Fluke 931B repair.... - Page 1
Forum section: electronics / general

author: Rax
author_rank: Super Contributor
timestamp: October 24, 2023, 04:28:34 pm
content: As soon as I pulled this 931B from the shelf (where it sat for a while), and though I could use it for another project posted on here, I am now noticing that its "fourth digit" doesn't want to adjust smoothly and I suspect R37 is not tracking well. It's a hermetic pot. I wonder if anyone has run into similar issues and figured out a solution. Other than getting a replacement - and I assume throwing out its cal/adjustment - I can only think of poking a hole and spraying some Deoxit in there (it's a ww, so no reason for FaderLube), and letting it dry. Then working it and hope it's smooth like a baby's buzzum.The other issue is the mechanical dial is not quite going from 00 to 100... And yes, I absolutely want to keep the calibration untouched!How would you tackle this?(I posted this initially on "Repair," but there's no apparent interest in it there, so I thought this would be a more appropriate crowd for it)

author: donlisms
author_rank: Frequent Contributor
timestamp: October 25, 2023, 04:07:15 am
content: If it's sealed and no (appreciable) dirt has gotten inside, I suspect (as I do with my Helipot collection) that it's corrosion. I have doubts that Deoxit will work, but I've thought the same as you about poking a hole and filling it with a cleaning slurry, even though it probably won't work. I think the fundamental issue is that the "wiping action" is good for bits of dirt, but that corrosion is a much more tough customer.I think for corrosion, some form of chemical attack might do it, but I also think that that would change the value a bit.I have a perspective on this:In parallel with whatever miracle cure you're contemplating, I would think about getting another pot, at least as an option. Since I don't have, and can't find, a schematic for the thing, I can't speak definitively to the calibration issue, but there's a fairly strong chance any tweaking you might have to do is still in the ballpark of the calibration of the thing as it sits. You have the advantage that error contributions from that digit(s) is a thousand times smaller than that of the first decade. The only thing that might matter is the total resistance, and if you can measure that with an accuracy of 1000 times worse than the 931b's specs, you can stay pretty close to "good" when you replace the pot. Tweak it with a big-old parallel resistor or whatever you like to get the total back where it's supposed to be; that will be part of the fun of it all.

author: Rax
author_rank: Super Contributor
timestamp: October 26, 2023, 05:13:51 am
content: Quote from: donlisms on October 25, 2023, 04:07:15 amGood points, thank you.I think Deoxit would cure the pot travel contact issue. I'm more concerned by:Plastic debris getting inside the pot as I drill through. How do I reseal after I'm done spraying inside? Do I try to aerate somehow, and then seal?Throwing off the calibration of the pot and its dialed value. But on further pondering on this, not likely, this being a wirewound pot. Conditioning the contact of the wiper shouldn't effect the overall value and/or adjustment at all, I wager.