How were "Outside Foil" connections on vintage foil capacitors documented?

Thread Starter

Analog Ground

Joined Apr 24, 2019
416
In another thread, I learned about the "Outside Foil" designation on one lead of some vintage foil capacitors. How was this connection specified on schematics or other documentation? Here is an example of the indication.

VintageFoilCap.png
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
21,438
It was a generally accepted form of manufacture to install decoupling capacitors etc with the outer wound foil end connected directly to the circuit GND or chassis point.
BTW, it is still indicated today on some types of capacitor that are not necessarily foil construction.
Max.
 

Wolframore

Joined Jan 21, 2019
1,797
@Analog Ground, this is new to me too but I watched the entire video and he recommends the foil to ground or to the lower impedance side... so output of one stage to the higher impedanCE of the next stage input was my understanding, did not need marking.
 

nsaspook

Joined Aug 27, 2009
7,594
We are trying to figure out whether the foil side of poly caps are marked on schematics
I've only see it marked on a few old tube radar devices (like radar repeaters) where pulses were being processed. The outer foil acts an electrostatic shield to help eliminate unwanted signal paths from circuit to circuit in the same chassis.
 

Thread Starter

Analog Ground

Joined Apr 24, 2019
416
I've only see it marked on a few old tube radar devices where pulses were being processed. The outer foil acts an electrostatic shield to help eliminate unwanted signal paths from circuit to circuit in the same chassis.
How was it marked? One post suggests a dot on the outside foil lead. I have been looking through vintage tube preamp schematics and don't find anything.
 

Wolframore

Joined Jan 21, 2019
1,797
Yeah been too long for me too... the one on the right looks familiar from the microwave circuits but it's been a while since I've seen my TM's... and that one reminds me a little of crystals.
 
I tried to locate an old schematic and I don;t think they are marked on the schematic. Usually there is a pictorial view and it could be marked there.

I just found this out.

I did do one test with an electrolytic cap. I had this test oscillator that can drive a speaker and a DSO150 scope, both battery powered. I connected the oscillator to the cap and I could identify the foil side. I plan to add a resistor in series with the oscillator and try again.
 
Top