Keithley 197A Repair - Page 1
Forum section: electronics / repair

author: vliwa
author_rank: Newbie
timestamp: September 21, 2025, 12:27:36 am
content: Hello, I picked up a Keithley 197A for not too much with the display missing segments and unknown condition otherwise. When I opened it up I noticed it also had the battery option which had leaked. I cleaned the display strips and added a piece of tape to put a little more pressure as well as cleaned up the battery leakage. From what I can tell all the traces are fine and still measure as shorts but some of the soldermask was eaten away. After powering it on the display showed all segments and it seemed to be measuring voltage fine. However ohms measurements would start significantly higher than the actual value and slowing come down over around 5 minutes until settling somewhat close to the marked value of a resistor. I left it like that last night.The next day I went back to it and on powering it on it would only display overrange in every measurement and range. I then used contact cleaner on the switches and cleaned the areas of the pcb that had battery leakage some more, but neither seemed to have any effect. The last things I have tried were to desolder and check the electrolytic capacitors which all seem to be okay, although I don't have something capable of testing the higher value ones properly. After soldering the capacitors back in the voltage range started displaying values again but does not seem to change when measuring anything and does not seem to be actually measuring anything. I checked the power test points as well on a scope and they all appear to to be the correct voltage and do not have any ripple I could notice.Currently both the ohms and amps still only show overrange, while the voltage range shows around -1000 volts on power up in the 1000V range, then increasing towards 0. While the other ranges only change the position of the decimal (-800V in 1000V range becomes -80V in 200V range and so on). The manual on Tektronix's site only showed basic maintenance and I think at this point it is past my knowledge of how to fix it so I figured I would ask here. Thank you in advance for any help or insights.

author: tooki
author_rank: Super Contributor
timestamp: September 21, 2025, 12:22:32 pm
content: FYI, while the 197A manual doesn’t contain schematics or anything, the manual for the 197 (without A) does, and they differ only in adding the display backlight. So 99.9% of the 197 manual applies to the 197A.Anyway, some circuit nodes are extremely high-impedance and require their areas of the PCB to beimmaculatelyclean. These are the ones surrounded by guard traces, which are bare (no solder mask) traces surrounding certain parts of the PCB. On the 197/197A, the guard traces are on both sides and have many vias connecting them, so they look like strings of pearls.*Those parts of the board are very sensitive to contamination from things like cleaning switches, where it’s super easy to get spent cleaning agent on the sensitive areas.The very best way to clean these is with an ultrasonic bath. Barring that, you need to do a very thorough overall board cleaning, then use high-purity solvent to generously flush the sensitive areas. (Perhaps others can chime in with optimum cleaning procedures for such boards.)*The guard traces are visible in the photos onhttp://www.kerrywong.com/2014/06/01/keithley-197-teardown-and-repair/. Look at the two images below the text “Here are two pictures showing the top side and the bottom side of the main PCB after it was taken out of the case. Note the guard traces in the high impedance JFET front-end circuit section.”

author: vliwa
author_rank: Newbie
timestamp: September 21, 2025, 07:52:13 pm
content: I was able to find the 197 manual although it does seem my version of the 197A changed the digital side quite a bit, moving to what I assume is a 6800 based microcontroller. I unfortunately don't have time at the moment to really dive into it but I did try cleaning the board again with isopropyl and it did not seem to have any effect. I did take a picture of the area that was exposed to the battery leakage and it seems like its the circuitry controlling the transistors that is effected.

author: tooki
author_rank: Super Contributor
timestamp: September 21, 2025, 09:48:47 pm
content: My 197 and 197A PCBs are essentially identical, other than the added backlight control. Your PCB seems to be significantly different in some ways — the hole visible in the foreground of your image isn’t present in either of my boards. And it looks like your resistor array has a 2004 date code? Is your unit actually that young? (I don’t know when they stopped making it.) My 197 is from 1986, my 197A is from 1992.Both of mine use a 6805 CPU (Motorola 146805E2P in the 197, Harris CDP6805E2E in the 197A). What’s in yours? Would you mind sharing some nice pix of the whole PCB?Your photo does seem to show some trace damage. There’s definitely corrosion under the solder mask of many traces.Also, I found a 197A manual with schematics! Someone uploaded this to archive.org in 2024, along with many others of thetens of thousands of manuals they rescued:https://archive.org/details/manualsplus_01597/

author: vliwa
author_rank: Newbie
timestamp: September 21, 2025, 10:18:59 pm
content: Here's a full picture of the board. I am debating if it would be worth trying to fix the corroded traces or not. It would be nice to have it working but I could always use the instruments in my schools makerspace as well.edit: Just peeled up the sticker and it is using a MC68HC711E9

author: MathWizard
author_rank: Super Contributor
timestamp: September 22, 2025, 11:52:03 am
content: I removed all the range switches on one of these before. It was not fun and I tore some traces. But I fixed it all back up, cleaned it all back up, and it all works perfect still. I have the 197 w/o the backlight, and I worked out the LCD codes, and switched to 7seg LED's. It works good. I bought even larger sized digit's, but so far I haven't cut up the front of the case to install them. But it will be worth it, I'll have to post a pic when I do it.I'd start probing around to see what's going on.That's a newer looking one than mine, there's no big square MCU/IC. The main voltage reference IC is the 8pin metal can near all the JFET's. Take care around it, not to short it.

author: tooki
author_rank: Super Contributor
timestamp: September 22, 2025, 04:35:11 pm
content: OP, your board seems to match what’s in the manual for the military version of the 197:https://www.radionerds.com/images/7/72/TM_11-6625-3277-14.PDF

author: vliwa
author_rank: Newbie
timestamp: September 22, 2025, 05:04:02 pm
content: Interesting, it does seem to be missing that extra shielding and toroid on the inputs though so I'm guessing someone swapped the board into a normal 197A then.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 22, 2025, 07:09:26 pm
content: In the K197 service manual schematics U102 is the main amplifier, not the reference. The reference is a zener diode at the voltage regulatur.There are not many references in a 8 pin metal can (an LTZ1000 would defintely be overkill).

author: tooki
author_rank: Super Contributor
timestamp: September 22, 2025, 08:52:21 pm
content: Quote from: vliwa on September 22, 2025, 05:04:02 pmWhat does your front panel say?? And more importantly, does the label on the bottom mention the military model number "AN/USM-486A"? If so, then you have the military version.Does your unit actually have a backlight?This page shows images of it:https://www.soriaudio.com/index.php?mid=m_eqp&document_srl=27919929Interestingly, in addition to the AN/USM-486A designator, the label also mentions "MODEL: 197-M". That matches up with the "197-M" label on your MCU. Almost everything points to your unit using the military board, but it isn't populated with all of the military upgrades. (See replies 15 and 23 onhttps://www.eevblog.com/forum/testgear/keithley-197a-owners_-corner/.)https://electroniccalibrators.tpub.com/TB-11-6625-3263-25/TB-11-6625-3263-250070.htmmentions it as the "197-MIL".I suspect this is how it worked:1. In 1984, Keithley releases the 197.2. In 1990, Keithley releases the 197A, adding the backlight (with switch), mains input filter toroid, and removable line cord. User manualrevision Aandrevision Binclude the schematic, theory of operation, and show the backlight switch on the back. (That link for manual rev B doesn't actually include the schematics, but I suspect they were originally there.)3. In 1994, Keithley makes a custom version of the 197A for the military, the 197-M, removing the backlight (probably due to noise) and backlight switch, adding a ton of shielding, a filter toroid on the input jacks, and a fuse on the 10A current jack. This version changes to the MCU. For whatever reason, they kept the 197's brown coloring. They probably kept the part number due to the lack of backlight?4. By 2001 (when manual revision C is released), Keithley removes the backlight switch from the civilian 197A. This hardware revision probably uses the 197-M board, but doesn't populate the fuse, and doesn't use all the added shielding.I suspect this is what you have.Manualrevision Cremoves the schematic and no longer shows a backlight switch.5. In 2005, Keithley discontinues the 197A , sometime between archive.org's April 23 and June 10 snapshots, when it disappears from the products page. (Perhaps it stuck around due to obligations to produce the 197-M for a while. The Keithley 2000 series had been out for many years by that point, so I can't imagine too many 197A's were sold.)Your unit contains many parts made in 2004, making it a very, very late 197A.As far as I know, nobody before you has ever shared pictures of such a late 197A. Could you take somesharppictures of the internals? (Your pictures so far are kinda blurry/shaky.) The internet's K197 fans thank you!

author: vliwa
author_rank: Newbie
timestamp: September 22, 2025, 11:44:50 pm
content: As far as I can tell its just a normal 197A front panel and has the backlight. No military marking I can see. The front panel PCB does have a 1999 date on it as well and there is no backlight switch so it seems like it is that 2001+ revision. I am in the middle of tests at the moment but after I can pull out my dslr to take some nicer pictures. If there's anything specific you want to see just let me know.

author: tooki
author_rank: Super Contributor
timestamp: September 23, 2025, 07:57:42 am
content: Quote from: vliwa on September 22, 2025, 11:44:50 pmThe more photos, the better. All the PCBs, all the chips, enclosure, etc. It seems likely that the vast majority of 197/197A's out there are older units, since as I said, there's no photo documentation of the late ones! :

author: MathWizard
author_rank: Super Contributor
timestamp: September 23, 2025, 04:22:30 pm
content: Ok Kleinstein I'm mis-remembering, it's output is all scaled to 2V and I think it's gain was 1x and 10x. It's Vref is a simple but clean zener(or a diode) circuit with an op-amp to make the 2V reference.Quote from: vliwa on September 22, 2025, 05:04:02 pmI'm doesn't have any mains input filtering before the rectifier, like a common-mode choke. How much difference does it really make ? I've never tried to calculate such things especially with magnetics like that.I don't remember seeing any chokes or anything on the DMM inputs either.Side by side, is there any real noise or performance difference between the 197, 197A, and updated versions of later years ?? Mine matches my SDS3065X pretty closely, it's just is slower updating, and seems more low-pass filtered that way. It seems calmer.Hey Tooki I know people have copied and modded the MCU code for these, how much better was the later bigger MCU's ? How much better was the M versions ? I'd buy some torrids if I knew what to get, but I don't. Also for more shielding, I have some scrap sheet metal I can cut up.I never really considered how much noisier my LCD to LED mod would be. But the LCD zebra strip is not fitting right anymore, and just wedging it didn't really work either. And the LCD is small, and dark , so I went to LED for now anyways. There's other ways I considered too, like OLED.