EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: tooki on August 06, 2015, 05:46:47 pm
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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.
(http://www.tooki.net/photos/eevblog/keithley197/IMG_4786.jpg)
(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.)
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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.
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Go for it. Keithley fan here approve.
Also post teardown pics, never saw one inside well enough :)
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I also want to see inside the 197, to compare it to the 197A. So please teardown for all to see :)
I'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 bottom
2) 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.
(http://i.imgur.com/41R4nlIt.jpg) (http://i.imgur.com/41R4nlI.jpg) (http://i.imgur.com/ckKiGkxt.jpg) (http://i.imgur.com/ckKiGkx.jpg)
[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 :)
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I also want to see inside the 197
I mentioned scullcom's channel before, but it seems not enough people know about it. His 197 teardown video
https://www.youtube.com/watch?v=b1F1f-ajyjY (https://www.youtube.com/watch?v=b1F1f-ajyjY)
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Re: scullcom's channel.. Nice! thanks.. I'll take a look.
Cheers!
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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.
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@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.
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@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.
Have you made any progress on that?
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Unfortunately not. As happens, projects tend to pile up :) I will see if I have some time in the next couple of weeks to throw together a prototype at least.
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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.
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@codeboy2k: I did the same fix to my Keithley 175A (I have pictures but not posted. I will do that tonight!)
I totally spaced and forgot to do this. Here are a few pictures of my replacement of the EL backlight in my 175A.
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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!)
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Looks good I really like it.
Might have to look into doing that to my HP..
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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!)
Of course the seller is not selling them, but I got something like this: eBay auction: #271234168462 Just 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).
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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!
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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?
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Has anyone found a source for suitable EL panels? My 175A backlight is very weak.
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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.
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OK. Ordered a white one off eBay.... and now a month or two of waiting.
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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?
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I just received my 175A from ebay and I came across this thread. Has any recently found a suitable EL panel for the backlight mod?
:-// The 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.
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:-// The 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.
The 197A does have a backlight but it is dim and barely noticeable. I was hoping the EL panel mod would be an improvement.
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OK. Ordered a white one off eBay.... and now a month or two of waiting.
Which EL panel did you order?
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OK. Ordered a white one off eBay.... and now a month or two of waiting.
Which EL panel did you order?
Similar 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 (https://www.ebay.ca/itm/ABS-Plastic-Electroluminescent-El-Panel-Light-Lamp-Sheet-Backlight-2-Colours/393103760354)
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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.
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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 (https://www.aliexpress.com/item/1005001977407178.html)
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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 (https://www.aliexpress.com/item/1005001977407178.html)
Excellent, I will do some checking tonight.
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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?
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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?
Can'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.
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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 (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.
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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.
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A 197 display upgrade has been a project on my to-do list for a few years. :p
I 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.)
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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.)
I'm intrigued.
Looks like someone has done it: https://www.reddit.com/r/electronics/comments/pkknvk/keithley_197_with_oled_display_upgrade/ (https://www.reddit.com/r/electronics/comments/pkknvk/keithley_197_with_oled_display_upgrade/)