Sometimes in TV and radio restoration we run across a custom OEM pot (potentiometer) that was manufactured with a center tap on the resistance element. These custom OEM pots were only available as replacement parts direct from the original maker of the appliance. These custom designed OEM parts are no longer available and unless you can find a donor chassis to cannibalize, you are up a creek without a paddle.
In some cases where a large wire wound pot was used, a center tap can be added to a standard 4 watt Clarostat wire wound pot. This process will not work for carbon element volume controls with a center tap.
The first step is to obtain a new old stock 4 watt, wire wound Clarostat pot of the correct value. Search the internet and ebay.
Once you have obtained a pot of the correct value, remove the back cover.
Next you will need to remove the shaft and wiper assembly by carefully spreading the brass “C” clip that holds the shaft in place. The “C” clip is located in front of the threaded neck which the shaft passes through.
With the wiper assembly removed, unsolder the wires from the end terminals that attach the resistance element to the terminals.
Now we need to grind off the heads of the rivets that fasten the end terminals to the body of the pot, using a Dremel tool and a small grinding stone. Do not attempt to grind off the rivets from inside the pot. It is too easy to damage the resistance element wire by accidently touching the wires with the Dremel tool. ALWAYS GRIND THE RIVET HEADS ON THE OUTSIDE OF THE POT.
When the heads of the rivets are removed, CAREFULLY push the rivets through the pot body to the inside of the pot and remove them. Save the flat washers that are on the stems of the rivets because we will need them at re-assembly time.
At this point the resistance element can be removed. Using a small jewelers screwdriver, carefully pry the resistance element upward. The resistance element is snugly wedged into the pot body, and will need to be carefully PRYED upward. Be sure to mark which end is which so you know how to orient the resistance element when we re-install it later.
After the resistance element has been removed, it is time to create a small slot in the Bakelite body of the pot where the new center tap terminal will be located. I do this using a Dremel and a very tiny burr. I carefully work my way through the Bakelite body creating a slot that is flush with the inner wall of the pot body.
Next you will need to make a new brass terminal. I start by cutting a ¼” by 2” strip of .010 brass shim stock. Place the strip of brass through the slot you made in the Bakelite pot body so it sticks through the slot the same distance as the height of the resistance element. Then bend the brass strip, outside the pot, at a 90 degree angle.
Now you need to re-install the resistance element orienting the ends so that the ends of the element go back in the same way it was originally. You will need to make sure that the new brass center tap terminal stays in place while you are pressing the resistance element into the pot body. The new center tap terminal makes it’s electrical connection by being tightly wedged between the inside wall of the Bakelite body of the pot, and the wires of the resistance element. You will find that the resistance element goes in rather tightly, and it is because of this tight fit that we are able to get a good contact between the brass of the new center tap and the resistance wire.
Once the resistance element is in place we need to re-install the original terminals that we removed during disassembly. To fasten the terminals we will use two 4-40 x ½”stainless steel pan head machine screws and nuts. Place one of the washers that were removed during disassembly onto the screw. Take a 2” long piece of 30 gauge copper wire and make a tiny loop just big enough to pass over the 4-40 screw. Place the loop over the screw and add another one of the flat washers you saved during disassembly.
Now pass the screw assembly through the hole in the resistance element and through the Bakelite body of the pot leaving the head of the screw in the inside of the pot.
Install the original terminal and secure in place with a 4-40 nut on the outside of the pot. Feed the end of the copper wire through the slot and secure under the tab on top of the terminal you just installed. Repeat this procedure with the remaining original terminal.
Then solder the copper wires to the terminal and snip of the extra length. At this point, using a carborundum disc on your Dremel, cut through the screws a few thousands of an inch away from the face of the nut to remove the unneeded excess length sticking out.
Now using a plier, bend the outside of the center tap terminal as shown in this photo and cut off the excess length.
Mix some JB Weld fast curing epoxy and secure the center tap terminal as shown in this photo. Use a piece of tape to prevent the JB Weld from dripping over the edge of the terminal. After the JB Weld is well cured, drill a small hole in the center tap terminal using a tiny ball burr in your Dremel tool.
At this point we can install the shaft and wiper assembly. Pass the shaft through the bushing and install the brass “C” clip you removed during disassembly. Using a pliers compress the “C” clip back to it’s original shape.
You can now use your ohm meter to test the new center tap. Place one lead of your meter on the center tap and one lead on the wiper. Starting with the wiper all the way at one end, rotate the shaft. When you have reached the new center tap terminal, you should read very close to zero ohms, and as you continue to the opposite end of rotation you should again reach maximum resistance. If all tests out OK you can proceed to the finishing step below.
Temporarily place the back cover on the pot body, and mark a line on each side of the new center tap terminal so you can cut a notch in the cover to accommodate the new terminal. Using a carborundum disc on your Dremel tool cut away an area as shown in this photo. After cutting the notch, install the back cover and you are finished.