Has anyone else seen this video. I didn't know that (non electrolytic caps) foil caps had a polarity to them. It's based on the outside foil lead vs the inside lead of the roll. If they are installed with the outside lead to the low impedance side of the circuit its cut down the noise sensitivity of the cap. .
https://www.youtube.com/watch?v=BnR_DLd1PDI
I would not call it polarity, but yes, in some applications this, lets say physical orientation matters.
Here's a video explaining why it doesn't matter:
https://youtu.be/hJVe_JTs4fU
Short version - the other components like resistors and wiring are more likely to pickup noise. Since no one seems to care about them, why should the caps matter ? Also, if it really made a difference, why don't manufacturers mark them ?
Its not always important, but the shielding effect of the outside foil does make a difference.
The video that Bob mentions is full of misinformation. Film caps have been marked as to the outside foil connection for decades because of the inherent shielding it provided, not for any risk of shorting to ground. Until recently, even yellow film caps made by Cornell Dubilier and Sprague were marked with a band for the outside foil end.
Contrary to what was said in the video, the scope test is an accurate measure of the sensitivity of the cap to stray signal pickup. When the outside foil end is connected to scope ground, there is less coupling to the inner foil from external fields, resulting in less noise indicated on the scope than when the leads are reversed. If the noise indicated is less with the proper orientation of the outside foil in the scope test, then it will also be less when properly connected in circuit. This is not magic or rocket science, just basic physics.
I typically use the scope test to mark the outside foil on all film caps I install. While it may not be critical in all circuits, RF and IF stages can be prone to self oscillation, and why take chances if you can replace a cap with the orientation that the design engineer originally specified.
Long time no post!
Good the hear from you.
Terry
Hi Terry. Yes I have been taking a break from radios for a while.
When spring came and the weather got nice, I started working on restoring old cars again and am just getting back into radio.
Its good to be back!
Mike K. WA2YGA
Agree, but the smaller physical size and construction of modern equivalent components obviates some potential orientation issues for most all consumer grade applications. It is still very much advisable to duplicate original lead dress even if it means lengthening a component's lead once in a while. Careful tack and reverse of suspected critical components is also a possibility should there be a problem.