Question about "repairing" wire wound resistors.  - Page 1
Forum section: electronics / repair

author: NaxFM
author_rank: Regular Contributor
timestamp: January 19, 2022, 11:08:42 pm
content: So, turns out that my precision 0.05% decade resistor box/voltage divider is not precise at all and some resistors are off by as much as 0.7%Probably someone put way too much current in those poor wire wound resistors.Anyway, few resistors are basically dead, but many are in a "trimmable" range.Resistance is always higher and never lower than what it should be, and the manufacturer was kind enough to leave plenty of resistive wire between resistors to trim at least two or three ohms (in the kohm range) from each resistor.Now i ask: if i trim them now to the correct value, they stay at that value, right? Or do I run the risk of wasting hours of careful soldering and measuring just for them to be out of spec later?My thought is that the damage has already been done, and now they are again stable like they should be.Attached pic of the current situation

author: SilverSolder
author_rank: Super Contributor
timestamp: January 20, 2022, 02:46:23 am
content: Nothing to lose by trying!- May make sense to use clamps/clips on the wiring so the heat of soldering stays local to the joint being soldered. That way you have the best chance of avoiding influencing anything inadvertently.

author: m3vuv
author_rank: Super Contributor
timestamp: January 20, 2022, 11:08:20 am
content: are they made with nichrome wire?,if so fat chance of soldering them!

author: NaxFM
author_rank: Regular Contributor
timestamp: January 20, 2022, 12:54:40 pm
content: Quote from: m3vuv on January 20, 2022, 11:08:20 amI don't really know, but it solders really well, the only thing i know is that with the diameter used for the kohm range, every couple of centimeters correspond to a couple of ohms (didn't really measured it precisely)One thing i noticed is that they used a very low melting temperature solder, my modest 40W soldering iron melts the solder instantly at 290C even with the high thermal mass of the studs which hold the resistors.Soldering is the last of my problems, i already soldered many times the same wires to get a couple of extremely accurate 1kohm resistors. Only problem is that it's very tedious, since you don't know at which point you have to solder and you also have to wait for everything to cool down every single time before taking a measurement.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 20, 2022, 02:54:22 pm
content: Beside NiCr wire there is constantan and manganin wire that are easier to solder, at least if not coated with a strong isolation.I would expect the solder to be kind of normal 40/60 leaded solder with the usual minor variations (<1% Sb or Cu).

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 20, 2022, 11:21:50 pm
content: HiFrom the visible appearance of your resistors they are low Ohmic value ones, say below 1K. If so the method that you are using to measure them is invalid, you must use a the four wire Kelvin method.The resistance wire looks to be cotton covered Constantan which can be easily soft soldered when the cotton insulation is removed.Nichrome resistance wire can only be soldered using hard Silver Solder at a temperature greater than say 600C.George Edmonds

author: NaxFM
author_rank: Regular Contributor
timestamp: January 21, 2022, 09:35:20 am
content: Quote from: George Edmonds on January 20, 2022, 11:21:50 pmYeah, i use four wire method for everything of course, in that pic there are only two clips because i removed the other two which were standing up, otherwise it would have been too cluttered to see.And you are also right about the cotton and wire type.