The AM Forum

   I'm in the middle of recapping my Viking II CDC and I happened to read parts of the V II assembly manual related to section I was working on. The manual says that the end of the capacitor connected to the foil should be connected to ground or low potential.
   My question is whether the polyester types I'm replacing these with have a similar "polarity" and, if so, how do I tell one end from the other?

Thanks in advance.
Ed

The 'outside foil' paradigm was such that in bypass service, the entire cap was shielded.

I test the new unmarked styles with a telco fox & hound.  Connect both leads to the fox, and hook a good earth ground to one side also.  Run the hound around it the case of the cap.  Switch the ground on cap leads, and mark that which gave the loudest tone as NOT the outside foil.

I've used begoodles of those things for recapping projects over the years. I've used them for coupling as well as bypass applications. I have used them in old broadcast receivers, communications receivers and transmitters as well. Like many others I would also like to have the outside foil grounded in bypass applications to take advantage of the built in shielding effect. However since they are not marked, I have never noticed any difference in operation whichever way they are connected.

After all would you worry about it if they were ceramic disk caps? Hell, no!
In a bypass application if the cap is sufficient for the frequency being bypassed, it doesnt matter anyway.

The big thing that I dont like about them is the stiff, unruly tinned steel leads that can be difficult to thread through some of the tight quarters that you have to deal with from time to time.

                                                the Slab Bacon

There are two types of film caps: metalized and film/foil.

They are not polarized, but in some cases the "outside" of the sandwich is marked with a stripe on the outside of the cap. The fox & hound is not a bad idea, and can be accomplished with a sig gen and a scope probe used to pick up the radiation outside the wall of the cap...

Doubt that it makes much of a difference in practice which way the caps go as far as shielding inside the rig - it's chock full of random RF anyhow...

                  _-_-bear

For bypass use probably perhaps not much difference. For RF coupling, it might be best to have the outside shield going to the lowest impedance point (plate). That could reduce AC hum induction into RF or IF stages at the CG.  This assumes a relatively small value cap with a very high reactive value at 60 or 120 HZ.

Pete

Once upon a time the rule of thumb was that the foil or band-side was the lead to the right of the writing. So, holding a cap in your hand while reading it, the lead to the right would be the band side. Haven't seen new gear with marked caps in some years, but that's the rule I go by.

The new foil caps have a better termination to leads yielding a lower ESL.
Old caps had a high series inductance so you are better off with the new parts.

ESL? Wazzat?

You meant maybe ESR?
Of course maybe there is an ESL, but I don't know it.
There's a lot I don't know...

             _-_-bear

Probably won’t mean much in a bypass application. I just recapped a BC-348 with “orange drops” and was just curious about the out side foil so I did this test. Nowadays the caps are printed with no relationship to the outside foil. So plug in an extension cord to your bench, to use as a signal source, wrap it around the cap equidistant from the leads. One turn. Connect your scope probe to the cap and look for the largest 60 Hz signal by swapping the scopes probe and ground. The larger amplitude will mean that the probe end is connected to the out side foil. Hope this helps.

Ted KC9LKE