Electrolytic Taxidermy

[igranic]

When electrolytic capacitors fail, stuffing the original container with modern capacitors is one of the few available options if no suitable replacement is available, particularly if cosmetic authenticity is important. In this case the victim is a 60uF/250uf reservoir/smoothing capacitor from a Bush TV22.
My apologies for the very poor picture quality - I was forced to use my mobile phone.

https://www.vintage-radio.net/forum/...chmentid=29589

The choices when opening the can are either to trephinate the rubber bung using a fine-toothed hole saw of the appropriate diameter, or slice through the aluminium can where it would normally be concealed by the chassis-mounting clamp. In this case the clamp fits at the closed end of the can, and the rubber bung, which is more readily visible, was showing signs of delamination, so the decision was taken to slice through the can near the closed end.

This could be done using a fine-toothed hacksaw or junior hacksaw, but, for neatness, the method chosen was to mount the capacitor on the lathe in a three-jaw chuck, with the outer end supported in another chuck mounted on a revolving centre in the tailstock. The can was then sliced through using a sharp, fine-pointed cutting tool. It was then possible to lift off the closed end to reveal the contents; a wound sandwich of aluminium foil and paper soaked in electrolyte, all encased in black pitch. As the formulation of the electrolyte is uncertain, rubber gloves should always be used when handling opened electrolytic capacitors, and the unwanted contents disposed of safely.

A large (100mm x No 12) chipboard screw was carefully screwed into the core. With the screw firmly held in the bench vise, the can was then gently heated with a 'soft' flame from the blowtorch to loosen the contents, then gripped with a thick rag and carefully pulled off taking care not to crush the the soft aluminium. At this point, it is possible to experience some difficulty due to the aluminium terminations attached to the tags in the rubber bung, but in this case they simply broke free.

https://www.vintage-radio.net/forum/...chmentid=29590

With the contents removed, the interior of the can can be cleaned out ready for stuffing, but first we need to find a way to fit the two parts of the can back together and reinforce the soft aluminium to prevent it being crushed by the chassis-mounting clamp. In this case, the can has a nominal outside diameter of 1.75 inches ( 44.5mm), and it just so happens that a piece of 43mm plastic waste pipe is a snug internal fit. A piece was cut to length so as to form a liner for the complete interior length of the can, and can be glued in place.

https://www.vintage-radio.net/forum/...chmentid=29591

The can is now ready for stuffing. However, a means is required to connect the capacitors to the tags already riveted to the rubber bung with aluminium rivets. One method is to drill a small hole through the centre of the rivet, but this does carry some risks, especially if the aluminium rivets have become corroded or brittle. My preferred method, particularly where larger capacitance values (and hence larger inrush and ripple currents) are involved, is to drill or pierce (with a hot wire) holes in the rubber bung immediately adjacent to the solder tags, thread fairly heavy tinned copper wires through the holes, connect the new capacitors and draw them into the can, then wrap and solder the TCW around the base of each tag. When stuffing is complete, the closed end can be glued in place on the end of the plastic liner.

https://www.vintage-radio.net/forum/...chmentid=29592

Edward

[Retired]

Hi Edward, A good explanation and a very useful tip. This is the method I also use. I agree both ends need suporting if using a lathe; I found this out the hard way when trying to cut one just mounted in the headstock chuck; it grabbed as the tool broke through. A pipe cutter also makes a neat job for those without a lathe but the inners take a bit more pulling out. Col.

[Station X]

A very nice how to do it post Edward. I restuff in the same way. If you can't find a bit of plastic pipe which fits exactly use an oversized piece and cut out a longways strip. The joint can be glued with solvent adhesive, but it isn't really necessary.

The joint can be covered with aluminium tape.

I was going to post a warning, but Retired has beaten me to it. The first time I tried to cut a can in the lathe the tool dug in and ripped the can to pieces. Ever since I've used a pipe cutter.

[Mike Phelan]

Nice job, Edward.

Just to be different, if I do this in the lathe, I use a piercing saw (the sort that dentists and jewellers use, with very thin blades) to cut it, if the clamp can cover the cut.

[YT2095UK]

I have to ask, and hope this isn`t too Naieve a question, but can`t you just Buy a replacement capacitor?
Surely it can`t be "I do it because it belonged to this radio" sort of philosophy, otherwise no one would change "That" cap with a bright Orange jobby.

[ppppenguin]

In many cases you can't buy a new electrolytic because they just don't make them any more. This certainly applies to most of the multi-section ones, with up to 4 caps in a single can. Even a single section modern type may be mechanically very different, usually smaller and able to fit in the old can.

Edward, thanks for a nice clear description of the method.

[igranic]

Originally Posted by YT2095UK

I have to ask, and hope this isn`t too Naieve a question, but can`t you just Buy a replacement capacitor?
Surely it can`t be "I do it because it belonged to this radio" sort of philosophy, otherwise no one would change "That" cap with a bright Orange jobby.

Many of the high voltage ( > 100 volt) electrolytics, especially multi-section types, such as the example I used, are simply no longer available.

Whilst it may be possible to functionally substitute modern electrolytics, they would not have identical mountings, nor would they blend visually with the other vintage components in the item being restored/repaired.

Some restorers do, indeed, remove the contents of 'that' wax capacitor and stuff it with a smaller, modern substitute in order to maintain the authentic look of a period radio.

Edward

[YT2095UK]

that`s a real pity, and I`d be willing to lay wager that todays caps wouldn`t last as well over 50 years as some of the older ones do/did.
it`s a wonder that No-one`s clocked onto this and set up a company that Does make identical caps as replacements though, there would be some real money in that I reckon!

and to keep on topic, for those of us (like myself) that don`t have a lathe or pipe cutter, are there any contra indications about using a Dremel?
that would be my 1`st choice.

[Brian R Pateman]

Originally Posted by YT2095UK

......for those of us (like myself) that don`t have a lathe or pipe cutter, are there any contra indications about using a Dremel?
that would be my 1`st choice.

I use a Dremel. It works very well.

My choice is to use the cheap fibre type discs. These give a neat thin cut and have the secondary advantage that they will break if you snag anything, rather than damage the case.

Be sure to wear eye protection of course.

[Boom]

Very tidy Edward. What capacitors did you use for the re-stuffing and where did you get them from? I have an identical smoothing can here waiting to be stuffed just as soon as I can scrounge use of a pipe cutter. The caps' kindly supplied by another forum member seem so small they will be lost in a TV22 can.

[OldBoffin]

Hi All,
Whilst I admire the work described above, I am very concerned that the purpose of using a large Ali can originally was to disipate the heat resulting from the high pulse currents with associated ESR, has been lost.
The does not seem to be any way for the heat from much smaller new capacitors to be disipated, with resulting short life of the new components!
Maybe some-one would like to comment from extended trials.
OldBoffin

[murphyv310]

Originally Posted by YT2095UK

that`s a real pity, and I`d be willing to lay wager that todays caps wouldn`t last as well over 50 years as some of the older ones do/did.

Hello.
Believe it or not today's polypropylene capacitors are far better than any waxy cap. the modern ones are hermetically sealed so moisture is not a problem unlike the ones produced 50 + years ago.
Electrolytics though are a different scenario, some are very good but often cheapo ones from China are very suspect with failures within a year. I have restuffed 50year old smoothers with modern caps only to find them fail within 2 or 3 years.
Cheers
Trevor

[DangerMan]

Yep, I'm with OldBoffin on this one.. I think he is correct to point out the "downside" of restuffing electrolytics.
As a rule of thumb, for every 10 degree increase in temperature of an electrolytic capacitor there will be a halving of its lifetime prviding it still runs within its maximum rating. Or to look at it another way... a doubling for every 10 degrees cooler it runs beneath that max rating. Lifetime figures for most electrolytic capacitors operated at maximum rated temperature are not exactly huge anyway... not even for so-called long life ones.
For a high capacitor carrying a large ripple current (e.g. a TV set's reservoir capacitor), there will be a degree of self-heating, and this will cause the case of the component to be warmer than the surrounding air, even when not hidden inside another can. The centre of the capacitor's winding will be hotter still.
This could easily account for Trevor's observed failures, especially when you consider the high temperatures that you often find inside valve television sets..
TTFN
Pete

[Dave Moll]

I find myself wondering whether there is anything to be gained by wrapping some fairly stout wire round the new electrolytics to make contact with the outer case (through a space made in the plastic lining) to provide some degree of thermal conduction (or heat sink) for the new caps - or would the amount of thermal conduction be too insignificant?

[DangerMan]

Hi Dave,
yes, I'm sure wire would help but even better would be to use flat aluminium or copper tape wound around the internal replacement, after first removing the plastic wrapper. This can be made a nice tight fit with good cross sectional and surface area for conduction, and so it really does the job. It has disadvantages if you would like to finish up with a capacitor that has its can iolated from the negative termnal (for use, say, in a HiFi amplifier) but it assures some extension in lifetime. I did this for a Leak Stereo 20, where original outside appearance mattered, and the position of the hidden reservoir capacitor can easily be felt from the outside after a little while in operation, because it is slightly warmer than the rest of the can.
The tar that is found in old capacitors was intended to provide improved thermal path as well as to stop the rolled-up capacitor winding from rattling about, and modern electrolytics intended for large inverters (variable speed motor drives) often have extensions of the negative foil which are intentionally forced on to ribs inside the base of the can to provide a conduction path to the outside world. These may be of the order 2000 to 4700 uF at 450v DC, so not really suitable for our purposes !
However, even a few degrees reduction is worth while, and this does give an idea of the lengths to which manufacturers will go to do this..
TTFN
Pete

[YT2095UK]

you could always part fill the cap with Woods metal and sink the new cap into that, it melts are roughly the same temp as a hot cup of coffee and conducts well as it`s a metal.
I use and have made my alloy variations of this before to "solder" in semiconductor crystals into tin pots, where a High temp would fracture/shatter these crystals.

just an idea

[Oldtimer74]

I consider it advisable when restuffing electrolytics to fill the can with good quality wax or even pitch to give thermal conductivity to the can. Electrolytics shouldn`t get hot as they should only be passing a mA or two. I`ve done a few in my time and not had a failure yet.
Pat G3IKR

[John M0GLN]

Originally Posted by igranic

Some restorers do, indeed, remove the contents of 'that' wax capacitor and stuff it with a smaller, modern substitute in order to maintain the authentic look of a period radio.

So one should actually check 'that' cap and not just dive in and replace it.

John

[DangerMan]

Most electrolytics only pass a mA or two, but reservoir capacitors do pass a lot of mA especially in a half wave rectified telly! I agree they shouldn't get hot from their own endeavours, so a cap with low ESR is very helpful. Then, any material to take the heat away is beneficial, and there are many options, even good old tar, wax, or silicone rubber provided the acetic cured version doesn't rot your capacitor's internals away. However, it has to be rememberd... you should provide a safety vent in case the electrolytic fails and vents gas... not to do so is asking for a mighty bang!
I have avoided woods metal as I never really wanted a conductor of electricity potentially wandering around where I can't see it... if it were to get too hot for any reason.
Also important to remember.... you might have to repeat the performace one day, so best not to use anything too permanent.
TTFN
Pete

[KeithsTV]

The caps I supplied to David are rated as follows

220uf 450v, -25C to +105C, max ripple current @120Hz 1120mA derated to 860mA @50Hz and endurance of 3000 hours.

68uF 400v, -25C to +105C, max ripple current @120Hz 460mA derated to 350mA @ 50Hz and endurance of 3000 hours.

According to the data sheet the capacitor has to be within 20% of its initial value, 200% or less for the specified tan delta and leakage current less than or equal to the specified leakage current with full working voltage, maximum ripple current at 105C for 3000hours.

I don't think even a TV22 could manage those conditions.

Now 3000 hours equates to 125 days but in practice the capacitor would not be run at 105C. If the temperature is 68C using the rule that the life doubles for each 10C temperature drop from maximum, the life would be approximately 4.5years. This is still at maximum ripple and being run continuously. So in practice these caps would last several years. Most capacitor manufacturers have derating factors for ripple current but I do not have this for these particular capacitors.

If these capacitors were being run at 45C their life without any ripple current derating would be of the order of 22 years, again continuous operation.

3000 hours is the typical life for this type of capacitor, you can get 5000 hour parts but they are more expensive. For manufacturers there is a compromise between lifetime and cost. A typical power supply, at least where I work, has a design life of 5 years minimum and for some customers it's 7 years.

I have not measured the ripple current in the main electrolytics of a TV22 but I don't think it is anywhere near the ratings of those capacitors. I'd be interested if anyone has the ripple current ratings and temperature specs of the capacitors originally used in the TV22.

Keith