Resources
« Which lamp works well for Marantz Models 1200 and 1200B | Main | I've added a HP8640B Signal Generator to my bench »
Tuesday
Jan152013

Restuffing a Multi-Section Can Capacitor

Saturday I was in the lab working on an radio from 1962. This little AM FM radio is a vacuum tube design and certainly was showing it age. When it came into the lab it had not worked for years and the owner wanted it repaired.

The most difficult component to replace turned out to be the original Sprague multi-section power supply can capacitor. This is a 4 section can capacitor with values of 100mfd @ 250 volts, 2) sections were 40 mfd @ 250 volts and the fourth section was 50mfd @ 25 volts.

These types of multi-section can capacitors use different symbols to identify which solder lug belongs to which section. In this case the symbols were:

Δ = 100mfd @ 250 volts

p = 40mfd @ 250 volts

m= 40 mfd @250 volts

- = 50mfd @ 25 volts

     

While there are several different companies that manufacture new multi-section can capacitors, none of them had the correct values for this tube radio.

I decided that the best method would be to “restuff” the original can with new modern capacitors. Sometimes it's practical to install individual capacitors in place of the original can, however this often looks messy and securing the components can be tricky. It's also important to remember that the original can capacitor has a common negative ground, this means that the negative lead from each section (capacitor) would need to be tied together and then jumpered across all 4 ground terminals on the circuit board, this can become quite messy.

The first step was to search for new capacitors that fit into the 1.5” diameter can properly. I found Nichicon CS Series which are pencil shaped capacitors. This series is a long and thin radial capacitor which only took up a small amount of the available space. I choose two 47mfd capacitors from the same series to take the place of the 40mfd sections. The original 50mfd section was replaced with a 47mfd from a different series, mostly for its small size.

 After removing the can capacitor from the circuit board, I took it over to the band saw and carefully cut the aluminum can right where it begins to flair into the larger diameter at its base. The aluminum is very thin and cutting it open was very easy. I think it would also be very easy to cut with a small hand held hack saw.

When you pull the cover off, inside you will find the capacitance roll. This is made up of a long thin sheet of insulating paper which has a thin layer of aluminum foil attached to it.

As you unroll the capacitor you will find points within the roll that have aluminum strips fastened to the aluminum foil by some type of crimping method.. These strips are connected to the solder lugs on the bottom of the can.

The more you unroll the capacitor the more aluminum strips your will find. Since this is a 4 section capacitor, there are 4 aluminum strips, plus 1 more which is the common negative and is attached to the can itself.

After the entire roll is removed you are left with the cans base and all of the aluminum strips.

One think to keep in mind is that you can not solder the leads of the new capacitors to the aluminum, it won't stick. If you look at the bottom of the original can capacitor, you will find 8 solder lugs. The 4 around the perimeter are the common ground lugs and each of the other 4 lugs are the 4 sections within the can.

Of course all of the 8 lugs were soldered to the circuit board. If you look carefully at the lugs, you will see when the aluminum strips have been crimped to the metal lugs. The aluminum strip for the common ground is crimped in between the aluminum can and the metal ring which holds the non-conductive base, this ring has the 4 ground solder lugs.

It will be necessary to install the 4 replacement capacitors onto the base of the original can. I carefully drilled (8) 1.25mm holes in the original base, 1 along side of every positive solder lug on the original base and 1 along side each of the common ground lugs around the perimeter ring for the negative terminal.

Drilling the holes in these locations will allow the leads of the new capacitors to align with the metal lugs on the base, where they will be soldered. All you have to do is push the leads through the holes, the positive lead can be tightly wrapped around the top of the lug and soldered. I bent the negative lead to match the curve of the cans base and solder it to the metal ring.

After all 4 of the new capacitors where in place, I used my LCR meter to verify that the each of the new capacitors where in the proper location, referenced by the symbols on the cans cover. I then used a multi-meter to make sure that the common ground was intact on all 4 ground lugs.

To close up the can, I simply placed the cover back on the base, matching up the cut marks and sealed it first with electrical tape followed up with a section or large diameter heat shrink tubing.

Once the can was remounted on the board, you really couldn't tell that it had been modified at all.

 

Reader Comments (2)

Chris,

Very clever ingenuity. I like! reminds me of how I had to use foam to 'hold' my new filter caps in the 'old' capacitor clamps.

Daniel

December 17, 2013 | Unregistered CommenterDaniel

Very nice work. How are the tuner voodoo lessons coming along?

April 21, 2014 | Unregistered Commenterlavane

PostPost a New Comment

Enter your information below to add a new comment.

My response is on my own website »
Author Email (optional):
Author URL (optional):
Post:
 
Some HTML allowed: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <code> <em> <i> <strike> <strong>