04-23-2018, 07:28 AM
Candohm - yes, the metal case is very tough. That is the first reason that rebuilding one could be difficult.
The concern regarding the use of the candohm contacts as a point to mount a new resistor arises from the prime reason most candohms fail, and that is an intermittent at the pressed-on contact/stud. If the connection is reestablished (it is intermittent) then the value of the resistor will be 1/2 of the desired value (old WW + paralell new = 1/2 assuming both are the same value).
Consider how they are assembled. A wire-wound resistor wound around an insulator/substrate is laid down on fishpaper which is laid down on a metal sheet. Then the sheat/fishpaper is folded around the resistor. In the process, taps were provided at points on the WW resistor. These taps are not soldered or connected except by mechanical means. These tap points are usually the problem. The connections become loose, probably because of heating and cooling AND it is likely that some oxidation also occurs between the tap and the resistive wire.
Now, when this whole mess is folded up there is still a seam on top. Both the contacts and some of the fishpaper emerge from the seam in the folded assembly. What I was doing was to push a razor knife in between the sheets of paper. The blade will contact the wirewound element. Dragging the blade along the element cuts the wire in many spots, making an intermittent connection very unlikely.
Again, if the candohm has been overheated then the insulation value if the fishpaper is compromised.
The concern regarding the use of the candohm contacts as a point to mount a new resistor arises from the prime reason most candohms fail, and that is an intermittent at the pressed-on contact/stud. If the connection is reestablished (it is intermittent) then the value of the resistor will be 1/2 of the desired value (old WW + paralell new = 1/2 assuming both are the same value).
Consider how they are assembled. A wire-wound resistor wound around an insulator/substrate is laid down on fishpaper which is laid down on a metal sheet. Then the sheat/fishpaper is folded around the resistor. In the process, taps were provided at points on the WW resistor. These taps are not soldered or connected except by mechanical means. These tap points are usually the problem. The connections become loose, probably because of heating and cooling AND it is likely that some oxidation also occurs between the tap and the resistive wire.
Now, when this whole mess is folded up there is still a seam on top. Both the contacts and some of the fishpaper emerge from the seam in the folded assembly. What I was doing was to push a razor knife in between the sheets of paper. The blade will contact the wirewound element. Dragging the blade along the element cuts the wire in many spots, making an intermittent connection very unlikely.
Again, if the candohm has been overheated then the insulation value if the fishpaper is compromised.
