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Capacitor outside foil - Printable Version

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Capacitor outside foil - bridkarl - 12-28-2023

I get the idea that installing caps with the foil to ground reduces the noise floor. New caps don’t come marked so you have to do this yourself.

I’ve not been doing this - and haven’t really noticed any issues on any of the radios I have repaired. 

I’m redoing this Heathkit O-12 oscilloscope right now. Would this matter more in test equipment?  I’ve got a Kenwood SM-220 station monitor with an oscilloscope function so it’s not an issue to take an afternoon and mark the foil ends.

My apologies if this is a dumb question. Perhaps everyone does this and I’m slow on the uptake.

462ron - 462ron - 12-28-2023

Hi, I found this article on how to determine which lead of a cap is connected to the outside foil, I’ve never tried it myself. Give it a try FWIW, seems quick enough!

https://www.nutsvolts.com/magazine/article/finding-the-outside-foil-lead

Ron

RE: Capacitor outside foil - Chas - 12-28-2023

Modern metalized capacitors do not expose an electrode to enclose the body like early paper capacitors. They are of a stacked construction, therefore there is no outside foil by definition. Consider if there were an "Outside Foil" the manufacturer would have indicated.

https://en.wikipedia.org/wiki/Capacitor_types

https://en.wikipedia.org/wiki/Film_capacitor

Though it matters little to restoration of domestic radio, choosing the correct replacement capacitor for the "all purpose" paper/foil capacitor is routinely the inexpensive metalized plastic film design. However, that type is not as universal as the paper once was...

Restorers are aware to use the "Safety" capacitors on AC line bypass, it that, it fails "open".

I am, too, squeamish about installing metalized without an outside foil. I "feel comfortable" by simply orientating the legend in the same direction as the paper capacitor it is replacing, silly, but I have not had a coupling/hum/noise issue, yet.

Common metalized cannot withstand serving in a pulse environment. The rise time of pulses that can appear in the output circuit of a radio can cause the metalizing to break-up, yet the film not puncture or short. This causes a loss of capacity and in some instances noises, (random ticking) actually a short train of pulses generated by the metalizing connecting/disconnecting.

For such purpose as pulse service, there are metallized of special internal construction or the use of a foil. Sounds all good but the foil/(plastic) can fail shorted and not clear the short like a metallized can.

Chas

RE: Capacitor outside foil - bridkarl - 12-28-2023

Yes - my experience has been the same. There are various online resources which declare this is a necessity but I’m dubious. It seems more important to make sure the caps are properly soldered and mechanically stable i.e. not hanging in space, using tubing over bare leads,etc.
I find interesting the variety of opinions on this. I’ve actually been told that replacing old caps is bad, “shotgunning” all the sixty year old caps is bad practice, recap kits are just a scam, etc. I don’t really get the antagonism in some places, not here in this forum, to people doing this.

RE: Capacitor outside foil - Chas - 12-28-2023

Quote:Yes - my experience has been the same. There are various online resources which declare this is a necessity but I’m dubious. It seems more important to make sure the caps are properly soldered and mechanically stable i.e. not hanging in space, using tubing over bare leads,etc.
I find interesting the variety of opinions on this. I’ve actually been told that replacing old caps is bad, “shotgunning” all the sixty year old caps is bad practice, recap kits are just a scam, etc. I don’t really get the antagonism in some places, not here in this forum, to people doing this.
Overall, the Phorum is a much "smaller place", because of its specialty.

I have "shotgunned" radios, Icon_eek but, I am a qualified (former MA licensed Radio/TV technician) as well as an inventor of several electronic devices. Meaning, if I dig my way into a hole, I can get out of it. I once took a A-K 55 down to the chassis, left it disassembled for some 6 years, then in a flurry, rebuilt with outstanding results, I never knew a TRF could be as sensitive as it was.

None the less, someone new to the hobby OR working on an unfamiliar chassis as well as not have "suitable" test equipment should NOT "shotgun".

That methodology does not do well for the person who just wants to have the radio/phono work.

I have mixed "feelings" leaving/using old caps. I have left some radios just that way with exception of filter caps and vital coupling caps. That I found is not always a viable cheat as I have had screen bypass caps fail and take out a nice, good dog bone screen resistor, grrr.

I have adopted a practice of replacing the most critical caps and far off tolerance resistors, but only after a suitable "triage", that is checking the difficult or unobtainable components first. The powering ( seldom use a "dim bulb or a variable transformer) that, depends largely on how much I "trust" what I have seen and triage repaired. Assess again, then proceed to replace two capacitors at a time and any adjacent (circuit) resistors, then power once again.

I only "shotgun" when I absolutely know the chassis/model and it potential issues.

My comment about capacitor kits, these too can be "two at a time" installed, unless the chassis is well known to the restorer. I do believe the cap kit is economical if one considers the time involved doing look up for capacitors used especially modern electrolytics for solid-state. I once re-capped a S-S phono, what a PIA! Not only the value but the dimensional information and the MTBF as well as operating temp had to be considered. Project went well but time at the desk was awful. At least two caps had to be laid down and silicone to the board... Icon_eek

Chas

RE: Capacitor outside foil - David - 12-28-2023

Some of the audio folks think any modern part is bad! The only way to have a perfect sound is to have 60 year old caps in a system. Some folks think the earth is flat. Going to stop this is a family Phorum. David

RE: Capacitor outside foil - Chas - 12-28-2023

Quote:Some of the audio folks think any modern part is bad! The only way to have a perfect sound is to have 60 year old caps in a system. Some folks think the earth is flat... David
Sound is everything... In a concert hall or practically any musical venue, there are delays in the sound as well as attenuation from clothing, tapestry and ceiling texture that the live listener cannot consciously detect with exception of echos. When an audio system is used in a home the environment can be similar but instruments and performance speakers in use do not exist, being replaced by the systems speakers. That, amplifiers, in order to more faithfully replicate the concert venue environment have timing delays, for a tube amp these are accidental. However, leaking components can accidentally" enhance these delays. But, there in, it is an inexact science and any two deteriorated capacitors are not going to leak at the same levels...

Musicians that use electronic instruments are well aware of the lack of delay in a solid-state amplifiers and often use devices that delay, color or what ever term change the tonal qualities, then send that modified sound to the amps. Such that the amp is FLAT with minimal delay now reproduces the sound without change.

That is how it should be accomplished for the home system. But so long as there are "Black Arts" and the associated jargon there will be folks who are happy to do and have equipment that is operating at some level of inexact reproduction... It is their ears/brain/pocketbook that has to be happy.

RE: Capacitor outside foil - Arran - 12-28-2023

In my personal opinion, I think it's a waste of time trying to find the outside "foil" of a film cap, if you look under the chassis of the average old radio the so called foil end was installed in all directions, no necessarily to the most negative end. If you want to do it go ahead, but I kind of view like restuffing the shell of a paper cap, I restuff them for looks but I don't expect the radio to perform better. 
Regards
Arran

RE: Capacitor outside foil - Brad Winder - 12-29-2023

I take the time to find/mark the outside foil on the new poly caps I install. Yes, I know that in most cases it won't make a difference...because up until about 6 years ago, I didn't bother to do that. However, I ran into a couple sets that it DID matter, and one took a heck of a time to find and correct the problem.
In reality...does it take much time to do this? It maybe adds a few minutes to every set restoration. But is that a lot when looking at all the extra procedures and rebuilds we do to the average chassis during a full restoration? No.

RE: Capacitor outside foil - radiorich - 12-29-2023

Hello Brad,
Yes, I agree with you it does not take too long!
Chas one the last capacitors that I can think of is Sprague orange drops.
as was talked about not every capacitor works best in every circuit and even the radios that I work on I mark or tag and use photos of my work so that I don't lose my place and like you some of my sets have been sitting for some time like a few years .

Sincerely Richard
P.S. great article !