Var resistor id - Printable Version
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Var resistor id - Martinj - 02-04-2025
What are these var resistors called. I cant seem to find one.
RE: Var resistor id - morzh - 02-04-2025
I can see something that looks like on right above it.
This one that you have circled....not sure what it is.
A general photo of things around, what it is attached to, both sides of the wall it is bolted to, would help.
A variable resistor in radio is typically called "potentiometer", if this is what you are asking.
RE: Var resistor id - MrFixr55 - 02-04-2025
Hi MartinJ,
It is in fact a wirewound variable resistor, sometimes known as a rheostat, but others call it a "pot" (Short for potentiometer). Potentiometer usually refers to a high resistance carbon resistor type, such as a volume control, and usually with a resistance between 5KOhm and 2MegOhm. Rheostat usually refers to a wirewound type of relatively low resistance (say, less than 1000 Ohm), similar to the ones used to control filament voltage in battery operated radios of the early 1920s. Edited 02/04/25 to normalize to Morzh's post which is a better definition.
The one in your set is likely of rather low resistance, wirewound and likely handles more than 1/2 Watt but likely less than 5 Watts. If only 2 connections (one end of the resistance element and the wiper), it is referred to as a rheostat. If it has 3 connections (both ends of the resistance element and the wiper0, it is referred to as a potentiometer. The physical type in your picture was somewhat common in some TV sets, especially in the convergence section of color sets of the 50s and 60s, and in other places where a low resistance variable resistor is needed with higher wattage dissipation than a carbon style pot, particularly one that will not be adjusted frequently.
Attached is a picture of one of these for an old RCA Color TV convergence board. In this particular one (and possibly yours) the wiper does not have its own pin; the wiper connects to the case. In addition, this type was designed to be mounted and soldered to a PC (Printed Circuit) board.
I suspect that given the rust, yours is total toast, but if you don't have the specs, take it out and ohm between the ends of the resistor. This particular style may not be available, but there should be a modern equivalent that will work.
What, specifically are you working on? We may be able to help with this and other issues and are very happy to do so!
RE: Var resistor id - morzh - 02-04-2025
Strictly speaking, the potentiometer is a 3-contact variable resistor (two ends and the wiper, so it could be used to scale a voltage as a divider) and the rheostat is 2-contact (1 end and the wiper, so it is not a divider in itself).
A potentiometer with only two contacts (one end and the wiper) used becomes a rheostat.
RE: Var resistor id - MrFixr55 - 02-04-2025
Hi Morzh,
I sent a post in response to yours, but it seems to have disappeared. I have corrected my post to conform to your definition as it makes more sense than mine.
RE: Var resistor id - radiorich - 02-05-2025
Hello Martin,
What model is this set !
Sincerely Richard
RE: Var resistor id - morzh - 02-05-2025
MrFixr
Informally people indeed call wirewound pots rheostats sometimes. Tom-ay-to, tom-ah-to.
Nothing wrong with it.
People used to call all off-road type SUV Jeeps.
I guess, because of the traditional lab rheostats were large wire-wound jobs, those porcelain wire-wound pots just like to be called that same name.
When I first learned of these, they were all (in Russian) called "variable resistors".
RE: Var resistor id - Martinj - 02-05-2025
Its a Sylvania 400 oscilloscope. here is a clip of r-100. Would be nice to know the current required. Not listed in parts list.
RE: Var resistor id - Chas - 02-05-2025
Trim pot, about 1/2 watt part of focus divider. As stated 2k ohms.
This appears to be a rusty chassis, resolder all grounds to chassis, remake or solder all mechanical joints as well.
GL
Chas
RE: Var resistor id - morzh - 02-05-2025
Considering what it is in series with, I doubt there should be any power requirements. Of course, we should
know where "A" goes to.