Old alma resistors characteristics?

[Nanitamuscen]

Does anyone have documentation or specifications of these Alma resistors?

[alm]

Does anyone have documentation or specifications of these Alma resistors?

I don't. From what I found it used to be a British resistor manufacturer in the sixties and seventies (and likely eighties given the date codes in your picture). It might be that your only source would turn out ads in old electronics magazines, like in the bottom right of page 36 here. Google Books finds a couple, but only shows snippets without useful information. I once went to a university library to look at paper archives of journals for ads of a particular instrument I couldn't find any information about. The electronic archives wouldn't archive ads .

It wouldn't surprise me if Alma was eventually acquired by Vishay through some other company.

[TimFox]

The only thing I could locate quickly is an old CERN report, which compared precision resistors from different vendors in section 3.
https://cds.cern.ch/record/2870418/files/MPS-ED-Note-73-13.pdf

[Nanitamuscen]

faulty 5kohm resistor

[Kleinstein]

The connection from the resistance wire to the terminals could give a hint on the wire material:
Manganin type (copper alloy) wire if often soldered.
NiCr type wire is usually welded in some way, as it is nearly impossible to solder.

[Nanitamuscen]

[Kleinstein]

Looks like a soldered contact and the color also points a little to a copper base alloy.

[Gyro]

Justt wondering, does Constantan (55%) have enough copper compared to Manganin's 85% to make it solderable?

[Kleinstein]

Usually Constantan is difficult to solder.  Likely a little better than NiCr, so maybe with a rather agressive flux. The point is not so much the copper content, but the formation of a stable oxide layer.

A downside of constantan for precision resistors is the high thermal EMF (some 40 µV /K). Both NiCr and Manganin are much better there (e.g. < 2 µV/K).

[Gyro]

Thanks Kleinstein. Yes I knew about the Thermal EMF but just wondered as I've never |(knowingly) tried it, I think all my resistance boxes etc are Manganin. I have some wire from a scrap resistor though that I tried to solder a while back and it wouldn't take, even with Alusol, so I suspect that it must be Constantan.

[coppercone2]

The ultrasonic soldering iron seems to be more and more a technology that we need for metrology.

The actual units are prohibitively expensive. But I wonder about a DIY one solely made for tinning loose wire.

The first idea I had was to cut a bifurication into a soldering iron and then stick a ultrasonic cutter in there. But there has to be some kind of cheaper contraption that can be made for this purpose. I thought a high temperature heat exchanger made from thin copper pipe, heated with hot air, that is brazed or even JB welded to a ultrasonic horn, possibly just cut out of a cheap ultrasonic cleaner. Then you have a little pool of vibrating solder covered pipe you can drag wires through.