Yes, I bought another Keithley (196) to repair (FIXED!) - Page 1
Forum section: electronics / repair

author: staze
author_rank: Frequent Contributor
timestamp: November 06, 2013, 10:54:21 pm
content: So, just took delivery of a Keithley 196 I got for dirt cheap ($40). Which, for a 6.5 digit meter, can't be beat.Initial diagnosis says it's probably something similar to what was wrong with my 199, which was the counts pathway between the analog and digital board (199 was the 74HCT04). Anyway, interesting that the 196 doesn't use optocouplers, but instead has pulse transformers (weird!).First question is, does anyone have higher quality schematics for these units? the ones in the online pdf manuals are terrible quality.Anyway, I'm working on physical repair of the unit at this point (shipping was un-kind to the unit), then I'll start (of course) with power rails, then I'm guessing TTL pathway, and I'm guessing the HC00's, or the LM311's that are in that same pathway. We shall see. Anyone else repaired one of these? =)

author: lowimpedance
author_rank: Super Contributor
timestamp: November 06, 2013, 11:53:41 pm
content: The only repair I have done was for one at work which was needed for a test setup (old software etc) and was powered 24/7.The failure was bad mechanical mounting of the mains transformer leading to eventual dry joint fracture and then burning of the PCB around that area. So my only input would be at least make sure yours is sound in that area.I did not need the handbook at the time and the owner probably would be unlikely to put his hand on it any time soon unfortunately.If you still need better scans I 'might' be able to find it !.

author: staze
author_rank: Frequent Contributor
timestamp: November 06, 2013, 11:57:26 pm
content: Quote from: lowimpedance on November 06, 2013, 11:53:41 pmIf you have the ability, that would be awesome. Judging by the scans, they seem to have been done with a really crappy scanner, or someone that just didn't care. So anything would be great... but don't go out of your way. I think I can get it fixed with what I have, but it sure would be nice to have better quality schematics. =)

author: rastro
author_rank: Frequent Contributor
timestamp: November 07, 2013, 09:03:18 pm
content: I also recently picked up a 196. It seems to be functional and maybe 8-10 counts out on DC voltage measurements. I still need to figure out accurate tests for ohms/amps/AC - certainly none are grossly out. I sure like the large LED display.I probably have the same documents you have from the internet. My firmware is B15 and I think the main PCB is revision "C". Let me know if you want me to cross check any test points or component values if that helps.One thing I noticed is that the front panel seems slightly warm (90F with room temp around 78F). I'm not sure if this is normal or there's a problem brewing.Anyway good luck with your troubleshooting.

author: tsmith35
author_rank: Frequent Contributor
timestamp: November 07, 2013, 09:37:23 pm
content: Would anything likethisbe helpful?

author: staze
author_rank: Frequent Contributor
timestamp: November 07, 2013, 09:38:46 pm
content: that's what I have that has terrible schematic scans... zoom in to see what I mean. =P

author: tsmith35
author_rank: Frequent Contributor
timestamp: November 07, 2013, 10:01:21 pm
content: Yeah, pretty standard for scanned manuals, unfortunately. Back when the manuals were printed, the print quality wasn't that great either.

author: staze
author_rank: Frequent Contributor
timestamp: November 07, 2013, 10:16:09 pm
content: Quote from: tsmith35 on November 07, 2013, 10:01:21 pmyeah... guessing just really crappy photocopies or scale-downs from the original engineering drawings. I kind of want to just fix the whole thing in Illustrator/Photoshop, but I'm not sure I care THAT much. =/

author: staze
author_rank: Frequent Contributor
timestamp: November 10, 2013, 06:13:11 am
content: So, power supply rails all check out (without the analog board hooked up), and TTL signals all look good.BUT, hooked the analog board up, and things go wonky again. Checked the analog rails, and the +5V and +15V are fine. -15V rail, is going between -15V and Gnd in a regular fashion, so something is pulling it up the ground.Will post a picture once I have a chance, and need to figure out what is pulling it up. Guessing it's when the measurement circles around to something, maybe the 0V reading. So, will put it in diagnostic mode next and see if I can isolate what part of the cycle it's doing this in.Wonder also if using my bench supply to drive the -15V rail would show much... wouldn't think so since if it was shorted, it would just sit at 0V rather than pulse back and forth. As a I said, with the analog board unhooked, the -15V rail is fine. And the +15V rail is fine either way, so it's not that the two are being shorted together.Any thoughts? Real annoyance is how bad the schematic quality is... printed them out full size (11x17 (A3)), and they're unreadable as far as numbers, etc. Might have to re-do them. =(

author: staze
author_rank: Frequent Contributor
timestamp: November 10, 2013, 06:50:56 am
content: So I just noticed C53, a 68pf 500V polystyrene cap looks melted/blown.I'm trying to find it in the schematics, but more importantly, I can't seem to find replacements through Digikey or Mouser. Can Mica's be used instead, or...?Thanks!

author: PedroDaGr8
author_rank: Super Contributor
timestamp: November 10, 2013, 06:54:28 am
content: Quote from: staze on November 10, 2013, 06:50:56 amI wouldn't; polystyrene have super low dissipation factors. You can source them online for a couple bucks. If I remember correctly those polystyrenes decouple directly to ground. So that could be the source of the failing -15V rail.

author: staze
author_rank: Frequent Contributor
timestamp: November 10, 2013, 07:08:19 am
content: Pedro,Good to know... that should help me find it.I happen to have a 68pF 500V mica cap in my bin... think I'd be safe replacing it temporarily with that? Then more permanently once I find a replacement polystyrene?Got any links to where they might be? All I've found is an audio parts place that sells a big batch for $100 of all difference sizes. =/

author: PedroDaGr8
author_rank: Super Contributor
timestamp: November 10, 2013, 07:30:35 am
content: I think it should be OK! Just don't be surprised if things read a bit off. It would still be helpful for diagnosing the problems on your analog board.I'll see if I can find my links. I do remember I had better luck with finding 630V polystyrenes than looking for 500V ones. Just like with other capacitors a higher voltage rating shouldn't affect things.FRI just in case you didnt know. Polystyrene capacitors are by far the most heat sensitive of all of the capacitor types. They can be easily damaged by soldering if you aren't careful.

author: staze
author_rank: Frequent Contributor
timestamp: November 10, 2013, 07:50:43 am
content: Quote from: PedroDaGr8 on November 10, 2013, 07:30:35 amyeah, noted that they're heat sensitive... it could be that this one was damaged by someone else trying to solder near it or something... they're a rather large chunk of it missing/melted.anyway, will try swapping in the mica one tomorrow (assuming I have time). I don't doubt it'll be off, but at least it'll tell me if it's that cap, or something else. I'll also snap a shot of the -15V waveform with the board plugged in.Thanks!

author: tsmith35
author_rank: Frequent Contributor
timestamp: November 10, 2013, 01:03:40 pm
content: Quote from: staze on November 10, 2013, 06:50:56 amIt's on the schematic "Analog Board" (page 134 of the PDF I linked above). It's at location "A2" at the bottom right corner.

author: staze
author_rank: Frequent Contributor
timestamp: November 10, 2013, 05:40:04 pm
content: Cool, thanks. And I do see a path between it and -15V. Hmm.That said, I would think a 68pf cap, should it fail, would fail open rather than short... Since those plates aren't very close together. Will post picture of waveform.

author: SeanB
author_rank: Super Contributor
timestamp: November 10, 2013, 06:04:58 pm
content: Polystyrene will almost invariably fail as a short circuit, just due to the construction, especially if it is hit with a soldering iron tip.

author: staze
author_rank: Frequent Contributor
timestamp: November 10, 2013, 07:07:24 pm
content: Oh. Good to know! I would think those plates would be pretty insulated given the size of the unit, and how low a capacitance.That said, I just checked it in circuit, and it shows 3.4k in one direction, and 5k in the other. So it doesn't seem shorted, unless they only short when there's more voltage/current than my 199 uses for resistance.

author: staze
author_rank: Frequent Contributor
timestamp: November 10, 2013, 07:49:52 pm
content: Well, I just fired up the meter again, and that cap is making a hell of a racket now (high pitched buzzing). So it's obviously bad.Will replace with mica and see what we get.

author: staze
author_rank: Frequent Contributor
timestamp: November 12, 2013, 02:43:16 am
content: So, replaced that bad PS cap with a mica one, and things are back to "normal" as in, what it was doing before the cap went sideways.Something is bringing the -15V rail up to ground (or just above) in pulses. Obviously the ramping is due to the filter cap on that rail, but any thoughts? Guess I still need to put into diagnostic mode and see if I can figure out if a specific part of the sample cycle is the issue.

author: edavid
author_rank: Super Contributor
timestamp: November 12, 2013, 03:15:13 am
content: The problem is that polystyrene caps aren't made any more. Usually the best substitute is polypropylene or NP0 ceramic.I can't figure out exactly what that cap is doing, so it's hard to say if mica is good enough.It looks like that cap is filtering the input, so I think they chose PS for low dielectric absorption, and I'd use polypropylene.

author: staze
author_rank: Frequent Contributor
timestamp: November 12, 2013, 03:21:27 am
content: Looks like it's just decoupling the signal ground. No sure that would matter that much, especially for testing.

author: staze
author_rank: Frequent Contributor
timestamp: November 13, 2013, 05:07:51 am
content: Diag mode stages made no difference, something is still causing the oscillation.Guess there's always the chance the 7915 can't feed any real load...Think I'd be safe injecting -15V into the analog board, and seeing if it stabilizes? Would imagine I'd just put the positive of my bench supply on the ground, and negative on the -15V rail... Or I guess I could be "safer" and inject it directly onto the pins for the 7915 (same basic idea though, negative lead to 7915 output, positive to analog ground.Thoughts, or is this a bad idea?

author: edavid
author_rank: Super Contributor
timestamp: November 13, 2013, 05:55:45 am
content: Quote from: staze on November 13, 2013, 05:07:51 amYou would have to pull the 7915 first.

author: staze
author_rank: Frequent Contributor
timestamp: November 13, 2013, 06:24:01 am
content: Okay. Maybe I'll just try loading it down first and watching the voltage. Got a couple 8R 20w resistors that I can use to put about an amp out of it.Out of curiousity, why would I have to pull the 7915 first? I know this is the practice... But why?

author: PedroDaGr8
author_rank: Super Contributor
timestamp: November 13, 2013, 07:37:15 am
content: Quote from: staze on November 13, 2013, 06:24:01 amI think because otherwise the vreg will fight the voltage from the power supply.Sent from my EVO using Tapatalk

author: staze
author_rank: Frequent Contributor
timestamp: November 14, 2013, 03:24:15 am
content: Bam. So, loaded down the existing 7915 output with 16R (disconnecting the analog board), and the voltage looked pretty similar. So, unsoldered the 7915, and wired in my bench supply for -15V, and fired up the unit. Viola, it worked! No random fluctuation at all, and the -15V rail was only drawing 70mA.So, going to have to look through my parts for a 7915, or just go pick one up. The PS cap though... I'll just have to order one... only supplier seems to be in Canada, unless someone knows a good way to find one in a scrap bin, or if the Silver Mica one would work just fine, I'll leave it in.Shame I didn't get to use the beautiful schematics scanned by lowimpedance! I will make sure to share them with the world though!

author: SeanB
author_rank: Super Contributor
timestamp: November 14, 2013, 04:40:53 am
content: Silver mica is not as good as polystyrene, but for this low value it will do, the 500v rating will keep unwanted DA and soakage to a very low level. Leave it in and get the regulator changed out and then check if the cal is still in spec.

author: staze
author_rank: Frequent Contributor
timestamp: November 15, 2013, 03:06:36 am
content: well, damn. it wasn't the 7915... which means it must be the filter "caps" on the bridge rectifier side of the ±15V regulators.Only issue is, the schematics show two 680uF caps, but the unit uses a single bi-polar 1500uF cap. I know you can make a bipolar by hooking two back to back, but I'm unsure how to do that in this case. Would I go negative to negative and just replace the bi-polar, or should I test first by just hooking a 680uF 25V cap directly to the 7915 and see if that stabilizes things? Other issue is, I don't have any 1200uF+ 25V caps, so making a bipolar is going to require a trip to the store.Any input would be great.

author: staze
author_rank: Frequent Contributor
timestamp: November 15, 2013, 03:50:30 am
content: Correction... I was looking in the wrong place. There are two 35v 680uF caps. The one for the negative rail is completely toast. Pulled it, and it shows 7nF. Awesome. ESR is seemingly infinite (Blue ESR just shows a dash).So... I'll go pick up a replacement for both of those caps tomorrow, as all I have are 25v 680's.So, guessing that's fixed.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 15, 2013, 04:03:55 am
content: Was about to post some thoughts on the said 680uF and bipolar stuff but you have already found the culprit, excellent.Another nice old meter saved. I have two 195a 's (5.5 DIGIT) which wont win any beauty contests but are good solid performers and best of all nice LED displays like the 196.Hopefully the two 196's at work will become redundant before me.BTW that scope shot is what you get with filter caps that have 'dried out' losing capacitance.With the 'new scanned' schematics you can zoom in and actually read the part details!. I printed one on A3 and its clean with no fuzziness as well as easy to read at this paper size.

author: staze
author_rank: Frequent Contributor
timestamp: November 15, 2013, 04:21:36 am
content: Quote from: lowimpedance on November 15, 2013, 04:03:55 amYes, pretty amazing. And good to know about that. I thought it was the VR, but I guess the filter cap makes more sense. So really the VR is only getting enough current when the BR is peaking. Will get those ordered, and also replace the ancient 1500uF cap on the 5V analog rail... the gigantic cap for the 5V digital rail appears much newer than 1986 (when that meter appears to have been built).

author: staze
author_rank: Frequent Contributor
timestamp: November 15, 2013, 04:40:55 am
content: Ordered some Nichicon HE (10000HR 105C) caps to replace the two on the ±15V rails, and the +5V analog rail. Those should last well past my lifetime (since I don't plan on leaving the unit on at all times).I don't plan to sell the unit, but I'm curious... anyone know what they actual go for? I see working ones on eBay for like $460, but I can't imagine they actually go that high.

author: staze
author_rank: Frequent Contributor
timestamp: November 15, 2013, 04:51:56 am
content: Quote from: lowimpedance on November 15, 2013, 04:03:55 amThough, please do post about the bipolar stuff. I know I'm curious.

author: staze
author_rank: Frequent Contributor
timestamp: November 19, 2013, 05:51:08 pm
content: Re-capped the power supply (replaced both 680uF 35V caps on the ±15V rails, and the 1500uF 25V cap on the 5V analog rail). Also put the original -15V regulator back (figured it worked, why replace it).Now I'm just waiting for the Polypropylene cap. I really hope the calibration is still good... though I guess it wouldn't be to bad to have it calibrated considering how inexpensive it was.

author: staze
author_rank: Frequent Contributor
timestamp: November 20, 2013, 05:19:12 am
content: So, all fixed.I dropped a polypropylene cap in (https://www.eevblog.com/forum/buysellwanted/wtb-couple-small-value-polystyrene-caps/) in place of the polystyrene 68pF that was bad, and it both fit perfectly, and I would guess should behave very close to the PS cap.Calibration seems very close, but I don't have a meter that makes a good transfer "standard"... it just compares well to my Keithley 199 that I had previously "calibrated" to my bench power supply. At some point, I'll send one of them off for calibration.Thanks for the input all!