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Capacitor Restuffing

Monday, April 11, 2016

When repairing/restoring vintage radios it's pretty common to end up replacing all the electrolytic, wax-paper, and molded paper capacitors, as well as several carbon-composition resistors. Usually it's simply a case of 'out with the old, in with the new' (while keeping any above-chassis components in place but disconnected to preserve the 'look' of the radio), but sometimes you might want to keep things looking as original as possible above and below the chassis.

That's where restuffing comes in handy…

Personally, it's not something I do often - however next in my repair queue are a couple of sets that I think are worth the little extra effort to keep looking as original as possible (or at least 'straight'; nothing obviously out of place at a casual glance). That means either (a) used or NOS, (b) newly-manufactured "old style", or (c) re-stuffing - hiding the new within the old.

Without going too deeply into the reasons/benefits/pitfalls of each:
  1. Used/NOS - just as old as the originals; just as likely to be faulty/fail (i.e. "very")
  2. Newly manufactured "old style" - expensive (but sometimes worth it)
  3. Re-stuffing - cheap, but takes a little time, effort, and skill.
In the case of resistors, I'll often go for (b) - many new wirewound resistors, particularly ceramic or silicon types, don't look too out of place in a vintage radio. I've also got a small stock of very high quality (telco/milspec) NOS carbon comp resistors that I occasionally use. For capacitors, though, I usually opt for re-stuffing.

It's worth pointing out that, while electrolytic caps from the 40's, 50's, and 60's were built like tanks,  not all modern electrolytic caps are created equal. In general, quality is reflected in the rated lifetime & temperature (higher lifetime & temp ~= higher quality), but is balanced somewhat by ripple current rating (higher ripple current rating ~= lower lifetime), especially at 50/60/100/120Hz (watch out for the ripple frequency derating factors in the datasheets!). Ideally, when restuffing old electrolytic caps you want a good mix of high rated lifetime, high temp, and high ripple current @ low frequencies.

My personal choice is Panasonic ED (now superseded by EE) series - 10000hrs @ 105°C, ~200mA ripple current rating < 120Hz for the values typically used in valve radios, and relatively cheap. But the other big manufacturers - Wiha, Nichicon, Vishay, etc - all have comparable series.

As is frequently pointed out in some places, those Panasonics have a [slightly] higher ESR than others - and so are seen by some as 'junk'. Without digging deep into the whole mess that is the general misunderstanding of ESR, (a) ED/EE series ESR is only slightly higher than equivalents from other brands, and (b) absolute ESR of an electrolytic is not an indicator of capacitor quality [although a too-high ESR is an indicator of a failing capacitor]. It's a non-issue - but if you disagree, feel free to buy another brand / series for your restorations ;).

Re-stuffing Electrolytic Capacitor Cans

I'm currently working on a little AWA 565MA. This set has 2 x 24μF 350V caps on the main HT & B+ rails which, despite being 60 years old and seeming OK on a quick power-up test, needed to be replaced since both were slightly weeping electrolyte. Normally I wouldn't bother restuffing the electros in such a common set, but I figured that since I almost never do it I'd get some practice in before I tackle those other sets.

Although the details differ from capacitor manufacturer to manufacturer, the basic process is this:

First, desolder the wires & remove the can from the chassis. In this case, the capacitor is a small twist-lock can mounted on a fibre plate, which in turn is screwed down to the chassis - simply remove the screws and lift out. Clean the solder tags, and straighten the twist-lock terminals.

Lifting off the fibre mounting plate reveals the actual base of the capacitor. In this case the can is made of aluminium (which is difficult to solder to) - if you inspect the base carefully, you may see that the mounting tabs (which are also the negative connections) are attached to a plated brass ring which runs around the circumference of the base & is crimped under the lip of the can, also holding the red end-plug in place. (Don't worry if you can't see that in the pics - on the caps in this set it was almost invisible even under magnification, and with the remains of 60 years of accumulated dirt in the seam looked like it was a single rolled-and-folded piece!).

Notice that this particular capacitor has a clear plastic sleeve over the can. Others of this style typically have a (usually grey) PVC sleeve which is rolled over the ends. In either case this can be softened with boiling water and rolled back out of the way…

Carefully pry up the rolled lip of the can all around. I find it easiest to start by tapping a small awl or very fine (but a little blunt) chisel into the edge between the rolled lip and the plated brass terminal ring. Once a small gap has been made, I then work a couple of passes around the edge with a small flat screwdriver to lift it all the way around, before flaring it out very slightly to allow removal of the end-plug. Lift out the plated brass negative terminal ring and put it to one side.

The end plug and guts of the capacitor can then be removed. The easiest way is to soften the wax inside the can by heating it in just-boiled water for a few minutes, holding the can in a towel, then grabbing the terminal with pliers and pulling straight out. You should end up with 3 pieces - the can, the capacitor 'guts', and the end-cap - all connected together by the capacitor foil.

Cut the foil at each end and toss the capacitor guts in the bin (being careful to wash your hands afterwards - the contents are waxy, but also slightly corrosive & nasty-smelling). Clean up the remaining foil from the inside of the can and the end-cap by scraping it away with a knife or screwdriver, tidy up the inside face of the end-plug with a small wire brush, and wash any remaining wax and/or electrolyte out of the can with boiling hot water. Wipe everything off with a paper towel, and leave it all to dry completely.

Using a small (~0.8mm) twist drill, drill two holes in the end-cap for the replacement capacitor leads. The first should be directly adjacent to the centre terminal, located so the capacitor lead pops up right behind the solder tab. The second should be at 90° or so, and spaced far enough from the edge of the plug that the lead emerges right next to where one of the terminals on the negative ring will be. (In this case, drilling through the existing pressure vent was close enough!).

Take your replacement capacitor - in this case, 22μF / 450V - and, taking care with the polarity, pass its leads through the holes you just drilled. Centre the capacitor above the end-plug, check the clearances between the leads and terminals and, if there's any possibility of shorting, sleeve the leads.

Wrap the positive lead around the centre terminal to hold it in place, and gently press the end-plug back into the can.

Replace the plated brass terminal ring and, pressing everything down into place, fold the rolled edge back into place. The easiest way to do this is using a small flat screwdriver blade to fold in 4 points at 90° around the circumference, then repeating that shifted by 45°. To complete, use a larger flat-blade screwdriver with a rolling motion to fold the remaining edge back over in a couple of passes around.

Finish off by replacing the fibre mounting plate (you may need to file or cut small slots to allow the new capacitor leads to come through). Twist the twist-lock tabs to hold everything in place, and solder the capacitor wires to the terminals. Of course, this is one time that the fibre plate decided to crack on me! I repaired it as much as possible with superglue before re-installing the capacitor in the radio…

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