
content: More photos!Here's the AC Amps/Option 1950 board after replacementsFirst photo of the innards I took right after getting it in the mailTop-down of the mother board before replacementsTop down after replacements

author: J-R
author_rank: Super Contributor
timestamp: August 29, 2021, 01:45:33 am
content: It's always nice to see writeups like this. Personally, I've been dabbling a bit lately with some Fluke 8840A and 8050A units. Fun times.Especially for board/component photos, higher resolutions might be helpful for some folks.Also, Dave needs to light a fire under the web admin. Still having the broken inline photos is just not cool. Fix it already!!

author: figurativelythedevil
author_rank: Contributor
timestamp: August 29, 2021, 01:52:01 am
content: Yeah, I'd love to post hi-res, but the upload limits are a bit restricting, especially when uploading a bunch of photos. I'll throw them all in an imgur album and put it in the first post!

author: figurativelythedevil
author_rank: Contributor
timestamp: August 29, 2021, 02:24:56 am
content: Added a link to my imgur album for the teardown photos!

author: J-R
author_rank: Super Contributor
timestamp: August 29, 2021, 03:16:35 am
content: How is the calibration after all the replaced components? Will you be calibrating it?

author: figurativelythedevil
author_rank: Contributor
timestamp: August 29, 2021, 03:30:00 am
content: The cal is definitely out after all the replacements. Most ranges are pretty close, but the high resistance ranges are considerably out. Once I'm finished fiddling I'm going to have it professionally calibrated as I don't have the equipment to do it myself.

author: bdunham7
author_rank: Super Contributor
timestamp: August 29, 2021, 04:26:32 am
content: Very nice! I haven't ever seen one of those in person, so if you don't need a 'professional' calibration I can do it for you for the cost of roundtrip shipping.

author: figurativelythedevil
author_rank: Contributor
timestamp: August 29, 2021, 05:35:09 am
content: I actually got a reasonable quote (~$95) from a reputable & local cal facility, so I think right now I'm inclined to take it, but I appreciate the offer!

author: AVGresponding
author_rank: Super Contributor
timestamp: August 29, 2021, 07:48:12 am
content: Quote from: figurativelythedevil on August 29, 2021, 05:35:09 amIs that quote for calibration and adjustment, or just calibration..?

author: figurativelythedevil
author_rank: Contributor
timestamp: August 29, 2021, 03:03:08 pm
content: Quote from: AVGresponding on August 29, 2021, 07:48:12 amCan you explain? Wouldn't a full calibration necessitate adjustments?

author: bdunham7
author_rank: Super Contributor
timestamp: August 29, 2021, 04:01:50 pm
content: Quote from: figurativelythedevil on August 29, 2021, 03:03:08 pmThe pitfalls of calibration services have been widely discussed elsewhere, particularly in the metrology forum, but the short answer is do not assume anything, ask exactly what the proposed service entails and don't deal with any calibration service provider that is unable to clearly answer your questions.Yes, I would expect the typical service quoted for your meter to include adjustments, but possible issues that you should ask about are:Does the service include adjustment to specification?Is before and after data provided? If not, are the specifications that the meter is adjusted to explicitly listed?What equipment is used and are the calibration lab uncertainties listed for each test point? (not so big a deal for your class of DMM)I think the most likely service that would typically be quoted for that price would be a non-ISO-17025-accredited with only OOT (Out Of Tolerance) data reported. In that case, you would get a short report stating that the meter is now within manufacturer tolerances (or a reject if it can't be adjusted to so conform) and a list of any ranges that were not in specification to begin with. Good enough for government work, as they say, but profoundly unsatisfying to the curious hobbyist.

author: figurativelythedevil
author_rank: Contributor
timestamp: August 29, 2021, 04:10:27 pm
content: Quote from: bdunham7 on August 29, 2021, 04:01:50 pmGotcha, I will definitely ask them! Thanks for the info.

author: figurativelythedevil
author_rank: Contributor
timestamp: September 05, 2021, 05:48:18 pm
content: Opened the Keithley back up for some last minute modifications! My new binding posts showed up, as well as some new resistors that connect the force & sense terminals together for the 4-wire resistance measurements, and new terminals to fit onto the new binding posts. I think that finishes up everything I wanted to do to this now. I spoke with the calibration lab, and they clarified that the initial quoted $95 price does not include adjustments, but if they are necessary, they will do them for an extra 50% charge (which this will almost certainly need). They do also provide as found/as left reports as well for everything, and it's NIST traceable as well. Not super important for my use, but it's nice to have. I also made sure that they'll follow the procedure in the service manual if I provide them with it, which I will be doing, so all and all, it still seems like a pretty solid deal.Right now I'm just letting it sit running, letting everything settle down before I get it calibrated. Went through some basic tests and everything appears to be working. It's actually surprisingly accurate measuring a 9v battery right now, even after all the changes I've made, but the resistance values are pretty far out, which tracks considering the modifications I made to the entire Ohms reference section.Next step for me is getting a USB-GPIB adapter so I can start experimenting with logging, so hopefully I can come back with some graphs for stability/linearity/drift/etc., but I imagine that will be some time.

author: figurativelythedevil
author_rank: Contributor
timestamp: September 05, 2021, 05:52:38 pm
content: A few more photos

author: figurativelythedevil
author_rank: Contributor
timestamp: September 05, 2021, 05:55:12 pm
content: This was a before shot of the analog section. This arrangement just looks really interesting to me.

author: guenthert
author_rank: Frequent Contributor
timestamp: September 05, 2021, 06:32:07 pm
content: Quote from: figurativelythedevil on August 28, 2021, 10:14:51 pmUh.Quote from: figurativelythedevil on August 28, 2021, 10:14:51 pmThere are definitively two schools of thought: the other one (to which I adhere) is to not fix it if it ain't broken (that probable is related to one's confidence in one's ability to actually fix things). The categorical replacement of all electrolytics might make more sense for devices suffering from the capacitor plague, but that came much later (around 2000). Older caps are often still good enough for the intended purpose. Better caps might mean lower ripple on the internal power lines, which might yield lower noise in measurements, but for that I need a low-noise amplifier to determine the ripple first ...Quote from: figurativelythedevil on August 28, 2021, 10:14:51 pmuh, why? Replacing everything indiscriminately assures that any stability the old instrument might have gained over the years is lost. To get most of the instrument, a calibration would be needed in any case, but now you might want to monitor its stability for some time before such would be worthwhile.Quote from: figurativelythedevil on August 28, 2021, 10:14:51 pmQuote from: figurativelythedevil on August 28, 2021, 10:14:51 pm*cough* Is this some kind of competitive soldering? The value of those resistors, which implies their tempco, is of virtually no concern (have a close look at the schematic to convince yourself of that). Any (thermal) EMF created on those connectors however, *will* falsify the 4 wire resistance measurements.Quote from: figurativelythedevil on August 28, 2021, 10:14:51 pmIn case it didn't become clear so far, I'm strictly against preemptive replacement, at least in the non-safety related precision analog domain.

author: bdunham7
author_rank: Super Contributor
timestamp: September 05, 2021, 06:38:01 pm
content: Quote from: figurativelythedevil on September 05, 2021, 05:48:18 pmQuite the project! My offer still stands if you want to try and save that 50%.