Welcome to Australia's only Vintage Radio and Television discussion forums. You are not logged in. Please log in below, apply for an account or retrieve your password.
Australian Vintage Radio Forums
  Home  ·  About Us  ·  Discussion Forums  ·  Glossary  ·  Outside Links  ·  Policies  ·  Services Directory  ·  Safety Warnings  ·  Tutorials

Tech Talk

Forum home - Go back to Tech talk

 Finding the outside foil lead of capacitors
« Back · 1 · Next »
 Return to top of page · Post #: 1 · Written at 8:48:32 AM on 11 March 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

I come across this link looking for something else, as you do, and decided to post it as the subject comes up from time to time.

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

I haven't studied this info closely, but hope it might be useful to someone, as it appears to be thorough.

I have wondered for a long time why the outside foil marking was considered necessary in the time of waxies, but is not now.


 
 Return to top of page · Post #: 2 · Written at 5:53:44 PM on 11 March 2023.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6720

It's a problem that I never knew I had (and am not yet convinced that I do).

Yes, there's quite a bit of discussion about it 'out there'. Here's just some ...


Carlson's solution:

https://www.youtube.com/watch?v=BnR_DLd1PDI

https://www.youtube.com/watch?v=ridVNKwDNPA&t=1313s


Restore Old Radio's version:

https://www.youtube.com/watch?v=PyBENsesbMM

https://www.youtube.com/watch?v=74I2iP0GkTk&t=5s


A timber version:

https://www.youtube.com/watch?v=Qu_FrHdUUNs


Or for just US$225 (!) + shipping you can have one of these:

https://www.youtube.com/watch?v=mx1Z_zyDhw4&t=348s

https://milspecdesigns.com/sniffkwik


 
 Return to top of page · Post #: 3 · Written at 8:50:07 PM on 11 March 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

I think that you might have capped the subject GTC.

Maybe outside foil lead orientation could be critical in difficult to predict cases. So a prudent designer might have said "always put the outside foil end at the earthy location" just to avoid trouble.


 
 Return to top of page · Post #: 4 · Written at 11:24:42 PM on 11 March 2023.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6720

I think that you might have capped the subject GTC.

It's a polarising topic, STC


 
 Return to top of page · Post #: 5 · Written at 9:06:42 AM on 12 March 2023.
BringBackTheValve's Gravatar
 Location: Linton, VIC
 Member since 30 December 2016
 Member #: 2028
 Postcount: 472

"I think you may have capped the subject----" plus "It's a polarising topic,---"

It's the banter that puts this forum light years ahead of other forums.


 
 Return to top of page · Post #: 6 · Written at 10:31:19 PM on 12 March 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

That merely high lights a problem that we have been spruiking about for years. We are well aware that the modern replacements for waxed paper caps are not marked like the old ones and when used in areas like the 2nd detector aka detection first audio; we can have all sorts of issues from radiation and induction.

Some of the disc capacitors are also apt to be susceptible to induction. All of which is part of lead dress.


 
 Return to top of page · Post #: 7 · Written at 9:54:58 AM on 13 March 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

Looks like the subject is undergoing a recap.

The lack of markings in modern capacitors suggests it is not seen as a problem by customers or manufactures, or they have a solution for it, by detecting the correct lead and/or designing out radiation/induction issues. In our little valve radio restoration niche, we are too small to count.

GTC isn't convinced that he has a problem; Marcc is convinced he does.........

Maybe a case for another micromite kit like the Silicon Chip IF alignment unit.

A PS on this: if you are stuffing caps, to do a proper job, detecting the outer foil lead would be necessary.


 
 Return to top of page · Post #: 8 · Written at 8:52:41 PM on 13 March 2023.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2405

Answer:

Modern caps have much lower ESR and self-inductance. Which makes the issue of Outside Foil irrelevant because the capacitor itself is much closer to being perfect, at least at the frequencies involved here.

Now when you work on PCBs with multi-GHz signals, 10 layers, ground planes and controlled impedance traces (as I do), it does start to matter.

That's not to say that lead and component dress doesn't matter. It does. But all things being equal, I challenge anyone to invoke instability by merely reversing a poly cap in the same position.


 
 Return to top of page · Post #: 9 · Written at 9:18:04 PM on 13 March 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

It does not take rocket science to work out which is the outside layer on those yellow axial ones, & the green caps, if there is a problem.

One gets an oscilloscope; probe, on "by one" and something "dirty" like a switch mode "wall wart" close. Hook the cap up so as its picking up the dirt then note the amplitude when you put your fingers around it, then reverse it. The lowest peak when you put your fingers on it is the end with the foil on the outside.

Todays crazy science lesson. Its also down right tedious and don't forget to mark the end with the outer layer. Hours of fun playing with this one.


 
 Return to top of page · Post #: 10 · Written at 12:28:06 AM on 29 March 2023.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 830

Using a probe meant for finding bad bulbs in Xmas light strings, See my page describing how I do this.

THE FOLLOWING CAN BE DANGEROUS AND DEADLY and should only be done by people experienced with handling high voltage electricity! Do this at your own risk!

I got one of those "mains sniffers" for troubleshooting Xmas lights, Home Depot (major hardware store) should have similar devices for detecting the presence of mains voltage on wires. A non-contact device that lights a LED in the presence of high voltage AC. An electrician's sniffing tool should work for this as well. Assuming the cap in question is rated for at least 200VDC (assuming you have 120V powerlines!), connect temporarily, but carefully!, across the mains. Use a long barrier strip and connect the mains to a pair of terminals at one end, and you can grab the other end reasonably safely. Then plug it in. Be careful you don't let the cap leads short to something! Use the sniffer on the leads to see which is the hot side, and sniff the body to see if you still get a hot indication. Reverse the cap leads (or reverse the power cord's plug), sniff again. The connection that gave a "hotter" indication on the body has the outside foil connected to the lead going to the hot side of the mains. If you get an ambiguous result, the cap may have foils from both leads exposed inside the insulated body. 1


 
 Return to top of page · Post #: 11 · Written at 9:28:36 PM on 30 March 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

I would rather do that Via a transformer, albeit that I have residual current trips, which will not trip on a secondary. Also note we use 240V domestic mains and that's rather unpleasant to the touch.

I would rather see an axial cap cradled on a forked support. Or; IC Clips are good, as the old tube tester here uses 225DC for leakage testing tube radio HV caps. If you touch the hot lead the leakage neon will light.


 
« Back · 1 · Next »
 You need to be a member to post comments on this forum.

Sign In

Username:
Password:
 Keep me logged in.
Do not tick box on a computer with public access.