Fluke 883A Differential Voltmeter: Teardown Pics + Mods - Page 1
Forum section: electronics / repair

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 11:06:49 pm
content: Hey all,Here are some pics of the Fluke 883A Differential Voltmeter I just acquired locally for $20. Once recapped this thing is damn near spot-on to my 6 1/2 digit Keithley! I didn't even let it fully warm up.I opened it up and replaced all the leaky electrolytics. A good number of them had this bumpy texture to them, where the electrolyte has leaked out and nearly uniformly coated the outside of each cap with a bump off-white substance. (will take a pic) They didn't damage the board much. I was able to clean most of it up with IPA. There were a few spots where it left some marks.The purists are going to hate me as I used radial caps I had on hand rather than the perfect sized axial caps. In a few places I had to extend the leads as well because they would not reach.I only replaced the caps that had leaked, and the main filter caps. I will go through and at least test the other ones. I may replace them too. This unit is from the early to mid 60s I believe. So they are bound to fail.I guess I ought to try to check the carbon comp resistors as well, but I am surely not going to desolder them. Just a quick in circuit check perhaps. But given how accurate it is things seem to be working correctly.Although this first picture makes it look backlit the meter is just being lit up from the top lighting without the cover on. I may install a backlight, though. UPDATE: The second photo with the maber glow is where I added a backlight to the meter with LEDs (see below).I am going to put together a video on this as well. Note that W2AEW did a great video on these here:Code:https://www.youtube.com/watch?v=9dXlPwQkOeg

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 11:09:36 pm
content: More pics inside. The big black thing in the foreground is the chopper.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 11:11:30 pm
content: More... This is the Null Detector board. You can see here I had to do some crappy splicing on the capacitor leads. Also you can see the white damage to the board. That is after I cleaned it. That gold trimpot looks like a replacement, and was there when I got this.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 11:12:32 pm
content: Bottom view...These are all the solder points for the Kelvin Varley divider section. That section is in a shielded can, which is kind of a pain to open up... So I didn't.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 11:14:12 pm
content: Sexy mechanical displays. The front knobs actually turn wafer switches back inside the shielded Kelvin Varley section. These shafts are then linked via gears to the display wheels.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 11:16:15 pm
content: Pretty amazing how accurate this guy is given it is over 50 years old!I love how all the components are clearly labelled on the boards.

author: Vgkid
author_rank: Super Contributor
timestamp: May 14, 2016, 11:20:07 pm
content: Nice job.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 11:21:04 pm
content: Quote from: Vgkid on May 14, 2016, 11:20:07 pmWell, a little sloppy, but it does work, and it does save this guy from the dumpster.Thanks.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 11:26:43 pm
content: One thing that drives me nuts working on old meters (and new meters I guess) are the crazy requirements for AC calibration. You need things like a 1000V AC 400Hz source, and a 20KHz source (I think).Is there any reasonably priced way to get these signals? I am always on the lookup for a Fluke calibrator, but they usually go for at least $500, which is way too much to spend on something I would only use to repair old stuff.

author: bitseeker
author_rank: Super Contributor
timestamp: May 15, 2016, 04:07:23 am
content: Nice job, FH, even with the alternative recap methodology.Earlier this year, I found an old electrolytic cap buried in a box. The electrolyte, also off-white, had bloomed through a hole in the bottom that made it look like a cauliflower. I should've taken a picture.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 15, 2016, 05:20:38 am
content: Thanks. I sometimes buy fancy axial caps for these old classics, but they usually end up costing more than the unit itself once you buy them all. Then they are too pricey to buy in quantity as well, so I rarely have any left over.I have been playing with this unit more and am truly amazed I have a 6 1/2 digit accurate meter that cost me $20 (plus maybe $5 in caps).It also has INFINITE impedance on the 1 and 10V scales (when nulled). Pretty amazing devices for the 1960s, or even today.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 16, 2016, 05:27:22 am
content: Today I went through and replaced the rest of the aluminum electrolytics. Seems to be working quite well.I went through the basics of the calibration and it checked out. I can't quite get the 1 volt range to trim properly (1 milivolt gain adjustment) when testing through the Kelvin Varley. I need to grab a precision (temperature stable, not just a metal film) resistor to replace one in the trim circuit-- R239 I think. It is supposed to be 453ohm or something oddball like that. It tests around 380. I can get it to calibrate within tolerance, but that is at the end of the trimpot adjustment, and I would rather get it right at the -1 mark. I think a 420 or so would be fine.There are two Tantalum electrolytics. I left those alone. Are those prone to failure after 50 years?

author: bitseeker
author_rank: Super Contributor
timestamp: May 16, 2016, 06:11:38 am
content: Yeah, at least a 420 gets you more than half way back.Not sure about those last caps. Tantalums are rather notorious for burning things badly, no?

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 16, 2016, 06:49:17 am
content: I really don't know. I have seen tantalum caps that are dipped in some resin before. I have not seen caps like these. The look like regular aluminum electrolytics, but they are listed as Tantalum. I wasn't sure if they needed to be replaced with other tantalums.Anyone? Bueler?

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 19, 2016, 04:49:26 am
content: I hacked in some backlighting to the meter on this differential voltmeter. I guess it is really side lighting. It helps quite well with reading it in dim light, or like the lighting in my lab, top light. I would have put an LED on top to light the midpoint of the meter more, but there is literally no space between the top of the encased meter movement and the top panel.I pulled off the +6V rail (just tapped the capacitor leads). This rail is unregulated, and was a little high anyway. This is probably because mains voltage back in the '60s was not what it is today. I twisted the wires and used more dropping resistance than specified for the LEDs just to be kind. It has no effect on the measurements.The lighting is smoother and more even in person than in the photo. I put white paper around some NTE amber LEDs (NTE part: NTE30042 )Anyway, I figured if this thing was going to take up a lot of bench space (it is quite larger) then it sure as hell better look cool.

author: bitseeker
author_rank: Super Contributor
timestamp: May 19, 2016, 05:19:52 am
content: That is very cool. I want to do a similar thing for some of my power supplies with analog meters.Regarding lighting from the top, what about using an LED filament? They require more voltage since they're serially wired, but they're thin and will give you really good light distribution.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 19, 2016, 05:43:39 am
content: That is a really interesting idea. Got a link to a good cheap source for these filaments? They would certainly produce the most even lighting.In the case of this particular meter I doubt even a filament would fit between the top of the plastic housing of the meter movement and the top panel. Perhaps you could route one or the other out, but I don't want to modify it too thoroughly.

author: Vgkid
author_rank: Super Contributor
timestamp: May 19, 2016, 05:50:03 am
content: Nice job.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 19, 2016, 05:50:24 am
content: I just ordered a few off eBay, but then I realized they take 50-60V DC. For some projects that could be done with a little boost converter, but I wouldn't want any switching garbage near this high precision meter.Will find some use for them, though.Know of any lower voltage models?

author: bitseeker
author_rank: Super Contributor
timestamp: May 19, 2016, 06:03:23 am
content: I'm not aware of lower-voltage filaments. They're a string of LEDs in series, so it takes a fair bit of voltage to drive them all. Probably getting shorter ones will have lower voltage requirement, assuming they use fewer LEDs.

author: sync
author_rank: Frequent Contributor
timestamp: May 19, 2016, 12:52:45 pm
content: Quote from: FlyingHacker on May 16, 2016, 06:49:17 amAre they looking like axial electrolytics with a metal can? Then they are likely wet slug tantalum caps. Very expensive stuff. Don't replace them unless they leaked their electrolyte.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 19, 2016, 04:12:18 pm
content: Quote from: sync on May 19, 2016, 12:52:45 pmYes, that is exactly what they look like. You would think they were axial electrolytics if you didn't read the BOM list in the service manual.I just wonder what properties of these caps would preclude replacing them with modern electrolytics. I have read the tantalums were supposedly ultra low ESR, but I don't read them any lower than what a new aluminum electrolytic would be.

author: sync
author_rank: Frequent Contributor
timestamp: May 19, 2016, 04:34:43 pm
content: I don't know. I don't think it's the ESR. Maybe the lower leakage or the stable electrical parameters. There must be a reason. These wet slug tantalums are really expensive. The case is made of silver (newer ones use tantalum too). At Digikey they start at $33 for one cap.

author: SeanB
author_rank: Super Contributor
timestamp: May 19, 2016, 04:49:12 pm
content: Simple test to see if you want to change them is to look at the end bung. Glass seal and it will last essentially forever, a rubber seal it is time to look for a replacement, as it will have high ESR. Kemet still sells them as new, though you would in most cases be able to replace it with the same value and voltage 105C Nichicon or Panasonic capacitor with no ill effect. They were used because they were a lot more reliable and had lower ESR than the wound axial aluminium capacitors in the time the meter was designed. The quality of electrolytics has improved in the interim, leaving wet tantalum there only for certain niche uses where you need either the multi decade lifetime or the insanely low ESR along with ultra low leakage.Cases are silver plated steel, or they use a tin outer plate with an inner silver plate over steel. The electrolyte is sulphuric acid. They are also made with high temperature solder, and the rubber bungs have the central wire that is showing before the steel lead made from pure tantalum metal, as it is the only thing that will not corrode in the acid bath.

author: bitseeker
author_rank: Super Contributor
timestamp: May 19, 2016, 05:47:47 pm
content: Quote from: sync on May 19, 2016, 04:34:43 pmYikes!Quote from: SeanB on May 19, 2016, 04:49:12 pmDouble yikes!Thanks for the background info. It's good to be aware of these little beasties.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 22, 2016, 04:25:19 am
content: Thanks for the info SeanB. With Sulfuric acid I may just replace them with aluminum electrolytics. But I have everything working really nicely now. So perhaps I will just inspect it every few months.

author: SeanB
author_rank: Super Contributor
timestamp: May 22, 2016, 05:07:07 pm
content: Less than 5 drops inside them, so not to worry. We used to use the dead ones as impromptu fireworks, just by applying 28VDC in reverse to them. Ones that were totally dead did nothing, but the dying ones would go off like a cracker. Mount in the vice and aim it, and you could get a good whack on somebody across the room.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 24, 2016, 05:04:55 pm
content: I wasn't so much worried about the acid hurting me, as I was worried about the acid hurting the PCB and other components if they leak.

author: crissterrian
author_rank: Newbie
timestamp: December 01, 2017, 07:36:28 pm
content: Enyone have the BOM for the fluke 883a and schematics?

author: Martin.M
author_rank: Frequent Contributor
timestamp: October 12, 2018, 10:23:17 am
content: I have buyed 883A todaygreetingsMartin

author: KK6IL
author_rank: Regular Contributor
timestamp: June 06, 2019, 02:59:26 am
content: Quote from: FlyingHacker on May 14, 2016, 11:26:43 pmOld thread, but I'll answer anyway. Back about 1968, I was working at Wavetek, when they were making differential voltmeters, trying to compete with Fluke. Initially for AC calibration, we were using a audio oscillator, into a Macintosh audio power amp into a transformer to step up the voltage. We used a Holt thermal transfer to determine what the output voltage was. Eventually replaced the kludge with an Optomation AC calibrator, which was a 4 foot tall rack mount oscillator, 6 digit ratio transformer and power amp. Still required the Holt to determine AC voltage.So there are ways to fake it with non-standard signal sources. Not too hard to get the high voltage.And unless you need to measure high v, high freq with accuracy, skip calibrating the 1KV range or calibrate at a lower voltage. Something like the AnEng AN870 for $30 can measure AC true RMS, 0.3% accuracy up to 3 kHz. A scope could be used to insure the signal source is flat with frequency. A scope with differential plug in, 7603/7A13, 545/W, etc could measure the signal directly with accuracy. Peak voltage X 0.707 = RMS. I'm using the 7A13 to measure AC signal source to verify my new HP 3456 is reasonably accurate on 1 and 10 V AC ranges. DC measurement of the comparision voltage terminal on the 7A13 verifies it's accuracy.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 05:35:39 pm
content: For a precision high voltage AC measurement for calibration purposes, I would try sampling at the high voltage reducing the problem to a high voltage DC measurement. (1)Sampling also works well to make RMS (and peak, and noise) voltage measurements at RF and low microwave frequencies because samplers can have a very predictable frequency response curve which can be verified by measuring the first null in their response.(1) I have a truly marvelous design for a high voltage sampler which this post is too small to contain.