Modding a Keithley 197 - Page 1
Forum section: electronics / testgear

author: tooki
author_rank: Super Contributor
timestamp: August 06, 2015, 05:46:47 pm
content: So I scored a Keithley 197 bench DMM on the local equivalent of eBay. It's the original version with the oh-so-1970s brown case and front panel, and no backlight. The internals indicate a 1986 vintage.(In the background you can see my DS1054Z that I just modded with a quiet fan.)It seems to work OK, not that I have anything to test it with. But it seems to agree with my 87V to the extent possible.So here's the question: Would it be sacrilege to mod it to put some kind of LED backlight on the LCD? (I assume something like some side-light LEDs, since the LCD panel itself is highly unlikely to lend itself to actual backlighting.)

author: AF6LJ
author_rank: Unknown
timestamp: August 06, 2015, 05:53:23 pm
content: I would say Go for It.I have been looking for a decent way to mod my HP-3478A.So please do and post lots of pictures.

author: TiN
author_rank: Super Contributor
timestamp: August 06, 2015, 06:25:23 pm
content: Go for it. Keithley fan here approve.Also post teardown pics, never saw one inside well enough

author: codeboy2k
author_rank: Super Contributor
timestamp: August 07, 2015, 03:14:36 am
content: I also want to see inside the 197, to compare it to the 197A. So please teardown for all to seeI've got a 197A which has an EL backlight already, but it was extremely dim when I bought it off eBay.So I did a repair to replace that, but I wasn't happy with the result:1) the new EL panel was narrower than the original, and it doesn't cover the entire LCD panel top to bottom2) it has a light blockage in the upper left corner where the electrode is. This blocks the word "AUTO".3) I asked for a white EL panel, but got a weird turquoise blue color instead .. ugly.. (it looks brighter and better on camera then in real life. In real life it's very, very dim. Almost not worth having there. I have not experimented with different voltages or frequencies to try to make it brighter, but since I don't like the color anyways, I never bothered )Now, I think I will try to get a larger orange EL, to get a look similar to the Agilent U1270 series DMM or the U8000 series PSU's (second picture below). I don't know if the Agilent PSU's use LED backlights or EL panels.[click for a bigger image]I also have another Keithley 197A that is in need of repair. It doesn't take a stable reading (measurements are all over the place and won't settle down). I'll post that repair when I get to it

author: retiredcaps
author_rank: Super Contributor
timestamp: August 07, 2015, 05:47:26 am
content: Quote from: codeboy2k on August 07, 2015, 03:14:36 amI mentioned scullcom's channel before, but it seems not enough people know about it. His 197 teardown video

author: codeboy2k
author_rank: Super Contributor
timestamp: August 07, 2015, 07:41:46 am
content: Re: scullcom's channel.. Nice! thanks.. I'll take a look.Cheers!

author: Lightages
author_rank: Unknown
timestamp: August 07, 2015, 08:18:38 am
content: I have a 197, and the two things I wish it had, continuity and a back light. I know, I could have purchased a meter with both, but not with the aged accuracy and for the price.I have considered adding a back light many times. Perhaps I will get to it in time. Please share any results you get as it would be very interesting.

author: idpromnut
author_rank: Unknown
timestamp: August 07, 2015, 04:20:20 pm
content: @codeboy2k: I did the same fix to my Keithley 175A (I have pictures but not posted. I will do that tonight!)@tooki: I have a project on the todo list that would replace the display from this line of meters (197, 175, etc) with an OLED display. I started the reverse engineering on the display controller and was at the point of figuring the best way to approach the mod.

author: tooki
author_rank: Super Contributor
timestamp: August 18, 2015, 11:16:32 pm
content: Quote from: idpromnut on August 07, 2015, 04:20:20 pmHave you made any progress on that?

author: idpromnut
author_rank: Unknown
timestamp: August 19, 2015, 01:09:31 am
content: Unfortunately not. As happens, projects tend to pile upI will see if I have some time in the next couple of weeks to throw together a prototype at least.

author: tooki
author_rank: Super Contributor
timestamp: August 19, 2015, 01:16:01 am
content: Lemme know what you figure out. I did a preliminary investigation of the 197's display, and it looks as though backlighting will not be trivial to add. So an entire replacement display could well be a better solution.

author: idpromnut
author_rank: Unknown
timestamp: August 19, 2015, 01:26:31 am
content: Quote from: idpromnut on August 07, 2015, 04:20:20 pmI totally spaced and forgot to do this. Here are a few pictures of my replacement of the EL backlight in my 175A.

author: codeboy2k
author_rank: Super Contributor
timestamp: August 19, 2015, 06:34:57 am
content: Thanks for those pictures! It turned out really nice for you.Which EL panel did you buy (if you remember) ?Mine was too narrow and as you see in my pictures it has an electrode on the front side that blocks the light (grrrr!)

author: AF6LJ
author_rank: Unknown
timestamp: August 19, 2015, 10:39:52 pm
content: Looks good I really like it.Might have to look into doing that to my HP..

author: idpromnut
author_rank: Unknown
timestamp: August 20, 2015, 01:21:22 am
content: Quote from: codeboy2k on August 19, 2015, 06:34:57 amOf course the seller is not selling them, but I got something like this:eBay auction: #271234168462Just look for a "cuttable" EL strip and you'll be fine (and of course you don't need one with an inverter, unless you're modding a unit that didn't come with a backlight originally).

author: codeboy2k
author_rank: Super Contributor
timestamp: August 20, 2015, 03:19:12 am
content: Great! thanks, I'll get one of those wider strips next time then.The 1.25" width is too big, I wasn't sure if I could cut them down on two edges and have it still work (both length and width). So in the past I opted for a narrow one and I only cut the length, but it was too narrow.I'll give it a try.Cheers!

author: tooki
author_rank: Super Contributor
timestamp: October 28, 2015, 02:14:08 pm
content: I really need to get back into this project, after successfully adding a backlight to a cheapie DMM I had lying around.Question though: EL strips require an inverter. Do the inverters produce noise that could affect the performance of a 197, given that they have microvolt resolution?

author: don.r
author_rank: Frequent Contributor
timestamp: October 18, 2019, 03:23:41 am
content: Has anyone found a source for suitable EL panels? My 175A backlight is very weak.

author: tooki
author_rank: Super Contributor
timestamp: October 18, 2019, 08:00:07 am
content: It’s fairly easy to find suitable ones on eBay, since you can just get an oversize one and cut it to size. (My 197A has the same issue.) There are both large strips and square panels that are big enough. The thing to look for is where the power connectors are, so as to be able to match the location of the original leads. (You don’t want one where the power goes in the very corner, but rather centered so you can trim to size and still have the leads centered.)That said, I’d love to experiment more with the diffusion films from thin edge-lit LCD panels. In particular, the film with the etched dots is very interesting, as it’s designed specifically to eject light evenly from just side lighting.)Well, that and with making a whole replacement display PCB using an Arduino decoder to drive some nice LED, graphical LCD, or OLED display.

author: don.r
author_rank: Frequent Contributor
timestamp: October 19, 2019, 02:14:59 am
content: OK. Ordered a white one off eBay.... and now a month or two of waiting.

author: akabek
author_rank: Contributor
timestamp: February 02, 2021, 10:58:49 pm
content: I just received my 197A from ebay and I came across this thread. Has any recently found a suitable EL panel for the backlight mod?

author: don.r
author_rank: Frequent Contributor
timestamp: February 03, 2021, 05:46:53 am
content: Quote from: akabek on February 02, 2021, 10:58:49 pmThe 175A already has a backlight. What mod are you referring to? My EL panel was fine but I had to get a new backlight driver off aliexpress.

author: akabek
author_rank: Contributor
timestamp: February 03, 2021, 06:25:31 am
content: Quote from: don.r on February 03, 2021, 05:46:53 amThe 197A does have a backlight but it is dim and barely noticeable. I was hoping the EL panel mod would be an improvement.

author: akabek
author_rank: Contributor
timestamp: February 03, 2021, 04:17:05 pm
content: Quote from: don.r on October 19, 2019, 02:14:59 amWhich EL panel did you order?

author: don.r
author_rank: Frequent Contributor
timestamp: February 03, 2021, 04:25:28 pm
content: Quote from: akabek on February 03, 2021, 04:17:05 pmSimilar to this one. Like I said, it was my driver at fault, not the panel.https://www.ebay.ca/itm/ABS-Plastic-Electroluminescent-El-Panel-Light-Lamp-Sheet-Backlight-2-Colours/393103760354

author: akabek
author_rank: Contributor
timestamp: February 03, 2021, 04:43:14 pm
content: I just realized I had the wrong model number mentioned. I have the 197a.I was looking at those EL panels. Were you able to cut yours to size? I thought I read somewhere the panel in your link could not be cut.Is your Keithley a 175a? I wonder if the driver may be the issue. To be honest I am not sure how bright the backlight should be in a 197a.

author: don.r
author_rank: Frequent Contributor
timestamp: February 03, 2021, 05:10:26 pm
content: Never got to the stage of cutting since I discovered it was the driver. Mine was a 175A. Measure the voltage going into the panel. It should be somewhere between 70 and 90V (AC?). Mine was 25V. I used a driver similar to this but not sure what the input voltage was (12V or 5V):https://www.aliexpress.com/item/1005001977407178.html

author: akabek
author_rank: Contributor
timestamp: February 03, 2021, 06:05:14 pm
content: Quote from: don.r on February 03, 2021, 05:10:26 pmExcellent, I will do some checking tonight.

author: akabek
author_rank: Contributor
timestamp: February 04, 2021, 12:21:33 am
content: I checked the AC voltage going into the backlight panel and it is 44.8v. Just curious how you know the AC voltage should be between 70 and 90v? Were you able to locate the schematics for the 197a?

author: don.r
author_rank: Frequent Contributor
timestamp: February 04, 2021, 05:31:02 pm
content: Quote from: akabek on February 04, 2021, 12:21:33 amCan't remember where I saw the voltage but I think it was the standard of EL panels. They light at 50V and get brighter as voltage increases. Mine was the 175A and has schematics in the user manual. I think the feed voltage to the driver circuit could have been 5V but you can measure that. Its the black box on the back of the front panel.

author: rwzahora
author_rank: Newbie
timestamp: August 02, 2022, 01:22:26 pm
content: My 197A had a very weak backlight so I thought I'd look into replacing it. The old panel was a Loctite LSI 37269-1 as shown in an earlier reply. It is 19mm x 86mm. I purchased a panel from eBay (https://www.ebay.com/itm/393103760354?var=662007375026) and cut it down on three sides and nail-polished the edges. It works and the light is much better than the original unit although I would have liked it even brighter. The voltage to the panel is 76Vac measured with a multimeter and the drive is triangleish at about 370 Hz.

author: jupiters_spot
author_rank: Newbie
timestamp: November 12, 2022, 05:14:40 pm
content: Reading through this thread it seems that:- the 197A has a backlight, and some here have needed to replace the inverter driver to supply the correct voltage to the electroluminescent backlight- there aren't any posts about adding a backlight to a 197 (as opposed to 197A) display. Did I miss such a post?Adding a backlight to a 197 appears to involve two primary steps: install appropriate inverter to drive EL strip, and trim/seal/install EL strip behind LCD display. All of the EL drive inverters I see either require 12V DC or 120V AC, and I'm not sure I see a convenient source of switched power of either sort in the base 197. I'd want the backlight to turn on/off with the meter's power button so I'd need to power the inverter somewhere after the meter's power switch. If anyone has installed an inverter and EL backlight in a base 197 (not the 197A), please share with us how you powered the EL drive inverter and where you managed to stuff the inverter into the meter's case.

author: tooki
author_rank: Super Contributor
timestamp: November 12, 2022, 05:52:38 pm
content: A 197 display upgrade has been a project on my to-do list for a few years. :pI don’t think there’s much sense in using EL these days, since LED is so much easier to drive.Long-term, an entire replacement display board, using a microcontroller to decode the original display signals and reformat them for some modern display, is likely a smarter way to go. (The work to decode the original display signals is already complete.)

author: jupiters_spot
author_rank: Newbie
timestamp: November 12, 2022, 06:23:31 pm
content: QuoteI'm intrigued.Looks like someone has done it:https://www.reddit.com/r/electronics/comments/pkknvk/keithley_197_with_oled_display_upgrade/