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

author: staze
author_rank: Frequent Contributor
timestamp: August 06, 2013, 06:02:48 am
content: All,After my success repairing a Keithley 199, I hopped on eBay and found a 197 for a song. It too is damaged (all readings show as oL). After some basic voltage probing, I THINK I have figured out the issue.It would appear the negative voltage zener is toast, but I'd love a second opinion.The manual can be found here:http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Keithley/Keithley_197.pdfSchematics are included. On the first page of schematics for the board, it appears that out of the bridge I get a filter cap, then through VR104 a positive rail, and VR105 a negative rail. On one side of VR105, I get 0V (makes sense, it's grounded), on the other I get about 0.8V. I confirmed off the bridge rectifier I'm getting a +15V, and a -18V.Everything I've checked that has a negative rail, all gets that 0.8V instead of the -15V ±10% it should be getting. Is it most likely that zener is toast, or am I overlooking something? Looking at the board, it appears it's a 1N47 15V zener (1N4744, or NTE145A). Is there a good way to test it short of what I've done? It does show 0.53V in one direction, and 1.96V in the other... so... guessing that's bad.Thanks!

author: Unknown
author_rank: Unknown
timestamp: August 06, 2013, 08:06:02 am
content: This copy lacks schematics. In general, it helps to be specific: at least give a page / figure number, and preferably post the relevant part of the schematic as an inline image. The easier you make it for people to help you, the more likely you are to get a response.Any semiconductor on the bus might be pulling the negative rail down (up). Desoldering the suspect component, measuring it out of circuit, and if necessary replacing it is a good strategy. Sometimes it may be necessary to isolate parts of the circuit (cutting a pin / track) if there's a lot attached to that rail. Some manufacturers will even include wire links that you can cut and replace for that exact purpose.

author: staze
author_rank: Frequent Contributor
timestamp: August 06, 2013, 09:10:11 am
content: Shoot. There are two version of the manual out there... guess I linked to the wrong one. =/Okay, schematics attached. This copy here:http://www.utwente.nl/tnw/onderwijs/Practica_TNW/doc/apparatuur/multimeters/keithley197.pdfhas the full schematics as well. The voltage rails are all on the first page, in the lower right quadrant. I see where the positive and negative rails are "created", as well as the +5V analog and digital rails, and the +10V rail (for many of the opamps on the board running on +10V and -9V), but I must be blind because I can't see where the -9V rail is created from the -15V that the zener in question should be providing.The digital stuff all seems to be working, since obviously it just runs on +5V.So is your thought that it's unlikely the zener has died? I figured something could be dragging it down, but given I can't find where the -15V actually goes to be converted to all the other rails...If I pull the zener, what's the best way to test? Just run 18V (or whatever) through it, with a 100R resistor in there and make sure it breaks down at ~15V?As always, thanks alm.

author: staze
author_rank: Frequent Contributor
timestamp: August 06, 2013, 09:28:32 am
content: Ah, looks like the -9V rail is created in F2, on page 1 of the schematics. It looks like VR101 may be the answer... but I can't read a model number, and keithley seem intent on not listing actual off the shelf part numbers in their manuals... =/

author: Unknown
author_rank: Unknown
timestamp: August 06, 2013, 11:45:57 am
content: The parts list from the first manual you linked to gives a description and part number for VR101: DIODE, ZENER 8.2V, IN756A  (DO-7).I'm not saying it's unlikely, just that anything connected to V- and a positive rail could be responsible for this. Were those diode measurements you gave for VR105 in circuit? That's not very reliable. You can indeed desolder one leg and test it with a resistor + voltage source.If you suspect a component, then cutting or desoldering its connection to V- should restore the rail to -15 V (this of course does not apply to VR105). I'm guessing R132 probably gets quite toasty with ~16 V across it?I haven't looked at the schematic in detail. If a lot is powered via the -9 V / -6.4 V rails, then you could try isolating the -9 V section and see if the V- rail comes up (goes down) to -15 V. For example by desoldering one leg of VR101.If there are tantalum caps in there, then a shorted tantalum cap would definitely be one of the candidates.

author: staze
author_rank: Frequent Contributor
timestamp: August 06, 2013, 07:16:37 pm
content: Okay, I must be blind. Where did you find the part number? The manual with schematics have hand written notes with keithley numbers. Oh... the manual without the schematic... weird. It's like one is pre-production.I'll check the diode this evening, but yeah, seems possible something is pulling it down. Would I see a short between the rails if something like an opamp was bad? There are a couple tantalems, but not sure if they are on that rail.

author: staze
author_rank: Frequent Contributor
timestamp: August 06, 2013, 09:39:18 pm
content: okay, it's not that diode. so, I'll have to check for what might be dragging it down. should be able to, like you say, disconnect VR101, see if that helps. Do some devices have their rails buffered so you don't get stuff like this happening (one device pulling the whole rail down)?

author: Unknown
author_rank: Unknown
timestamp: August 06, 2013, 10:36:41 pm
content: You can of course have multiple power rails, like the analog and digital rails, but in something like a DMM there's not very useful to keep supplying the uC with power if the input stage is not working. Like I said, some equipment (I recall seeing it in some Tek scopes) feature jumper links to make it easy to isolate sections.

author: staze
author_rank: Frequent Contributor
timestamp: August 07, 2013, 02:32:19 am
content: alm,if you take a look at the schematics, could you please explain how I could separate the negative rails? I can disconnect VR101, or VR102, but that seems like it would just raise the negative rail up, rather than potentially isolate them from each other. I'm slowly looking at everything attached to the negative rail to see if there's a short between ground, and the negative rail... so far nothing. I don't see the tantalem's being connected to the negative rails at all, so that seems unlikely. One thing I note is there's a tantalem cap on the board that doesn't exist, according to the schematic. It's right below R103, and is listed as COK 1.5C 1KV. =/

author: PedroDaGr8
author_rank: Super Contributor
timestamp: August 07, 2013, 02:52:07 am
content: Quote from: staze on August 07, 2013, 02:32:19 ampretty sure that's NOT a tantalum as IIRC they don't go up to 1KV. Likely a ceramic instead.

author: staze
author_rank: Frequent Contributor
timestamp: August 07, 2013, 02:53:32 am
content: ah, okay. it LOOKS like a tantalem (orange blob), but guess it could be.Pedro, How's the 199 repair? Get the replacement zener?

author: Unknown
author_rank: Unknown
timestamp: August 07, 2013, 09:13:38 am
content: Quote from: staze on August 07, 2013, 02:32:19 amUnless I'm missing a connection in the schematic, it seems to me that if you disconnect VR101 the -6.4 and -9 V rails should go to ground and should be disconnected from V-. If something on the -9 V rail is shorted, then this should allow V- to return to -15 V.Quote from: staze on August 07, 2013, 02:32:19 amMaybe I misinterpreted something, but I thought you mentioned the V- rail was +0.8 V. How would you get +0.8 V from a short between ground and V-?

author: staze
author_rank: Frequent Contributor
timestamp: August 07, 2013, 06:52:36 pm
content: Quote from: alm on August 07, 2013, 09:13:38 amTrue. It seems likely it's more like a -9V and +10V short... but I'm not seeing anything that would indicate that's the case. I'll try unsoldering one leg of VR101 and see if that brings the negative supply back up. If not, I'll try VR102 (with VR101 reattached), which should give me my -9V, but kill the -6.4V rail. Really need a board size panavise for this type of thing... this board is a pain in the but to work on. =PAlso of note, and I still can't figure out why, I'm seeing some (what looks like 60Hz) ripple on the cathode side of VR104. I checked C124 (out of circuit), and it seems fine for both ESR and capacitance. So not sure why it's not filtering that out. Could it be due to the issue with the negative rail? Anyway... one thing at a time.

author: Unknown
author_rank: Unknown
timestamp: August 07, 2013, 07:08:55 pm
content: If lifting VR101 does not help (then lifting VR102 won't do anything either), then I would check CR107 and S101. I wonder if it's something stupid like bad switch contacts or an open rectifier. Do the signals on S101 pins 11 and 5 (or any convenient location connected to the same net) look reasonable with the power turned off? Pin 5 should give a smooth ~ 15 VDC signal, pin 11 a rectified sine with ~ -18 V amplitude relative to ground. What about resistance with the cord unplugged and the power switched on?Yours doesn't have the battery option, does it?

author: staze
author_rank: Frequent Contributor
timestamp: August 07, 2013, 07:12:55 pm
content: Quote from: alm on August 07, 2013, 07:08:55 pmIt does not have the battery. I was wondering about the switch, actually. My main question is, how the hell are the pins numbered? There's one tall pin, which I gather is pin 6, but does it continue around clockwise, or does it return to the top and go down again? I would have liked to see pin numberings on the switch itself, but alas, there are none, and the drawing doesn't show any either. =/

author: Unknown
author_rank: Unknown
timestamp: August 07, 2013, 07:45:10 pm
content: You can also measure at CR107 or some other convenient point.

author: staze
author_rank: Frequent Contributor
timestamp: August 07, 2013, 09:13:30 pm
content: Ah, sorry, I forgot CR107 was the rectifier. Yes, I get +15V rectified on the positive side, and -18V rectified on the negative side. So that is working. Like I mentioned, I'm seeing that -18V on R132, so it's definitely coming out of the rectifier.From my looking around, it seems like the pinout of the switch is:top7 18 29 310 411 512 6Where the "top" is the pushbutton, and this is looking down at it on the component side, rather than the solder side. A bit later I'll disconnect VR101 and see what we get (if the negative rail comes back up/down or not).

author: staze
author_rank: Frequent Contributor
timestamp: August 07, 2013, 11:28:02 pm
content: Disconnected VR101, no change. -15V rail is still being pulled down. So it's not anything on the -9V or -6.4V rail, it's got to be something on the -V rail.So, looks like the stuff on -V are: Q127, Q128, U107, and S101... that's pretty much all I can see. What's the deal with Q127? It's just a NPN base attached to -V, and a Collector attached to the circuit, with the emitter not connected.Thoughts?

author: Unknown
author_rank: Unknown
timestamp: August 08, 2013, 07:53:50 am
content: Quote from: staze on August 07, 2013, 09:13:30 pm+15 V rectified? This should be pretty much DC, since there's a cap (C124) and no load.Looks like they use Q103/Q127 as protection diodes to prevent the voltage on the sense line from exceeding +V + 0.6 V and -V - 0.6 V. Remember that a BJT looks like two diodes from B to C/E (the other way around for PNP), although the collector-base diode is reverse-biased during normal operation. No idea why they use this instead of regular diodes, could just be something simple like BOM size. This could actually be where the +0.8 V is coming from, but that means something else is pulling it high. What's the voltage on the collector of Q127?What's the resistance between -V and +V? I see nothing obvious between the various positive rails and -V.

author: staze
author_rank: Frequent Contributor
timestamp: August 08, 2013, 09:58:18 pm
content: Quote from: alm on August 08, 2013, 07:53:50 amThe positive rail has about 1.2Vpp ripple around 15V. The negative rail has 2.2Vpp ripple. Both of which seem pretty high since that filter cap is in there. I kind of figured the ripple was due to the negative rail being "shorted" so the cap was basically being loaded down.Quote from: alm on August 08, 2013, 07:53:50 am0.08V. Little noise, but otherwise, nothing.Quote from: alm on August 08, 2013, 07:53:50 amStarts at about 15kR, and ramps up to 25-30kR. Not positive why that would happen? Capacitance? That's measured from the anode of VR105 to the cathode of VR104.Also of note, on power up, the negative rail is about 0.2V, and it slowly ramps up to 0.6V or something (let it sit about 3 minutes and that's how far it got). I'm guessing it would keep going if I left it long enough. I'd almost wonder if C127 was shorted, but then I wouldn't get the -18V on that side of R132... so, hell if I know.I've hooked VR101 back up at this point... since it didn't seem to make any difference. Arg...

author: staze
author_rank: Frequent Contributor
timestamp: August 08, 2013, 10:19:53 pm
content: Okay, interesting... was just basically touching random things with my multimeter, and interestingly, the vent on the top of C127 is at -15V.... pretty sure, that shouldn't happen. As C124 is a bit below 0V, but nothing major.So... ?Yeah, just tested with a new cap, same thing. So, is the vent supposed to be attached to the negative lead?

author: Unknown
author_rank: Unknown
timestamp: August 08, 2013, 11:50:01 pm
content: Yes. Also the case for caps without foil around them. I once drew some sparks and slightly scorched a scope probe because I didn't know the cap was for the negative trails, so the negative terminal was not grounded.

author: staze
author_rank: Frequent Contributor
timestamp: August 09, 2013, 02:45:30 am
content: Quote from: alm on August 08, 2013, 11:50:01 pmOkay... that's good to know. So, thoughts were I should check next? At this point, I'm thinking of pulling Q128, or at least the collector. It's a pretty jellybean part (3906), so just cutting it off would work too. Because at this point, it's either that, U107, or I guess Q127. I would think if Q127 was it, I would see a short between it's collector and base... And with Q128... I don't see how it would be grounding out the rail (or bringing it down), since it should only be connected to other negative rails, but I'm just guessing. Is there something I'm missing? Some other component attached to -V?I don't have a thermocouple (yet) so I can't really measure temperature of components, but C127 and/or R132 do seem warm. Guessing R132 is heating up C127.

author: staze
author_rank: Frequent Contributor
timestamp: August 09, 2013, 05:31:50 am
content: Well, it's not Q128 or Q127. Disconnected the collector on Q127, no change. And removed, then replaced Q128, no change with either. So, it's down to two options.1. It's U107, which I don't have a replacement on hand. It's a TCL271CP. Or...2. Removing VR101 didn't kill the -9V and -6.4V rails since I didn't also remove Q128.

author: Unknown
author_rank: Unknown
timestamp: August 09, 2013, 08:33:16 am
content: Quote from: staze on August 08, 2013, 09:58:18 pmI originally suggested measuring it with the power turned off. Then C124 is energized, but disconnected from the load.Quote from: staze on August 08, 2013, 09:58:18 pmCapacitors charging up. Nothing interesting, 15k is way too high to pull the rail up to 0.8 V.I don't really see the point of replacing parts before you've fixed the problem, it's not like putting them back is going to help. What you can do is cut the V- pin of U107 close to the body, but leave a little metal sticking out of the body. You can then easily resolder this connection later on. If this doesn't help, remove VR101, Q127 and Q128 again. If that doesn't help, go searching the PCB (is easier than the schematic) for other parts connected to -V. For example a short on the PCB, or some other component that we missed in the schematic.

author: staze
author_rank: Frequent Contributor
timestamp: August 09, 2013, 03:35:18 pm
content: Quote from: alm on August 09, 2013, 08:33:16 amMainly replaced it because I couldn't get the original back in (the legs were all bent at the factory to be lower profile, but it made for re-installation to be rather problematic.I've traced the -V rail pretty well at this point, and those components seem to be it (save S101). I don't see any shorted/broken traces, and the resistances work out. I'll see if I can snip that lead (it's in a bit of a bad location). Any harm in snipping it, yet leaving Q128 attached (it would have -18V on it's collector, but nothing on it's base).Thanks!Thanks!

author: Unknown
author_rank: Unknown
timestamp: August 09, 2013, 03:52:44 pm
content: Leaving Q128 should be fine as long as it's not shorted.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: August 09, 2013, 06:17:52 pm
content: Well,This and The Older 199 Thread Is Pure #$%#%#% I Love It Guys .You Are Making Me Buy These And Get It All The Way To India For Repairs .I Hate You Buggers.Lets See How You Crack This One ,Would Be a Awesome Effort Like Last Time" Best Of Luck "

author: staze
author_rank: Frequent Contributor
timestamp: August 10, 2013, 01:47:27 am
content: Bingo. Cutting the negative lead (pin 4) on U107 (op-amp) brought the negative rail back up (down). It's only at -12V, but that's certainly better than it was.Know of a good replacement, or should I just try to find a replacement? They don't seem to be a very common part. Mouser has them, but shipping and all that.Any thoughts before I order the part? I'm going to need to no matter what, since I pretty well mangled the pin trying to cut it (my flush cutters aren't really small enough to get in that space.Though, just realized that I should have used an X-Acto knife, or the like, to cut the pin... probably would have mangled it less. Ah well.

author: Unknown
author_rank: Unknown
timestamp: August 11, 2013, 12:21:20 am
content: Given its function and location, I can't see this component being very critical for either correct operation or accuracy. Almost any op-amp that accepts this supply voltage would probably work fine in this position, at least for testing.

author: staze
author_rank: Frequent Contributor
timestamp: August 11, 2013, 12:39:19 am
content: Quote from: alm on August 11, 2013, 12:21:20 amMakes sense. I don't think I have anything that'll take that range. I'll just order the replacement, and swap out the current op-amp for a 8 pin socket.Here's hoping!

author: staze
author_rank: Frequent Contributor
timestamp: August 11, 2013, 04:12:43 am
content: So, removed the dead op-amp, and installed a socket. Tried dropping a jellybean LM741 in there, and no love (not sure if that LM741 likes single supply or not). So, guess I'll wait for the true replacement from Mouser. Hopefully that brings it back to life.Does it make sense that without that op-amp, the voltage across VR102 would be +6.2V rather than the stated -6.4V? Guessing it would be because Q128 isn't pulling it down without the op-amp driving it's base.

author: Unknown
author_rank: Unknown
timestamp: August 11, 2013, 01:24:55 pm
content: Something could be pulling up the -9 V rail (or -6.4 V rail) through its ESD protection diodes. With VR101 disconnected, feeding 9V from a current-limited bench supply to the emitter of Q128 should restore the proper voltages, I think.

author: staze
author_rank: Frequent Contributor
timestamp: August 11, 2013, 05:14:27 pm
content: Quote from: alm on August 11, 2013, 01:24:55 pmAny idea what I should limit the PSU to (current wise)? 50ma or so?

author: staze
author_rank: Frequent Contributor
timestamp: August 17, 2013, 12:29:16 am
content: So, good news and bad news.Good news: opamp was bad. Doesn't pull the negative rail up/down to ground anymore.Bad news: something is still wrong. The -6.4V reference is a +6.2V, and the -9V rail is up around -4.6V. The -V rail at the op-amp is at -12.8V, but at the filter cap, it's at -16V or so. So something is still pulling stuff down/up. Guessing this is due to the -6.4V reference being all wonky, and wondering if something broke when I had the opamp out (or the 741 in) and powered things up. Not sure why that would be though.Any thoughts?

author: staze
author_rank: Frequent Contributor
timestamp: August 17, 2013, 04:11:22 am
content: interestingly, I'm seeing about 2.5V on the collector of Q127 (when in volts mode), or about 4.8V in ohms (which seems normal). Am trying to figure out where that voltage might be coming from.I'm just going to go back through the voltage checks and see what's within spec, and what's outside. I also note, according to the schematic, all the buttons should be "out". I'll do that, and see if that changes anything as well. But obviously something else is blown, and maybe that took out the opamp to begin with. =/

author: staze
author_rank: Frequent Contributor
timestamp: August 17, 2013, 05:20:30 am
content: So, still issue with negative rails all around. For some reason, the negative rail seems to be at -19V on Pin 12 of the power switch, but once it gets to U107, it's at -12.8V, and there's a fair (about 400mV+) ripple on that rail on U107. I would think that should all be smoothed out by C127.Other negative rails are as described before. So... I have next to zero clue what is going on. Part of me wants to replace VR105 with a LM7915, but I'm not 100% on how I would wire that. Could VR101 just be toast?

author: staze
author_rank: Frequent Contributor
timestamp: August 17, 2013, 06:12:26 am
content: I keep looking at U104 as a possible, as well as the Q103/Q127 pair.U104 is interesting because the resistance between pin 3, and pin 6 is only 36KR. Which, looks like it should be higher than that since I don't see how they're connected. Also, pin 4 to pin 7 is only 7.5KR. Which also seems wrong.The interesting part of the Q103/Q127 pair, other than why the heck they're there (I know, you said, but it still doesn't make a whole lot of sense), is why there's +5.6V on the base/collector of Q103/Q127 respectively. That's leaking in from somewhere... but hell if I can tell where. But this could all be a big while goose chase... and the real issue is staring me in the face.I'm off to bed.

author: staze
author_rank: Frequent Contributor
timestamp: August 17, 2013, 11:37:49 pm
content: well, not Q127.so... try pulling VR101 again? All the positive rails look perfect. U101 is the only thing I can see on the -6.4V rail... but it doesn't seem right that it would be flipping the -6.4V to +6.2V, unless it's ALL screwed up and taking that 2.5V on it's pin 5, plus it's +10V on pin 7, and dumping it ALL into it's negative rail.I'm having a hard time finding anything that says what one should expect, impedance wise, between the rails of an opamp, or the input and output, etc.Thanks!

author: staze
author_rank: Frequent Contributor
timestamp: August 18, 2013, 12:56:30 am
content: U101, for interest, shows the following on it's pins.1. 6.308V2. 6.308V3. 5.8V4. 6.304V5. 5.864V6. 6.125V7. 6.125V8. 9.995VNow, it LOOKS like a dual op-amp, but the manual lists it as an IC, and I can't find anything with the numbers on the chip. What's interesting is it seems to be a bit "off", when it comes to input/output. Guessing that's just because the negative rail is not right, so it's slamming into that limit (though, it seems to be using the lowest voltage available for it's negative rail, rather than the real negative rail (since the output on pin 3 is lower than the true negative rail).

author: staze
author_rank: Frequent Contributor
timestamp: August 18, 2013, 03:17:27 am
content: actually wondering if the integrator might be the issue now. Looking at the "ripple" on the negative and positive rails, it kind of looks like integration. Will fire up the scope tonight, tomorrow, or next week (as baby allows) and take a look. But please, if you have other ideas/suggestions to check, let me know. I'm kind of bouncing around, I know. I'd cut all the negative rails on devices (one by one), but I haven't figured out a good way to do it (x-acto knife didn't work very well, and flush cutter just mangled the lead).Thanks alm!

author: free_electron
author_rank: Super Contributor
timestamp: August 18, 2013, 03:31:01 am
content: TLC27L2i have a 197 so i opened it to see what mine says.

author: staze
author_rank: Frequent Contributor
timestamp: August 18, 2013, 03:50:49 am
content: Quote from: free_electron on August 18, 2013, 03:31:01 amSorry, U101 (by the relays) is a TLC27L2 for you? I know U107 (by the input jacks) is a TLC271CP in mine. Any chance you want to check what you get on pin 4 of U107? Or the emitter of Q128?

author: staze
author_rank: Frequent Contributor
timestamp: August 18, 2013, 04:55:31 am
content: For interest, this is what my -9V rail looks like at this point (measured on Pin 4 of the integrator). But, obviously this isn't the problem, but may be a symptom.

author: staze
author_rank: Frequent Contributor
timestamp: August 19, 2013, 02:45:38 am
content: So, interesting symptom...Put the unit in diagnostic mode, DC, 2V range. Shorted in inputs for good measure, and looked at pin 6 of U102 (the multiplexer).My understanding from the block diagrams is that the mux flips between signal, ? (says 0V for 20V range, doesn't say what it is for 2V range), the 2V reference, and 0V). Great.Problem is, on every diag mode, pin 6 sat at -2.08V or so. No change. So, it would seem that maybe U103 (which seems to be what controls the Mux's FETs) isn't working? Is this just because the -9V rail isn't working, or could it be that U103 is the cause? It's just a jellybean LM339, so cutting a pin wouldn't be that big a deal, but it does seem odd that NOTHING would happen.

author: timb
author_rank: Super Contributor
timestamp: August 19, 2013, 07:35:00 pm
content: Cool thread and very timely! I just scored a 197a on eBay for $85USD, looks pretty clean from the picture. Should be here end of the week!I couldn't resist at that price. If it works, I'll calibrate it with my Tek 4030 as a transfer standard. A DMM with microvolt resolution is always good to have around! (At least that's how I justify these type of purchases.)

author: staze
author_rank: Frequent Contributor
timestamp: August 19, 2013, 07:50:01 pm
content: hopefully it does work! I do wish I could get mine running. My Keithley 197 is great (though I could use to have it calibrated).Just need to figure out what's f'ing with the negative rail.

author: Unknown
author_rank: Unknown
timestamp: August 21, 2013, 02:17:52 am
content: Anything connected to the -9V / -6.4V rail may act dodgy as long as that rail is out of spec. You can either cut pins / tracks to isolate sections, or try to figure out where the current is going. To pull up the negative rail, a fair amount of current needs to be flowing from a more positive rail. Maybe you can find this point? The more positive the negative rail, the closer you are to the culprit. Of course if the negative rail is positive, ESD protection diodes may start conducting and make life interesting. I would consider powering the -V or -9V rail from a lab power supply again (make sure you disconnect the zener). This will allow more current to flow. You may then be able to use voltage drop (the more positive the closer to the culprit) to find the culprit. If only you had a high-resolution DMM. It might be something stupid like a short somewhere on the PCB.If the -9 V rail is that unstable, then either something's drawing a serious amount of current (does any part feel hot?), or the -9V rail has a high impedance. Does the same happen to the -6.4V rail? Labeling the circuit around U107/Q128 (where these rails are generated) with voltages and studying that may provide some clues. Resistors are convenient gauges to indicate current direction and quantity. Also check with a scope: is anything oscillating? What's the cause and what's just an effect? Any way to verify the operation of the -9V / -6.4V circuit without the load connected?

author: staze
author_rank: Frequent Contributor
timestamp: August 21, 2013, 04:54:04 am
content: Quote from: alm on August 21, 2013, 02:17:52 amalm,Thanks very much. I'll check this once I get my "lab" back (mother-in-law in town). I kind of think the -9V rail looks like integration from the integrator (which makes me look at anything that could act like an integrator that's on that rail (U107 (just replaced), U102, U110 (the actual integrator))... but maybe that's just me. Nothing feels hot except R132 (since it's dropping like 7V at this point), Q123 (guessing that's why it's in a TO-220 package), and Q122 (a FET that drives the +10V rail, so maybe that's a symptom of the current draw).I do note that the -6.4V rail is completely reversed to +6.2V, which makes me wonder if something that's flipping it is getting both the +10V rail, and the +2V reference (which makes me look at things like U102 and U101).Hadn't thought of just looking for where the most positive part of that negative rail is.I don't know of a way to remove the load on the rails... I could pull VR101, but that would kill the whole shebang except the -V rail itself (well, and whatever U107 and Q128 would do with their remaining -V connection.Hmmm... grumble.

author: Unknown
author_rank: Unknown
timestamp: August 21, 2013, 09:49:37 am
content: Are you sure you didn't configure U107 as an inverting amplifier?

author: staze
author_rank: Frequent Contributor
timestamp: August 22, 2013, 05:21:23 am
content: Not unless something very weird happened. =PThat said, I haven't tried checking to make sure the TLC271CP I replaced (U107) isn't bad. *shrugs*

author: staze
author_rank: Frequent Contributor
timestamp: August 23, 2013, 12:40:02 am
content: Quote from: alm on August 21, 2013, 02:17:52 amOkay, in a position to work on this a bit more, and a couple questions.1. When you say power the -V or -9V rail via a PS, and to disconnect the zener, I would assume you mean disconnect the cathode, then hook the PS up to the circuit referenced to gnd (which is easy on the 197 since they're all a shared ground). So for the -V rail, try disconnect VR105, or with the -9V, disconnect VR101? What should I limit the current to?2. Measuring the voltage relative, as in, where it's the most positive, I would measure that relative to the cathode of the zener for that rail, yes?So, for example, let's say I disconnect the -9V zener, hook the negative rail of my PS to that spot, positive size to ground. That should leave the -9V and -6.4V running (though I'm still confused as to what U107 and Q128 are going to do in that case, since they're getting power from -V). Then, clip the positive probe to the negative rail of the PS, and go and probe all the -9V rail, and see where things get higher than at the PS lead. Highest spot should be my problem.Or if I wanted the -6.4V rail, just disconnect VR102, and do the same.My biggest question is, what current do I limit the PSU to (though, I suppose if I DON'T limit it, I could do the whole "smoke it out" method, though my PSU only does 2A, which might not be enough to smoke anything)?One question I have is the bridge rectifier is measuring +15V on one side, and -19V on the other. Is that "normal"? I would think the zeners for the rails would pull both down to ±15V.Okay, think that's it for now. I'm going to try doing some probing tonight, but it all depends on my son's sleep patterns. =)

author: Unknown
author_rank: Unknown
timestamp: August 23, 2013, 12:09:08 pm
content: Quote from: staze on August 23, 2013, 12:40:02 amFor V- I would disconnect VR105 and R132, for -9V disconnect VR101 and Q128. Incrementally increase the current, monitor the voltage and check if anything gets hot. If anything gets hot, then you've likely found a/the culprit. A thermal imager would also be very helpful for this.Quote from: staze on August 23, 2013, 12:40:02 amI would measure relative to ground, although since you're only looking for relative differences, it doesn't really matter as long both are stable. Note that I would try measuring voltages before going through the trouble of disconnecting rails. Maybe the voltage drops are already large enough with the current current (no pun intended). Especially if you could use some sort of 5.5 digit DMM.Quote from: staze on August 23, 2013, 12:40:02 amYes, you should also disconnect Q128. Don't really care about U107 at that point.Quote from: staze on August 23, 2013, 12:40:02 amI would also disconnect R125J. I would probably start with the -6.4 V rail and work my way up. It may very well be that the -9 V rail recovers if you power the -6.4 V rail seperately, then you've localized the problem to the -6.4 V rail.Quote from: staze on August 23, 2013, 12:40:02 amThe only thing I would be worried about killing are PCB traces. You're limiting the voltage to that nominal for that rail, so no component should damaged. Anything drawing enough current to be damaged is likely already dead anyhow. But based on the schematic I would expect the current draw to be very low: all current for the negative rails has to flow through R132, a 330 Ohm resistor, so they are unlikely to be designed to draw more than maybe 20 mA. You can measure the voltage across R132 to calculate the current it's drawing now. I certainly wouldn't go up to 2A, I expect things to get hot well below that. Something like 100 mA would be unlikely to damage any PCB traces, but would produce a ridiculous voltage drop of 33 V across R132.Quote from: staze on August 23, 2013, 12:40:02 amVR104 will pull the voltage on its side of R131 to +15V, and will drop the rest of the voltage across R131. So I would expect the voltage at the rectifier output to be a little higher that +15V. I would expect at least 100 mV or so drop across R131, possibly with an AC component.

author: staze
author_rank: Frequent Contributor
timestamp: August 23, 2013, 06:03:19 pm
content: Quote from: alm on August 23, 2013, 12:09:08 pmA thermal imager would be very helpful for many things. =PQuote from: alm on August 23, 2013, 12:09:08 pmYeah, figured I'd start that way. =)Quote from: alm on August 23, 2013, 12:09:08 pmGood point.Quote from: alm on August 23, 2013, 12:09:08 pmThat becomes a bitch, if I have to pull that resistor network.Quote from: alm on August 23, 2013, 12:09:08 pmokay, I think.Quote from: alm on August 23, 2013, 12:09:08 pmOkay, but what about the negative side of the rectifier producing that -19V. Is that just because the -V rail is running at -12V vs -15V?okay, so, I'll start with just looking at voltages on the -6.4V with everything still connected. -6.4V isn't connected to much (U101, R117C). Though, like you said, since that rail is completely reversed at this point, things are going to be conducting weird. Pretty sure U101 is going to be where that voltage is higher (unless I'm missing somehting else that aril is attached to).-9V should be easier. Though, now that I think about it, the closer to the issue I look, shouldn't the -9V be MORE negative rather than positive? So if at the zener it's -4.8V or whatever, at the source of the issue, it might be down at -5V (or whatever)? There are quite a few things on the -9V rail.I'm really hoping it's U103, U112, or U109, etc. Some jellybean part. I was looking at options for replacing something like U110, and having a hell of a time finding something that would work (main issue being slewrate). *sigh*I'll poke at things tonight. Try to keep good notes.

author: Unknown
author_rank: Unknown
timestamp: August 23, 2013, 07:05:23 pm
content: Quote from: staze on August 23, 2013, 06:03:19 pmThe transformer is producing a higher voltage than the regulated voltage, and the excess is dropped across R132, that's how a zener regulator works. As long as the voltage is high enough so enough current can flow through R132 to keep VR105 conducting and the rest of the circuit powered, you don't care. The magnitude of the voltage is likely higher because the load is much lower (compare R131 to R132).Quote from: staze on August 23, 2013, 06:03:19 pm-6.4 V is the reference for the -9 V rail, so if the -6.4 V rail is at +6.2 V, then Q128 is not conducting, and both the -6.4 V and -9 V rails will be off. Something is still pulling current from the -9 V rail, so VR101 is conducting and is dropping about 8 V, resulting in a -9 V rail that's 8 V above the V- rail (which is also overloaded through VR101). Of course with -4.8 V across the series combination of R125J and VR102 will not make VR102 conduct, so that rail is essentially floating. Most likely some current is flowing from the devices that are connected to this rail through R125J, dropping -4.8 - 6.2 V across R125J (about 1 mA). So it seems that something is loading down the -9 V rail by feeding current into the -9 V rail from a more positive rail (ground or one of the positive rails). The closer you get this something, the closer you get to the positive rail, and the more positive the voltage will be. After this analysis, I think you can ignore the -6.4 V rail. It's likely one of the many devices on the -9 V rail that's the culprit.The idea behind using a sensitive multimeter and forcing more current through it is that PCB traces will drop more voltage the more current flows through them. So you are using the PCB traces as current shunts.Did you already make a thermal image with the Mk. I finger to see if anything gets hot?

author: staze
author_rank: Frequent Contributor
timestamp: August 23, 2013, 07:52:37 pm
content: Quote from: alm on August 23, 2013, 07:05:23 pmOkay, that makes way more sense. I thought it would be more negative since things go back to ground, but I guess since the offset is coming from the positive rail... =/I measured around with my multimeter temp sensor, and the only things that were hot that I found were listed earlier (though, I was mainly focused on ICs and diodes).So, should I just skip trying things as they are, disconnect VR101, and Q128, and feed the negative -9V rail with my PSU with like 50mA or so, and slowly ramp that up (to 100mA or so)? Or start how we are and just see where I get looking at voltages? I just printed out the schematic on 11x17 (A3) so I can easily mark voltages.Yay having a 5.5 digit multimeter.

author: Unknown
author_rank: Unknown
timestamp: August 23, 2013, 08:06:39 pm
content: The ground is also positive relative to the negative rail.I would probably start measuring voltages. Start at the cathode of VR101, and see if you can measure a voltage drops across any of the traces originating from there. If they're all essentially zero, try disconnecting the zener/transistor and increasing the current. Current is now approximately 20 mA, so 50 mA sounds like a good starting point. Any part that gets hotter as you increase the current is suspect.

author: staze
author_rank: Frequent Contributor
timestamp: August 23, 2013, 08:16:07 pm
content: Quote from: alm on August 23, 2013, 08:06:39 pm"part" being IC's, I would assume. I would imagine resistors are going to get hotter. =)Does it make sense to measure at VR101 cathode, then hit all the IC's connected to that -9V rail? I would assume one should be higher. Then start looking around the trace between that and VR101 to make sure it's the highest point? Or am I just giving myself more trouble since I'm skipping resistors, and the like? Can I assume that the point of "leakage" is going to be substantially (1V or more) higher than the -4.8V that I'm seeing at the cathode of VR101, or are we thinking it's going to be rather minimal and the balance point just happens to be around that -4.8V?

author: Unknown
author_rank: Unknown
timestamp: August 23, 2013, 08:41:03 pm
content: Quote from: staze on August 23, 2013, 08:16:07 pmTechnically yes, but I don't expect any of the resistors to get hot, since I don't expect any low value resistors across the -9 V rail.Quote from: staze on August 23, 2013, 08:16:07 pmYou can also probe the ICs. But keep the PCB traces in mind, since these are going to drop the voltage. Power pins on multiple ICs all connected to the same plane are going at the same potential. I haven't checked the PCB layout. Long skinny PCB traces make this much easier than fat traces and polygon fills.Quote from: staze on August 23, 2013, 08:16:07 pmEven if that 20 mA is all flowing through the same part, then 1 V drop would require 50 ohm of resistance. No PCB trace is going to be 50 ohm. So you're most likely looking at millivolts, which is why I'm suggesting that you may need to increase the current.

author: staze
author_rank: Frequent Contributor
timestamp: August 23, 2013, 09:31:22 pm
content: Quote from: alm on August 23, 2013, 08:41:03 pmOkay, that makes sense. It's a doubled sided pcb, so there's no power "plane". But it's not a star topology really, so I'm not going to see one leg then another. I guess I'm in for some tracing. Most of the traces are long and skinny, and snake from one part to the next, basically. I wish the component layout image was a bit higher detail... looks like of like a scan of a copy of a copy. The traces are pretty much illegible.So you're saying U104 pin 4 is going to read the same as U103/112 pin 12, or U110 pin 4? I don't doubt you, I just don't understand how that could be the case. I would think the problem component would have it's -9V connection be up at say -4.75V, vs others being down from that. It should be the "high" spot. Or at least direct me to look at the traces in that area (since it could be a short, or something... though I would think a true short would do more than just bring it up to -4.8V).Sorry, I know I'm asking all these questions without the board/meter in hand... I'm just trying to understand.

author: Unknown
author_rank: Unknown
timestamp: August 23, 2013, 10:13:53 pm
content: There is no -9 V point, there is -4.8 at the cathode of VR101, and it only goes up from there, assuming my analysis is correct and Q128 is not conducting. If two pins are connected by a trace, than the voltage between those two pins follows from Ohm's law. So if it's 20 mA through a short, fat trace with less than 1 mohm resistance, then the voltage drop between those points is going to be less than 20 uV. A long skinny trace with a resistance of 0.1 ohm will create a much larger voltage drop, however.From the PCB layout it looks like these parts are not that close together, so I think you may be able to measure a drop between those. Though maybe not without increasing the current.

author: staze
author_rank: Frequent Contributor
timestamp: August 23, 2013, 10:48:21 pm
content: oh, one other question...how long should I let the unit warm up before doing the measurements? just so things, hopefully, stabilize? I've been worried about leaving it on TOO long in it's current state thinking something hard to replace might get toasted...

author: Unknown
author_rank: Unknown
timestamp: August 23, 2013, 10:55:47 pm
content: I don't think you need much of a warm up. You're mostly looking at voltage drops across traces, not absolute voltages. And parts that overheat will get hot within seconds.

author: staze
author_rank: Frequent Contributor
timestamp: August 25, 2013, 06:48:30 am
content: Okay, so, measured all the -9V egress points, and they all are basically -4.2V (some a bit higher, some lower). Hard to tell, becasue the bitch is, that rail is oscillating like crazy, so it's hard to get a good reading. So... going to have to power directly, I think. Guessing it's oscillating since the -V rail is as well. Doh!The only thing definite I found is VR101, is pretty warm. After a couple minutes running, it was over 45C (everything else I measured except Q123, and R132 were down around 27C (a bit higher than the room temp (~25C)). That seem normal?Once I get a chance, I'll pull VR101, and Q128, and try powering the -9V rail directly from my PSU at like 20mA or so, at first. See where that gets me. Hopefully I can do that tomorrow...Thanks!

author: staze
author_rank: Frequent Contributor
timestamp: August 26, 2013, 01:44:24 am
content: Alrighty, progress, I think.So, seeing things oscillate, I pulled Q128, and VR101 (anode side), and hooked up my bench PSU to the former location of Q128's emitter.I set the PSU for 20mA, and fired everything up. PSU goes into current limit mode, and is only pumping about -1.8V. I have to crank it up to 40mA to get a solid -9V out of it. Hmmm. Though, PSU says only 30mA, but I'm guessing that's because it doesn't have that 1mA resolution. Hooked the 199 in series, and yeah, it's drawing 27.5mA (and change). Guessing my PSU is not 100% accurate on it's amp reading.So, checked a few of the -9V rail points, and they're all at -9V, and not oscillating. Cool. The -V rail is back at -15V too.But, meter still shows OL, until I switch to the 200V range, in which case it reads 63-70V oscillating all over. Awesome. (note, after about 5mins running, it's settled around 58V, with a couple volts oscillating... and yes, the inputs are shorted).Check the -6.4V rail. +6.2V. That's... still not right. Which, might make sense since U101A/B look like they're involved in the mux switching. I also note that R125J is relatively warm. It also only reads 1.02K rather than 1.1K. Obviously that's only 80R, but I'm not sure how accurate these resistor networks should be.So... thoughts? Am I looking at U101 being bad (I hope not, since I can't seem to find what it actually is). Schematic seems to indicate a dual op-amp, but the parts list just says it's an integrated circuit. It LOOKS like it's labeled SN103152P (TI, obviously). I don't see anything else on that rail, but I could be blind.So, then I went back through the voltage checks. U109 pins 2 and 3 look good, +6.4V on both pins 2 and 3. The -6.4V shows up on pin 2 of U107, but I see +6.2V on pin 3. Which makes sense since it's coming from that zener. Could that zener be bad? Also interesting, pin 12 of S101 shows -19V... is that just because I'm largely bypassing the -V rail at this point?I did see that -6.4V at +6.2V with both U107 in, and out of circuit. So that doesn't seem to be it. Either U101 is bad and pushing it up, or that zener is bad. =/ I don't have a second bench PSU to feed that rail with though... so, maybe hook VR101 and Q128 back up, and disconnect VR102 and inject -6.4V?Thoughts?

author: staze
author_rank: Frequent Contributor
timestamp: August 26, 2013, 02:15:55 am
content: So I just realized... why the hell are pins 2 and 3 on U107 different? I'm no opamp expert, but my understanding is they should pretty much always be the same (assuming they're within the rails, etc).I'm guessing this must be because U101 (or some other device I'm not seeing) is pushing back hard enough that it's swamping out the ability for pin 3 to come down to that voltage (since pin 2 is set at the -6.4V by a voltage divider off the negative rail)?If that's the case, I'm guessing that's what's causing the -9V rail to oscillate (if those two inputs are that far off, and U107 is basically set up like an integrator, it's pushing Q128 to be on/off ramped like that. So that would explain why the -V rail (and -9V rail) look like integration, no?

author: Unknown
author_rank: Unknown
timestamp: August 26, 2013, 02:31:49 am
content: I don't have the schematic in front of me, but I don't think U107 has feedback with Q128 disconnected (its output is floating), so pins 2 and 3 may be different (and the output will be close to either its positive or negative rail).So my initial analysis was wrong, and it's not just that VR101 did not have enough voltage across it. So there's a huge voltage drop across R125J? This means that something is drawing too much current from the -6.4 V rail. I wouldn't worry about the accuracy of R125J, it just has to provide an approximate current for the zener and whatever else is connected to the -6.4 V rail. If that's just U101 (I haven't checked, but I expect you did a thorough job), then that is the likely culprit unless there's a short between the -6.4 V trace and some other trace. I'm guessing no low resistance from the -6.4 V rail to any of the positive rails? Low as in much less than 1 kohm.~27 mA on the -9 V rail is quite high, though, given that it all has to flow through R132, which would drop about 10 V across this resistor.You keep mentioning oscillation, is one of the opamps actually oscillating (eg. U101)? Did you check with the scope? I forgot if you had access to a scope. Something oscillating could easily explain the high current draw.

author: staze
author_rank: Frequent Contributor
timestamp: August 26, 2013, 03:11:47 am
content: Quote from: alm on August 26, 2013, 02:31:49 amI mention oscillation because the -V rail is/was oscillating. See picture attached tohttps://www.eevblog.com/forum/testgear/keithley-197-repair/msg278450/#msg278450.Attached I've included the schematics. To me, it looks like U107 DOES get feedback through C139, which makes it kind of look like an integrator to me, which is what I thought the -V rail looked like in the picture referenced. So, oscillation, yes?I don't see any low resistance between the -6.4V rail and any other power rail (closest is between it and the -9V, which is the value of R125J).I also see, now, the -6.4V rail is hooked up to R117C (second page, top, where the input comes in from the mux), pin 10 of S101. I'm not quite sure why...So... next step? re-install Q128 and VR101, and try injecting -6.4V at... ?

author: staze
author_rank: Frequent Contributor
timestamp: August 26, 2013, 06:07:45 am
content: Though, thinking about it, I'm not sure how I can disconnect the -6.4V rail... other than cut the trace, or cut the pins on U101, and R117C.Otherwise, I'd say hook the -9V back up, and disconnect the -6.4V and see what happens. But looking at the schematic more, the -6.4V rail is kind of tied to a lot through that second page. It's not a rail for anything, but it seems to be involved in offsetting the input somehow. It's connected to U105, U117 (both relatively easy to replace), and Q121, which I can't quite read the label other than P842. Schematic makes it look like a dual fet. It's a 6 pin can package. ?Update: It's an MP842 (just looked at the parts list).

author: staze
author_rank: Frequent Contributor
timestamp: August 27, 2013, 04:56:37 am
content: So, figuring the issue might be with the input on the other side of the buffer, it looks like R117C is supposed to be ~28K, but it's reading 2.2K. So I'm guessing there's some other pathway that I'm not seeing. Would this be low enough for the rail to be offset by a more positive rail?On a related note, what if I hooked the -9V rail back up (Q128, and VR101), then used my bench PSU to basically push the -6.4V rail harder? Is that feasible, or likely to break something? I would think it might help "smoke" whatever has failed.

author: staze
author_rank: Frequent Contributor
timestamp: August 27, 2013, 11:56:30 pm
content: I just don't understand this schematic sometimes... I think I'm going to try cutting pin 4 on U101 soon, since I found that the TLC27L2CP should be a good replacement (as noted in the pictures here:http://kuzyatech.com/keithley-197-microvolt-dmm-in-pictures/olympus-digital-camera-188But I'm not sure that's the issue. It SEEMS like the issue is on page 2, where the -6.4V rail is attached to the main signal path, and R117C showing as 2.2KR. I also note that R117 pin 4, to ground is only 30R, which seems to be because Q119 is partially conducting when off, which seems weird since as an N channel FET, it should only conduct when it's base is positive. Guess I could ground the base and check again.Also, of note, if I get all the buttons to be in their "out" position except the power, I do get some voltage fluctuation on the -6.4V rail (very minor). So something is feeding back into it slightly, rather than it being completely floating.I'm just not really sure where to go at this point.

author: staze
author_rank: Frequent Contributor
timestamp: August 28, 2013, 02:36:22 am
content: BAM! So, knowing that I could replace U101 with a TLC27L2CP, I threw caution to the wind and cut pin 4 on U101. Guess what? The -6.4V rail came back, and the meter reads 0.0000V!So, ordering a replacement as we speak.Guess the question is, do we think U107 blew out U101, the other way around, or do we just think shit happens?Thanks alm for the help! Once I get the parts I'll put everything back together and post back.Thanks again!

author: Unknown
author_rank: Unknown
timestamp: August 28, 2013, 10:54:17 am
content: I was planning to spend some time to actually study all your results as opposed to just some quick remarks, but you already found the problem, darn.If you're careful cutting pins can be kind of reversible. I received an instrument like that once that had all positive power pins cut in an attempt to track down a shorted opamp. Only the person doing the repair didn't solder them back together properly, so it still didn't work after replacing the damaged part. Worked fine after resoldering the connections.Was U101 actually shorted or was it just oscillating/driving a heavy load? The 7 mA current it was drawing doesn't seem totally insane to me for a healthy opamp. Can you supply 15V from a bench supply to the power pins and short its inputs and see what current it draws, or is it too far gone for that? U101 is driving the guard, so I can also see a short on the PCB causing high current draw or oscillation. Is the resistance between the guard traces around the voltage and resistance traces and the traces around it as high as you would expect based on the schematic?

author: staze
author_rank: Frequent Contributor
timestamp: August 28, 2013, 03:11:45 pm
content: Quote from: alm on August 28, 2013, 10:54:17 amI'll try hooking it up on a breadboard tonight. I only cut that one pin with an x-acto (needed two blades), the rest I unsoldered pretty cleanly. I don't have a split bench supply (though I have a rail splitter I bought a few weeks ago that I've been meaning to try).I'm not sure what you mean by it's driving the guard... but if I recall, the resistance between it's inputs (on the board) seemed reasonable, as well as the feedback loop being intact.Just checked, the inputs, on the PCB, for U101A (pins 5,6) show 1MR between them, and for 101B (pins 2,3) show 3.72MR. Haven't checked voltages as I'm getting ready for work, but can do that tonight. I was actually kind of curious what they all should be since it seems like U101 is involved in the system for switching FETs on and off (tied to the comparators), but not exactly sure what they were doing.Will post back tonight with breadboard results, and voltages.

author: Unknown
author_rank: Unknown
timestamp: August 28, 2013, 04:10:10 pm
content: A quick glance at the schematic suggested to me that they were driving the guard (see Fig 23 ahere): driving the guard trace with a buffered version of the input signal. Could have been wrong, though.Why do you think that you need split supplies to drive an op-amp? An op-amp does not have a ground pin. It can't tell whether you connected it to +15 V and ground, ground and -15 V or +7.5 V and -7.5 V. You can just short the inputs or configure it as a buffer and feed it a voltage from a divider between the power rails (eg. pot). If it's really broken, then I expect it to draw several mA regardless of the input and output connections.It's mainly the outputs that are interesting, since any current flowing from the output will directly affect current draw.

author: robrenz
author_rank: Super Contributor
timestamp: August 28, 2013, 04:39:42 pm
content: Nice paper on shielding and guarding alm, Thanks.

author: Unknown
author_rank: Unknown
timestamp: August 28, 2013, 04:45:11 pm
content: I must confess that that was mostly lucky. I just searched for driven guard on Google and picked the first hit that showed what I wanted to show.

author: staze
author_rank: Frequent Contributor
timestamp: August 28, 2013, 04:56:47 pm
content: Quote from: alm on August 28, 2013, 04:10:10 pmAh, that makes more sense. I guess I just always figured guard rails were more passive than that.okay, I can do that. I'll just feed it with a single rail (assuming it works with a single rail). I would assume, then, with no input, the output should sit at 7.5V (assuming a +15V rail)? Then I guess I'll short the rails and see what I get. Guess I'll also take a close look at the traces around there... but after I put in the DIP socket, I checked all the pins and they all seemed to be reasonable. Also checked that chip out of circuit and there didn't seem to be any shorts between pins either. But, given the two chips I've replaced were both marked as "static sensitive". I also know that when I had U107 out for replacement I turned the unit on... I wonder if U101 didn't like having a +10V rail, and a floating gnd/neg rail. e.g. U107 was the fault the unit had to start with, and in the process of replacing/testing it, I killed U101. =/

author: staze
author_rank: Frequent Contributor
timestamp: August 28, 2013, 04:57:45 pm
content: Quote from: alm on August 28, 2013, 04:45:11 pmHuh? Was there a link I missed?Update... nevermind. Cool!

author: Unknown
author_rank: Unknown
timestamp: August 28, 2013, 05:13:44 pm
content: Quote from: staze on August 28, 2013, 04:56:47 pmThe advantage of active guards is that the voltage between guard and signal is very tiny, so very little current will flow, resulting in very low leakage and very high impedance. Very useful if you're aiming for Gohm input impedances. The AD paper discusses this in more detail.Quote from: staze on August 28, 2013, 04:56:47 pmThere is no such thing as a single supply vs. dual supply op-amp. The difference is just how close the inputs and output can get to those rails. An op-amp that can't go lower than V- + 2V is not very useful with a single +5V rail, for example.Quote from: staze on August 28, 2013, 04:56:47 pmUnlikely. There's always a tiny offset between the input, and since the amplifier is operating open loop, the output will likely be close to one of the rails.Quote from: staze on August 28, 2013, 04:56:47 pmShort the rails?Quote from: staze on August 28, 2013, 04:56:47 pmI'm not discounting the possibility that U101 is dead, but I'm not completely convinced that it's dead either. So it's possible that replacing U101 won't fix the problem. I wouldn't really expect a floating V- pin to kill the chip, although I guess it's possible that it would blow up one of the ESD clamping diodes between the inputs/outputs and the V- pin. Does one of the pins on the U101 that you desoldered have a low resistance to V-? If the voltage from your meter is high enough to forward bias the ESD clamping diodes, then reverse the leads so they're reverse biased.

author: staze
author_rank: Frequent Contributor
timestamp: August 28, 2013, 05:25:45 pm
content: Quote from: alm on August 28, 2013, 05:13:44 pmSorry, inputs. But I would assume that if I short the inputs, I should see an output that either oscillates (bad), or gives me 0V (good), but draws a lot of current in it's rails (bad).Quote from: alm on August 28, 2013, 05:13:44 pmokay. I'll take a look. I would assume "low resistance" would be less than 1K, or more like, 100R or less?

author: Unknown
author_rank: Unknown
timestamp: August 28, 2013, 05:29:07 pm
content: Well under 1k.

author: staze
author_rank: Frequent Contributor
timestamp: August 28, 2013, 05:36:25 pm
content: Wow, this IS a great reference. huh.

author: staze
author_rank: Frequent Contributor
timestamp: August 29, 2013, 03:50:36 am
content: okay, so something definitely happened testing U101 out of circuit.At first, I just fired it up with a 9V rail, shorting the inputs. As you said, the output sit close to the rail at 8V. So I ramped up the voltage and at about 12V, it seems to have latched, destructively, because it's now drawing about 35mA, the PSU is in CC mode at 2.4V (too low to power it, and it's outputting all of 68mV). Turning the power to it off and on doesn't change anything, so it would seem to be toasted at this point. Guess I should have limited the PSU to only supply about 10mA or so. =/ So, chip could have been dead before, or I killed it, but given it's behavior, it seems reasonable that it dragged that -6.4V rail, and therefore the -9V rail down due to current draw.She's dead jim. Checking resistances on the chip:2-3: OL5-6: OL4-8: 120R. BINGO!Back to the board of the 197, in the U101 socket.4-8: 13kR2-3: 4MR5-6: 1MR1-7: 4MR5-7: 1MR1-3: 4MRObviously 1-2, and 5-6 are 0R.Now I just wait for the replacement. Hopefully by Saturday (don't want to wait until Tuesday). I got other stuff to work on! =)

author: Unknown
author_rank: Unknown
timestamp: August 29, 2013, 09:14:50 am
content: 12 V is unlikely to have killed the op-amp, since it was running at about 15 V (10 V + 6.4 V) in circuit. The higher current may have been the final blow, but the op-amp is probably rated for something like 30 V.

author: staze
author_rank: Frequent Contributor
timestamp: August 29, 2013, 06:31:53 pm
content: Quote from: alm on August 29, 2013, 09:14:50 amThe replacement is only to 18V max... but, yeah, 12V shouldn't have killed it.Looking online, it sounds like if you short the rails together, it can saturate the output at one of the rails... but my meter is 10MR input, so it shouldn't have been loading the amp down.So, think the op-amp was failing, and I just threw it over the cliff? Replacement should be here Saturday.

author: Unknown
author_rank: Unknown
timestamp: August 29, 2013, 06:52:51 pm
content: Yep, the combination of the circuit suddenly starting to work when you removed it, no obvious faults in the traces connecting to it and it self destructing when exposed to less than 18 V suggests certainly a failing U101. Guess we'll know in a few days.I just checked the datasheet of the TLC27L2. 3 - 16 V supply voltage and 18 V absolute max, that's kind of tight on a 16.4V (nominal) supply. If the original had similar specs than I could certainly see something weird with the supply rails (eg. U107 developing a short between one of the inputs and V-) killing it. Hope those zeners are accurate.

author: staze
author_rank: Frequent Contributor
timestamp: August 29, 2013, 06:56:28 pm
content: Yeah, that's what I saw for that op-amp too.But I can't find any details on the original one I pulled, so I can't find specs.Attached is the image for it. If you could find specs, that would be awesome... but I keep turning up empty. I guess for all I know, Keithley had TI build it for them... I only used the TLC27L2CP because I saw that other image, and yeah, the absolute max was 18V... obviously in the meter it's going to be running at about 16.4V, which is just above it's nominal rating. =/

author: Unknown
author_rank: Unknown
timestamp: August 29, 2013, 07:11:05 pm
content: Most likely an (selected) off-the-shelf part stamped with a Keithley p/n by TI. The only way to figure that out is if you find the same Keithley p/n in another manual together with the TI p/n. I'm not aware of any cross references for custom Keithley parts like those available for HP and Tektronix parts.

author: staze
author_rank: Frequent Contributor
timestamp: August 29, 2013, 07:26:41 pm
content: Quote from: alm on August 29, 2013, 07:11:05 pmAh, could be. Weird. Stupid unobtainium parts.

author: staze
author_rank: Frequent Contributor
timestamp: August 31, 2013, 10:21:20 pm
content: Replacement op-amp worked great, and meter is up and running. I'll get some pictures here in the next day or so comparing readings to my 199. Certainly don't like the display as much (non-backlit, LCD). Guess they fixed that with the 197A (it has a backlight).

author: Unknown
author_rank: Unknown
timestamp: August 31, 2013, 10:58:09 pm
content: I'm glad it's now fixed.Yep, those low contrast LCDs without backlight are not great. HP also used a bunch a lot of them, including in some fairly expensive equipment like the 3468a, 3478a and even the 3457a. Although I still prefer LED to even LCD with backlight. Some of the earlier 160/170 series meters used LED, but battery power was an option for many of these meters (including the 197), and LED hurts your battery life.

author: staze
author_rank: Frequent Contributor
timestamp: September 02, 2013, 03:59:18 am
content: Quote from: alm on August 31, 2013, 10:58:09 pmYeah, I love the LED as well now. Before I found out about the 199, I was looking at the venerable Fluke 8840, and the HP 3478a, both of which have their issues. The 199 is awesome. The 197 would be a great, and cheaper alternative to the HP.So, comparing the 197 to my 199 and my Fluke 189, I'd say pretty darn good. Let both the Keithley's warm up for an hour before doing the testing.Sadly, I think I'm going to sell the 197. I don't have much use for it. =/ Also sucks that it doesn't have an IEC socket (just an attached power cord).

author: SeanB
author_rank: Super Contributor
timestamp: September 02, 2013, 05:50:31 pm
content: What opamp did you put in there to replace the house part?

author: staze
author_rank: Frequent Contributor
timestamp: September 02, 2013, 05:56:27 pm
content: Quote from: SeanB on September 02, 2013, 05:50:31 pmA TLC27L2CP. See post here:https://www.eevblog.com/forum/testgear/keithley-197-repair/msg282942/#msg282942

author: Tepe
author_rank: Frequent Contributor
timestamp: September 02, 2013, 08:59:59 pm
content: Quote from: staze on September 02, 2013, 03:59:18 amWould there be room for adding a socket? (If it doesn't have a GPIB board)

author: staze
author_rank: Frequent Contributor
timestamp: September 07, 2013, 06:57:22 pm
content: alm,Okay, one thing still isn't right, and I'm not sure why. Pin 12 of S101 (power switch) reads -19V rather than the supposed -15V ± 10% it's supposed to. The diode reads ~-14.8V on the correct side, as does R132.Could this mean the rectifier is bad, or that the transformer is bad? I mean, VR105 is doing it's job, but I worry about it's, and R132's longevity if they're dropping 4V rather than a few hundred millivolts they're supposed to.Thoughts? And thanks!

author: staze
author_rank: Frequent Contributor
timestamp: September 07, 2013, 07:26:13 pm
content: So, the rectifier does seem a bit odd.It's 4 pins (normal) in a DIP type layout.Left inputs are AC, right outputs are negative on top, positive on bottom.So basically:UL: ACLL: ACUR: -RectifiedLR: +RectifiedMeasuring with black probe on UL (diode check), I get:UR: 0.51VLR: 1.25Vblack probe on LL:UR: 0.52VLR: 1.25VHmm...Red probe on UL:UR: 2.25VLR: 0.52VRed probe LL:UR: 2.25VLR: 0.52VAnd, just because I should: Red UR, Black LR: 1.05V. Reversed: OL.Seems to me the testing should show the same between the neg and positive.Thoughts? (this was all done in circuit)

author: staze
author_rank: Frequent Contributor
timestamp: September 08, 2013, 07:29:17 am
content: Here's what my scope has to say... this is on the AC side of the rectifier... I have... no explanation for the clipping at the top, or the artifact on the waveform, but I'm guessing something has failed in the rectifier in some weird way (not a short, and not an open).

author: staze
author_rank: Frequent Contributor
timestamp: September 11, 2013, 05:59:09 am
content: so, it's not the rectifier (surprise). Pulled it, and it tested fine out of circuit.I'm still getting the weird waveform on the AC though. And still seeing the weird integration looking waveform on the negative rail.What worries me though is the negative rail is at -19V at pin 12 of the power switch, which means R132, and VR105 are dropping ~5V, rather than the 1.5V or so they're supposed to.What I don't understand is how could that supply be LOWER than it should be. I could see closer to ground... but being 4-5V BELOW where it should be defies logic in my mind.So, I got nothing. The unit works...

author: Unknown
author_rank: Unknown
timestamp: September 11, 2013, 06:21:48 am
content: Did you test the rectifiers with reverse voltages up to 20 V? The AC waveform looks like one of the diodes is breaking down like a zener.As long neither the resistor or zener diode is getting warm, I wouldn't be too worried. R132 might be dissipating ~0.1W (forgot the exact value)? That's not something to be worried about, since the 197 is unlikely to be using 0805 or smaller resistors. VR105 might be dissipating 1/4 W max.

author: staze
author_rank: Frequent Contributor
timestamp: September 11, 2013, 06:42:46 am
content: Quote from: alm on September 11, 2013, 06:21:48 amI didn't. Didn't really think of that... also not sure how I would. Apply 20V to the DC side (output) and see if there's anything on the AC side (input)?R132 and VR105 are both getting over 38C (100F+ or so), so, warm... but not hot. R132 (330R) LOOKS like it's at least a 1W resistor, if not 3W or so (they're old school brown cylindrical ones (carbon?))... not sure why they never labeled wattage on resistors. So, they're getting warm, but not hot. I local shop has a replacement BR that I can pick up easily and swap in... see if that helps. Would a leaky diode result in the lower negative like side like this? I did take the BR out, and looked at the waveform coming from the transformer, and it looked perfect (as expected).

author: Unknown
author_rank: Unknown
timestamp: September 11, 2013, 07:04:36 am
content: Apply 20V (positive and negative) to the AC terminals and see if you get anything odd (like excess current/heat, or odd voltages on the DC side). Not sure how this would be related to the excess voltage, though. Based on the scope screenshot (a 20 Vpp AC signal) -19V seems perfectly reasonable. I think the only thing that's going to fix it is is changing the transformer winding or adding a load resistor to load down this winding. It sounds reasonable to me that they specced identical windings for the + and - 15 V windings, but since the current draw is much lower on the -15 V rail, it's at -20 Vpp instead of -15 Vpp.

author: staze
author_rank: Frequent Contributor
timestamp: September 11, 2013, 07:19:39 am
content: Quote from: alm on September 11, 2013, 07:04:36 amHuh, good point on the load... Just weird the manual indicates it should be -15V ± 10%. I guess my replacement opamps could draw that much less current... =/When you say apply that voltage, do you mean VAC, or VDC? my PSU isn't dual rail, not sure that matters though. The waveform is actually 48Vpp.I'll try swapping the BR (and testing the current one once I get clarification), and maybe call it good. All just kind of weird. =/

author: Unknown
author_rank: Unknown
timestamp: September 11, 2013, 07:37:53 am
content: I don't expect the replacement op-amps to make much of a difference. A replacement bridge rectifier of this size costs sub-$0.50, so it would be an easy thing to try. There's not a whole lot that affects the voltage from the transformer: input voltage, winding ratio, core, wire gauge and load. Since this is a shunt regulator, the current draw is going to be fairly constant at the voltage across R132 divided by 330 ohm for VR105 to maintain regulation.You don't need a dual rail power supply. Just apply +24 VDC to the AC terminals, measure, and then apply -24VDC. Essentially a diode test on steroids, although you might as well measure the voltage on the DC side while you're at it. The scope screenshot suggests maybe conduction between the two AC terminals at +24 VDC. Or an open in the diodes conducting at the negative part of the cycle.

author: staze
author_rank: Frequent Contributor
timestamp: September 14, 2013, 07:59:21 pm
content: Quote from: alm on September 11, 2013, 07:37:53 amNew BR has the same waveform on the input. =/ Weird. Any other possibilities? Otherwise, I'm just going to call it good.

author: Unknown
author_rank: Unknown
timestamp: September 14, 2013, 11:57:03 pm
content: I don't see any reason to worry about this as long as nothing gets excessively hot and the regulated DC rails are in spec as far as levels and ripple is concerned.

author: staze
author_rank: Frequent Contributor
timestamp: September 15, 2013, 02:58:39 am
content: Quote from: alm on September 14, 2013, 11:57:03 pmNo idea what spec is as far as ripple. The rails are "out" in that according to the spec, pin 12 of the power switch should be -15±10%, but it's at -19V. But like you said, that's likely due to there being that much less load on the negative rail. So who knows.Thanks for all your help! I'll just call it good since it works, and I don't think anything is getting "hot".

author: Unknown
author_rank: Unknown
timestamp: September 15, 2013, 04:36:27 am
content: I said regulated rails. I don't particularly care about the voltage of the unregulated rails as long as they are high enough that the regulator stays in regulation (clearly the case here) and not too high that it produces excessive heat in the regulator.

author: staze
author_rank: Frequent Contributor
timestamp: September 15, 2013, 06:09:43 am
content: Quote from: alm on September 15, 2013, 04:36:27 amAh, then yes, they're all within spec. And R132 and VR105 do get warm, but not hot. Like you said, they're not dissipating much in terms of heat.Thanks!

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 19, 2013, 07:50:36 pm
content: *Hello thread, are you awake?*Just got myself a 197 for € 20 in an unknown state. Looks like new but has a simular problem, OV on all ranges and no negative rail. Cutting pin 4 of U107 brought it back so I ordered a handful of TLC's.To be continued...

author: staze
author_rank: Frequent Contributor
timestamp: September 19, 2013, 07:53:24 pm
content: Quote from: PA0PBZ on September 19, 2013, 07:50:36 pmCool!

author: rwzahora
author_rank: Newbie
timestamp: September 24, 2013, 08:23:08 pm
content: I recently picked up a used Keithley 197A. The unit tests fine but the backlight is quite dim. I read somewhere that this is a typical problem with this model. Does anyone know if repairs can be done here short of replacing the display board? Also, does anyone have schematics for the 197A which would show the backlight circuitry? I have the manual for the 197 that was referenced earlier in this string which has the schematics for that unit. I also have two versions of the 197A manual but neither comes with schematics. If there is no easy backlight solution I suppose I'll go low-tech and do something like tape a key-chain LED light to my meter to illuminate the display when I need to read it. I have to agree with other posters that an LED display is more useful on a multimeter (the 197A replaces an older Keithley 177 I had been using).

author: staze
author_rank: Frequent Contributor
timestamp: September 24, 2013, 08:28:35 pm
content: Quote from: rwzahora on September 24, 2013, 08:23:08 pmCheck voltage rails first and foremost.I haven't been able to find a schematic for the 197A... but you might be able to contact Keithley and get a copy. It's referenced in the 197A manual toward the end (there are specific diagram numbers).Might also want to start a new thread...

author: rwzahora
author_rank: Newbie
timestamp: September 26, 2013, 11:30:59 am
content: I contacted Keithley tech support about a schematic for the 197A. They said that because of intellectual property issues they don't include schematics in their manuals. Funny, but the schematic for the 197 is included in their manual athttp://www.keithley.com/support/data?asset=11252.

author: staze
author_rank: Frequent Contributor
timestamp: September 26, 2013, 03:25:02 pm
content: Yeah, that's BS. Could they supply the schematic at all?Section 6.5 (page 6-1) says you'd want drawing number 197A-116 for the display board schematic, or 197A-110 for the component layout. -100 and -106 are the component layout, and schematic for the motherboard, respectively.

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 28, 2013, 05:57:49 pm
content: Quote from: PA0PBZ on September 19, 2013, 07:50:36 pmReplaced U107 and got the same as staze, the -6.4V was at +6.2. After that also replaced U101 (with TLC271, not the low power one but I don't think it makes a difference) and the meter is fine again. Looks like a common error with these meters. Checked the calibration and it is still spot on (last cal was 1993 according to the sticker).Thanks staze for doing all the hard work!

author: staze
author_rank: Frequent Contributor
timestamp: September 28, 2013, 06:08:53 pm
content: No problem! Glad it's working. Could you, by any chance, check the -V rail off the bridge rectifier? Should also be pin 12 on the power switch.Thanks!

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 29, 2013, 05:36:29 pm
content: Quote from: staze on September 28, 2013, 06:08:53 pmOf course, just opened it again and the -V on the bridge is -18.64V. The + on the bridge is 16.68V.

author: staze
author_rank: Frequent Contributor
timestamp: September 29, 2013, 05:55:23 pm
content: Quote from: PA0PBZ on September 29, 2013, 05:36:29 pmPerfect, or rather, right inline with what mine is (if not off from what the manual thinks they should be).Thanks!

author: timb
author_rank: Super Contributor
timestamp: November 05, 2013, 12:37:05 am
content: Quote from: rwzahora on September 24, 2013, 08:23:08 pm*Casts Raise Dead (Level 12, Prepared) on Thread*I got one of these awhile back on eBay, tore the whole thing down. It seems there's an EL strip [DD-48] that's glued to the display PCB which the LCD [DD-50] simply sits on top of. The whole thing comes apart easily because the LCD is sandwiched between a pair of zebra strips [CS-460-2] and the the main bezel [175A-305].I think you could get someELtapeor apanel, cut it to size and install a small inverter (possibly even directly off the old backlight power rail) and it would be better than new! If the older 197 non-backlit display uses this same type of bezel arrangement, I suspect you can even add a backlight to it.I might give this a go on mine (using red EL tape, retains night vision) and make a thread about it if anyone is interested.

author: rastro
author_rank: Frequent Contributor
timestamp: November 05, 2013, 06:08:31 am
content: Great idea, EL tape sounds like a good replacement. I just took my 197a LCD apart to get some dimensions. This component looks exactly like the EL tape in the vendors pictures. One side has a pink color that's similar to the white EL tape I saw in a video.There was some info printed on the part:LSIP/N 37269-1 Rev E014840 08/05/98I was not able to locate any supply doing Google search.Strip dimensions: 19.5mm X 87.0mmActive area: 17.5mm X 83mmThe selections from Radio Shack/Adafruit/sparkfun/eBay seem to be about 14mm wide and one eBay seller indicated 8mm wide active area. This comes up a little short for our purpose. Perhaps it's enough to light only the center covering most of the numerals. I'll probably stop by RS this week to see what their EL tape looks like.

author: timb
author_rank: Super Contributor
timestamp: November 05, 2013, 11:02:33 am
content: Why not two of the 15x100mm EL tapes from Adafrui and trim each one to 10x90mm? Since you'd be trimming the non-glowy side off each one you shouldn't be able to notice a seam in the middle of the LCD (especially since it's semi-opaque and EL is very soft light).Or you could just cut one of the 100x100mm panels and call it a day:http://www.adafruit.com/category/50_81

author: rastro
author_rank: Frequent Contributor
timestamp: November 05, 2013, 02:08:58 pm
content: I prefer the panel. Less cutting/soldering and more consistent coverage.

author: timb
author_rank: Super Contributor
timestamp: November 05, 2013, 02:32:32 pm
content: That's what I'd do too.I'd really love to design up a little PCB that form fit in place of the LCD with an LED or VFD on it instead. You could use a small micro to convert the data without a ton of effort, I'd imagine.A bit off-topic, but I don't really have a need to undertake something like that right now since I've actually got an old ISA based GPIB card on the way from eBay that I'm going to be hooking into an old Compaq Portal III lunchbox PC (the one with the awesome orange gas plasma display), so I'll most likely be using that to display (and log) data right from the 197A anyway. Turns out the card was made by CEC (sometimes included with old Keithley gear) and they have a really awesome driver/library/example package that shows how to easily write custom GPIB apps in everything from Turbo C, Visual Basic for DOS/Windows, Qbasic, QuickBasic and pretty much any other MS-DOS/Windows 3.11 language.Since you can find these cards in ISA and PCI variants for well under $50, I'd like to write a nice little modular GPIB control suite that'll run under FreeDOS. That way you could turn anything from an old 286 to Core2Duo into a command center for vintage gear!

author: SeanB
author_rank: Super Contributor
timestamp: November 05, 2013, 03:53:00 pm
content: Is the display one with part no MD400F640BDT8409 as I have one that came out of an old Panasonic Executive Partner. If you need one i will post it to you.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 29, 2014, 09:09:11 pm
content: *defibrillating this old thread*smear gel....clear!!!.....zap!!!I just got a 197a from a seller marked dead.. does not power on. Paid $40 on the 'bay and received it today. I like buying things that are sold as "does not power on" since it's usually just a fuse or voltage regulator problem or something else equally simple.So I opened it up, fuse was blown. Replaced the fuse... works like a charm! All functions test normal and it appears to be in conformance with my Tek TX3.However, its EL backlight is very dim. So, did anyone get the schematic for the display and backlight, and/or manage a replacement mod? I like LEDs on the bench tooThanks!

author: rastro
author_rank: Frequent Contributor
timestamp: April 30, 2014, 12:42:38 am
content: Hello codeboy2k,The patient is resuscitated...beeeeepI went the "EL" route. I found and EL-panel that could be trimmed down to fit. Although it was an improvement I probably would not make the effort in retrospect. It would be nice to use some smd LED's. However I really don't see clearance behind the LCD.The "EL" panel I used:http://www.ebay.com/itm/300935081746?ru=http%3A%2F%2Fwww.ebay.com%2Fsch%2Fi.html%3F_sacat%3D0%26_from%3DR40%26_nkw%3D300935081746%26_rdc%3D1The power module was functional so it's just trim and solder.The available 197A manual as you may have discovered is missing schematics so the next closest is the 197 manual with schematics. If you do locate 197A schematics I would like to get a copy.Also what kind replacement fuse did you use? The original main-fulse from the factory is a glass fuse with a series resistor inside.The 197's are great meters, enjoy!Regardsrastro

author: codeboy2k
author_rank: Super Contributor
timestamp: April 30, 2014, 09:34:26 am
content: Thanks @rastro.. I'll take a look at the EL panel you linked. I would really like LEDs here too, but then you have the problem of the symbols like V, A, DC, AC, omega, mu, etc. One would have to decode the protocol on the display cable and also create a new display with all those symbols too as backlit individual leds. It all seems like more trouble than it's worth.For the fuse, it's just an old style 1/8A / 250V slow-blow fuse. The resistor takes some time to heat up. If there was an in-rush current it won't overheat, but if it's a prolonged over-current condition, the resistor heats up and melts the solder at its end. The spring pulls it back and away, breaking the circuit.I just replaced it with a modern MDL 1/8A slow-blow, as I had them in my parts drawers.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 30, 2014, 09:52:08 am
content: Oops.. I just realized you meant SMD LEDs for back lighting , not SMD 7-segment LEDS as I was thinkingbut I do like 7-segment LED meters.Anyways, does the EL panel you linked to work off the original back light inverter on the display board? I presume it will.Did you pick a different color ?

author: idpromnut
author_rank: Unknown
timestamp: April 30, 2014, 11:21:49 am
content: I actually recently replaced the EL backlight on my 175A without any issues. The inverter measures 75VAC, so pretty much anything on flabby should work a treat. I choose electric blue as the replacement, but red was a runner choiceA 7-seg LED might be doable. The supply to the inverter for the EL strip is 5VDC if I remember correctly on the 175A (I can't imagine the 197 is much different, I suspect they used a similar PSU). I didn't check, but I seem to remember that the LCD is driven by a controller on the PCB. I think I might need to pull apart my 175A and sniff the data going to the displayKeep in mind that there are a few other display items that would need to be replicated as well (dB, Recall, Store, Cal, etc) but those could be done with a few SMD LEDs. Sounds like a neat project!