Fluke 883A Differential Voltmeter: Teardown Pics + Mods

[FlyingHacker]

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: [Select]

https://www.youtube.com/watch?v=9dXlPwQkOeg

[FlyingHacker]

More pics inside. The big black thing in the foreground is the chopper.

[FlyingHacker]

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.

[FlyingHacker]

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.

[FlyingHacker]

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.

[FlyingHacker]

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.

[Vgkid]

Nice job.

[FlyingHacker]

Nice job.

Well, a little sloppy, but it does work, and it does save this guy from the dumpster.

Thanks.

[FlyingHacker]

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.

[bitseeker]

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.

[FlyingHacker]

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.

[FlyingHacker]

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?

[bitseeker]

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?

[FlyingHacker]

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?

[FlyingHacker]

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. 

[bitseeker]

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.

[FlyingHacker]

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.

[Vgkid]

Nice job.

[FlyingHacker]

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?

[bitseeker]

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.

[sync]

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.

Are 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.

[FlyingHacker]

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.

Are 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.

Yes, 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.

[sync]

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.

[SeanB]

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.

[bitseeker]

At Digikey they start at $33 for one cap.

Yikes!

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.

Double yikes! Thanks for the background info. It's good to be aware of these little beasties.