My Delta variable speed grinder (GR 450)(5 amp) has stopped being variable speed. It only wants to run flat out. I like it slower. I'm thinking a new pot. Radio Shack has 5k, 10k and 1m pots. I have no idea what to get. The manual isn't helpful. The sites for Delta parts show the speed controller to be obsolete and not available. It's probably a long shot to fix but I would like to give it a try before buying a new grinder. Any ideas would be appreciated.
Remove the speed control thingy and look on the back for its rating shown as a number followed by letter K, such as 5K, 10K, etc. which indicates the maximum resistance it offers in the circuit. Then google with the number followed by the word "pot" (short for potentiometer). You'll get plenty of hits (I got 2,580,000 matches, 0.43 seconds) and there will be photos which will help you select one that is a size/shape/connection match for what you have.
If all else fails pull it out and send it to me. I should be able to.figure out what it is and if i dont ha e one i can get a pot locally at an electronic store.
Thanks for the responses. The existing pot has no markings.
A google search showed a Radio Shack 25 miles from me connected to a Ace hardware store. I thought they were gone also. They show the pots I am asking about are "on Sale" for $2.00.
Understand that potentiometers have more specs than just the resistance value in ohms (10K, etc). Some have a linear taper (meaning half-way through the turn a 10K pot will read 5K) and some have a non-linear, e.g., logarithmic taper. Some have a carbon film, others could be ceramic or wire wound to handle more current. (A wire-wound pot will feel different when turning.) If you replace it with the wrong type it might not work the same (i.e., could change the speed too slowly or too fast on one end of the rotation) or could even burn up from too much current. A pot intended for audio control may be different from one in a grinder. Note that I have never looked at what type of pot is in such a grinder - the type used depends largely on the speed control circuit in the device.
With no markings I'd probably take it apart and analyze it - usually there are metal tabs you can pry up to remove the cover. Once open you can easily see if it is wire-wound, verify the taper, and might even be able to see what the problem is, for example, the resistive element might be cracked or burned where riveted at one end. Either way, you can probably measure the resistance of what is left and determine the resistance of the original pot.
I agree that the problem is most likely a bad pot. Keep in mind that it could be something else in the circuit. If the disconnected pot reads a smoothly increasing/decreasing resistance when measured from the center terminal to both of the outer terminals, the pot is probably OK.
Note that a replacement pot (of the correct specs) does not have to be the same physical size as long as it will fit in the space and fit through the hole.
You mentioned you like the lower speed. Until you find a replacement pot one simple thing you can do is substitute a fixed resistor (or a pair, if the circuit was connected to all three terminals), picking the resistance values to get the fixed speed you want.
If unfamiliar with such things, someone with experience could test and fix this easily (assuming the problem is in fact the potentiometer). If they are like me, they will have spare parts on hand and be glad to help. To find such a person, one place to check is with a local Maker's club.
....use of the pot for turning a lathe off and on. The needless extra wear and tear on what is a rather delicate part accelerates the potential for failure.
The Pot is about 1/5 the cost of the switch on my lathe. Maybe even less. commercial heavy duty switches are fairly expensive. Potentiometers are usually much less.
IF the pot checks out ok and you suspect the circuit board, before buying one consider repairing it. Problems are sometimes obvious and simple to fix. You can often see where "the smoke came out" of some component or where there is a broken wire or bad conductivity at a soldered connection. I can't remember how many things I've fixed over the years by simply remelting "cold" solder joints - one day it was air conditioners on two separate family vehicles. (The hard part is finding the problem point!)
If lacking experience in testing and repair locating a friendly local electronics geek might result in a cheap or free fix, just for fun. Besides the Makers club mentioned, school teachers are a good source of referrals, especially those who work with robotics competition.
