Electrolytic Can Capacitor Re-Stuffing

I’m often asked how I go about “re-stuffing” electrolytic can capacitors. A number of restorers, are hesitant to take the time to do this procedure, and either choose to solder new capacitors under the chassis, or just try “re-forming” the old electrolytics. I have found “re-forming” to be a waste of time on extremely old sets because these very old electrolytic cans are almost always dried up inside. In addition, soldering individual capacitors under the chassis becomes unsightly and confusing.

Once you master this re-stuffing technique, you will find that it doesn’t take much time, and your restoration is better off for the effort.

A number of tools that I find essential to make the job a lot easier:
…….140 watt and 250 watt Weller soldering gun (the 250 for un-soldering mounting tabs)
…….solder sucker and solder wick
…….miniature diagonal cutter
…….duck bill pliers
…….needle nose pliers
…….small hammer
…….insulated gloves
…….propane torch
…….utility knife
…….electric drill with a 1/16” drill bit

CANS WITH A CARDBOARD COVER………….
If your electrolytic can has a cardboard cover, you may be able to use the following method of re-stuffing. This method does not require the removal of the entire aluminum can from the chassis, and you don’t need to un-solder the wires from the can terminals under the chassis.

First step is to remove the cardboard cover. The cardboard cover is fastenend to the aluminum can inside with tar. Use a heat gun to soften the tar which secures the cardboard cover to the aluminum can, When the heat softens the tar, you will be able to remove the cardboard cover. Save the cardboard cover to re-install later. After the cardboard cover is removed, cut through the aluminum can, just above the rib at the bottom of the can using a hack saw blade or a Dremel tool. Remove the entire can, leaving only the bottom of the can with it’s terminals still attached to the chassis. Now drill a hole next to each terminal and one or more holes for the negative lead of the capacitors. Mount new individual electrolytic capacitors onto the base of the old can sticking the positive lead wires through the holes you drilled. Make sure you observe correct polarity and that you have the correct values attached to the appropriate terminals. Underneath the chassis wrap the lead wires around the original can terminals and solder them in place. Connect all the negative wires together and run a negative lead wire through a hole in the terminal board. Attach the negative wire to one of the twist tabs under the chassis and solder it in place. After you have the new capacitors installed, re-install the original cardboard cover over your new capacitors. Use a small dab of silicone sealant in a couple spots on the original base of the can to secure the cardboard cover in place.
I consider the above method to be a shortcut, and although easier, I prefer to treat cans which have a cardboard cover, with the same method I use on a bare aluminum can. The only difference is; after I have removed the can from the chassis, I place it in a microwave oven as in step 3A below to remove the cardboard cover.

BARE ALUMINUM CANS WITHOUT CARDBOARD COVERS……
In most instances, your electrolytic can will not have a cardboard cover, and you will need to follow the method described below which requires the removal of the entire can from the chassis, and installs new individual capacitors inside the original aluminum can.
1. First step is to take many good close up photos of the wiring connected to the terminals of the electrolytic can. Your photos will be invaluable when you are re-installing the re-stuffed can. I print out a 8” X 10” full color print of the photos and make lots of notes, labeling where wires and resistors are connected. I also make sure to label the terminal symbols on the drawing (triangle, semi-circle and square) so that I get the terminals back in their original positions during re-instalation.

2. Next, carefully unsolder all the wires connected to the terminals of the can. I use a 140 watt Weller soldering gun, a solder sucker and a needle nose pliers.

3. After the wires are disconnected and moved aside, we need to unsolder the mounting tabs, if they have been soldered to the chassis. I use a 250 watt soldering gun for this. Sometimes you will find that the mounting tabs are mounted on a phenolic insulator. In the event the can is attached to an insulator, be extra careful. It is very easy to break the insulator when attempting to remove the can. The best tool to use for un-twisting the tabs is a large “Duck Bill” pliers. Where the tabs have been soldered, I sometimes need to use a very sharp 1/4" wide wood chisel to carve away at the solder on the tab. IF any solder blobs are left on the tabs it will be impossible to remove the tab from the slot it sticks through.

The duck bill pliers has a narrow profile and is easier to get into tight spaces. It also has a wide enough jaw that will allow you to FLATEN AND STRAIGHTEN the mounting tabs so they will slide back through the slots in the chassis or phenolic insulator. The tabs must be perfectly straightened in order to get the tabs to slide back through the slots. Once the mounting tabs are nice and straight, you can usually remove the can without much problem. If the can doesn’t come off easily, you need to work more on straightening the mounting tabs and make sure all the solder has been removed from the tabs.
Note: If you brake one or more tabs during removel, or have damaged them to the point that they are likely to brake off during the re-instalation, I strongly suggest that you obtain a replacement mounting tab ring from a new old stock doner can.

3a. If you have removed a can which was mounted to a phenolic insulator (common on RCA sets), the can probably has a cardboard cover on it. If your can has a cardboard cover, the cover must be removed. The cover is held in place with tar. The easiest way I have found to remove the cardboard cover, is to get the can warm enough to soften the tar. I do this by placing the entire can into a microwave oven. I heat the can in increments of 2 minutes. Between each 2 minute heating, I remove the can and check if the tar has softened enough to let the cardboard cover to come off. Once the cardboard cover is off you can proceed to the next step.

4. The next step is to remove the mounting tab ring. You will notice that the bottom edge of the aluminum can is rolled over to hold the mounting tab ring in place. Using a high quality sharp pointed miniature diagonal cutter, work at the rolled edge until you are able to pry up a small section. Once you have a small section started, work your way around the entire edge of the can, rolling back the rolled aluminum edge. You will be able pull off the mounting tab ring when the edge is completely rolled back,

5. Now it's time to remove the capacitor bundle and the electrical terminals that are attached to the bottom of the can. The capacitor bundle is being held in the can with tar. We need to heat the aluminum can until the tar liquefies. When the tar liquefies, you will be able to pull the entire capacitor bundle out of the can quite easily. I use a propane torch as a heat source. A heat gun can be used, but I prefer a propane torch because it's faster. Put on a pair of insulating gloves so that you can grab the body of the can when it becomes hot. Hold the can by one of the electrical terminals using a pliers. Place the aluminum body of the can in your heat source (torch flame or heat gun). Move the can back and forth rotating while heating it. I usually heat until I can tell the pressure inside the can is starting to push the bottom of the can out. When I see this happen, I grab the body of the aluminum can with the insulated glove and pull the pliers and the can in opposite directions. (A couple of times I overheated the can and it built up enough pressure to blow the can and insides apart. Don’t worry the aluminum can is strong enough that it is not going to explode) Liquefied tar will run out of the can when you remove the capacitor bundle, so do this over some cardboard because the tar will make a big mess if you are not prepared. Place the can open side down so that the tar drains out onto the cardboard. A coating of tar will remain on the inside of the can. I usually leave the tar coating on the inside of the can.

6. Now you need to separate the electrical terminal assembly from the capacitor bundle so the terminal board assembly can be re-used. Place the blade of your utility knife between the terminal board assembly and the capacitor bundle, and cut all the way through slicing through the aluminum terminal leads that come out of the capacitor bundle. Trim any remnant of the aluminum leads flush to surface of the terminal board assembly.

7. Now it’s time to drill holes through the terminal board assembly using an electric drill and a 1/16" drill bit. I place the drill bit as close as possible to each terminal on the side of the terminal that faces the center. You will also need to drill one additional hole for the negative common lead that attaches to the mounting ring.

8. We are now ready to assemble a new capacitor bundle using new individual electrolytic capacitors you have purchased. In some cases you are going to have to use a little ingenuity to assemble the new capacitors in a way that will allow them to fit inside the can. Sometimes you will find that the new individual capacitors will not fit inside the old can because the old can is not tall enough. If you find the old can is too short to accommodate the new capacitors, you will need to obtain a taller can as a donor. Over the years I have collected 100's used and NOS can capacitors of different sizes. I buy them at swap meets and scrounge them off of old junked chassis.

Caution: Watch your polarity carefully when assembling your new capacitors.
The easiest mistake is getting your polarity wrong. Also be sure to connect the capacitor with the correct voltage and capacitance, to the correct terminal. You will find that there is a symbol next to the terminal that corresponds to a legend of capacitor values embossed onto the outside of the aluminum can or the cardboard cover. The symbols are “triangle” “semi-circle” and “square”. One terminal usually does not have any symbol. Place the positive lead of the capacitor through the hole you drilled and wrap the lead around terminal. In some cases the original leads on your new capacitors will not be long enough and you will need to lengthen the leads with some hook up wire. Solder the positive capacitor leads to the base of each terminal. Connect all the negative leads together and attach a piece of hook up wire wire to the negative leads. Extend the negative wire through the hole you drilled in the terminal board back in step 7. Make sure the negative lead sticks through the hole at least 1”. At this point the negative wire is not connected to anything. The negative lead will get connected after the can is mounted on the chassis. Make sure to dress your leads in such a way that they will not short out and that they will not short to the inside of the aluminum can. This is a photo of the way your new capacitors will look.

9. At this point, and before installing the new capacitor bundle into the can, we should straighten the unrolled edge of the can. I made myself a small anvil that I place in my bench vice. It is made from a piece of ¾” thinwall conduit. I cut one end on an angle so that there is a “tongue” that sticks out to use as an anvil surface. Place the edge of the can on the surface of the anvil, and using the small hammer, flatten out the rolled edge of the aluminum can to remove all the kinks.

10. Now it’s time to install the new capacitor bundle into the can. Slide the new bundle into the can and push firmly on the terminal assembly so that it seats as deep as possible into the bottom of the can.

Next, install the mounting ring over the terminal assembly. You will see that there is a small indexing finger sticking out of the ring on the side that faces the terminal assembly. If you look closely you will see a small corresponding notch in the phenolic material of the terminal assembly. Make sure you place the index finger of the mounting ring into the notch in the terminal assembly. Using a screwdriver and a hammer tap around the perimeter of mounting ring to make sure the terminal board is seated down into the can as far as it can go.

11. At this point we can roll back the aluminum edge of the can to re-seal the can. I usually use the flat side of a diagonal or needle nose pliers to start folding the edge back. In some cases there will be just a small amount of aluminum to fold back over. It will vary from one can to the next. If you don’t have enough aluminum to fold over, you probably have not pushed the terminal board hard enough into the bottom of the can to get it seated all the way down. Once you have the aluminum edge of the can folded back over the mounting ring, take your small hammer and “peen” the aluminum nice and flat, best as you can to seal the can.

11a. If your can had a cardboard cover, now is the time to re-install the cover over the aluminum can. I place a small amount of silicone rubber sealant on the inside surface of the cardboard cover right at the bottom edge. Then just slide the cover over the aluminum can all the way down so that the aluminum can is seated all the way inside the cardboard cover and so that the aluminum rib at the bottom of the can is completely hidden inside the cardboard cover.

12. Now, re-mount the can onto the chassis in the same exact position it was prior to removal. Look at your photos to make sure the symbols are in the original position like the factory installed it. Insert the mounting tabs through the slots and CAREFULLY twist the tabs ¼ turn using your duck bill pliers. Grab the tab near the tip of the tab. If you grab the tab close to the chassis, the tab will be more likely to snap off. The tabs are designed to be twisted only once, so this is the 3rd time we are twisting them (once at the factory, once to remove and now a third time to re-install.) If your can had the mounting tabs soldered to the chassis, you should re-solder the tabs now.

13. Now connect the negative wire sticking through the bottom of the can, to the closest mounting tab and solder the negative wire to the tab. Finally, re-connect the chassis components and wires to the terminals on the can as documented in your photos.

Good luck on your project.