Same-size replacement for older potentiometer? - Page 1
Forum section: electronics / beginners

author: elroy
author_rank: Regular Contributor
timestamp: August 01, 2019, 10:48:49 pm
content: Recently I bought a used HP 3586B selective level meter. Overall it's in great shape. The only minor nit is that the volume control for the audio out didn't quite survive the shipment, and the small potentiometer's shaft sheared off clean with the threading on the front of the pot's body. I'm not seeing any way to adapt a knob to control it as-is, so it seems as though the best option is to replace the pot.The pot has the following on its side:2100-0669 CLARO50K [ohm symbol] 19-8431 USAI did a web search and couldn't find any relevant hits from the above.Obviously there's no problem with finding a 50K pot (audio taper, I imagine), but the challenge will be to find one (via online shopping) that will fit the fairly tight space occupied by the old one.So, two questions. First, is there any way using the above information to find an essentially identical pot?If not, what is the quickest/easiest way to find a compatible-size replacement? I guess I can measure the dimensions of the old pot (diameter etc) and dig through the spec sheets for pots in online searches. Is there a faster way to do that?

author: james_s
author_rank: Super Contributor
timestamp: August 01, 2019, 10:59:08 pm
content: Measure it, then hit the parametric search on DigiKey, Mouser, etc. posting a picture here may help you get some suggestions.

author: Nusa
author_rank: Super Contributor
timestamp: August 01, 2019, 11:12:28 pm
content: Well, given no information from the numbers provided (I'll take your word on that), the measurement route should get you there.If you want to reuse the original knob we need shaft size, shaft length, and shaft end type. Otherwise, whatever matches the knob you do want to use.Mounting hole size. Width and depth of area available for pot. Or at least measurements of the old pot.Picture of parts against a ruler would probably go a long way.

author: elroy
author_rank: Regular Contributor
timestamp: August 01, 2019, 11:27:46 pm
content: Quote from: james_s on August 01, 2019, 10:59:08 pmThanks. See the picture below. My imperial micrometer indicates the body is 0.5" diameter, the height is 0.475" from the back to where the threading starts, or ~0.71" from the back to the top of the threading. The shaft (retrieved from the loose knob) is 0.125" diameter and ~0.385" long. (Given that it seems to have been spec'd in imperial units, I wonder if the shaft would be considered 0.375"?) The fact that the terminals are on the back rather than side seems as if it would be an important point to look for.

author: Nusa
author_rank: Super Contributor
timestamp: August 01, 2019, 11:59:18 pm
content: Here's something that looks like it could work:https://www.bourns.com/docs/Product-Datasheets/3862.pdfYou can probably find other brands with equivalent products.If you file a slot in the end of your broken shaft, you can probably adjust it with a screwdriver until you can replace it.

author: edpalmer42
author_rank: Super Contributor
timestamp: August 02, 2019, 01:56:50 am
content: Claro = Clarostat who is a well known manufacturer. I did a quick check on ebay for <clarostat potentiometer 50K> and came up with these:https://www.ebay.com/itm/143193067606https://www.ebay.com/itm/192891650576There may be others.I'd compare the specs to what's in your hand. Measure yours to see if it's linear, log, or anti-log taper. Even if it's not quite right, one of these might be 'good enough for now' while you keep looking for a better one.Ed

author: james_s
author_rank: Super Contributor
timestamp: August 02, 2019, 02:46:28 am
content: The important mechanical bits are the shaft dimensions and style, the diameter of the mounting hole and the overall size fitting within the constraints of the space you have available. It doesn't really matter what style terminals it has or where they are located since it appears to be connected to flying wires rather than needing to fit into a PCB. As was already mentioned the taper type also matters, I suspect it is audio taper though as you mentioned it's a volume control.

author: elroy
author_rank: Regular Contributor
timestamp: August 02, 2019, 03:30:34 am
content: Quote from: edpalmer42 on August 02, 2019, 01:56:50 amThanks! Those look like excellent options, and I'll check them out.

author: soldar
author_rank: Super Contributor
timestamp: August 02, 2019, 11:36:52 pm
content: What material was the shaft? Because it is a strange mode of breaking. If you can replace it then, obviously, that is the easiest repair. If you cannot find a replacement It should not be difficult to make a new shaft on a lathe and maybe even without a lathe.

author: james_s
author_rank: Super Contributor
timestamp: August 03, 2019, 02:20:41 am
content: I've had pot shafts get broken before, usually they are carbon steel that is strong but somewhat brittle. Unless I were very desperate I would not try making a replacement shaft for a pot, my time is worth a lot more than that even at minimum wage relative to the cost of a new pot. Of course if you're itching for an excuse to use the lathe then it may ave some merit.

author: soldar
author_rank: Super Contributor
timestamp: August 03, 2019, 10:56:51 am
content: Quote from: james_s on August 03, 2019, 02:20:41 amIt seems you missed the part where I saidIf you cannot find a replacement.And, it may seem strange to you but some people enjoy tinkering with mechanical parts just like some people enjoy tinkering with electronics. It's a waste of time, I know.And if that shaft is similar to some I have seen it is just a cylinder with a flat end like a screwdriver and can be made in very little time, even without a lathe.Let me repeat,If you cannot find a replacement.

author: mikerj
author_rank: Super Contributor
timestamp: August 03, 2019, 12:42:01 pm
content: Quote from: soldar on August 02, 2019, 11:36:52 pmIf you look at the new ones the shaft is retained with a circlip (E clip) right at the end of the threaded section. This means the shaft has a nice stress raising groove machined into it at exactly the point that it will experience the most stress if the control knob is knocked sideways.

author: elroy
author_rank: Regular Contributor
timestamp: August 06, 2019, 01:48:13 am
content: Quote from: edpalmer42 on August 02, 2019, 01:56:50 amThanks again for the links to the Clarostat pots. I ordered the one below, which arrived today. The part numbers aren't an exact match, but this looks as identical as anything is going to get. Much appreciated!