[TerryMSU]

What is up with C21 on the Philco 46-350?  It has a knot of wire around it.  The note says to duplicate it if you replace the capacitor.  The problem is that I will be using a different physical sized part.  I am tempted to either just slide it loosely around the new part or maybe just not replace that capacitor altogether.  It is a 0.1 uF paper cap.  A yellow Mylar will be a smaller diameter.

Thanks,
TerryMSU

[Warren]

What is up with C21 on the Philco 36-350?  It has a knot of wire around it.  The note says to duplicate it if you replace the capacitor.  The problem is that I will be using a different physical sized part.  I am tempted to either just slide it loosely around the new part or maybe just not replace that capacitor altogether.  It is a 0.1 uF paper cap.  A yellow Mylar will be a smaller diameter.

Thanks,
TerryMSU

You could always cut the leads off the original and leave the "knot of wire" in place then solder the new cap at the contact points. You will be leaving the wire as is and have a new cap too.

[Radioroslyn]

http://www.philcorepairbench.com/tips/svctip37.htm

[Warren]

http://www.philcorepairbench.com/tips/svctip37.htm

OK, reading that sounds like my idea wouldn't be the best answer.

[ttmiller]

Sometimes when a cap has wire around it, it works like a choke. Ck Schematics.!!
Open the wax paper cap and slide a new one inside. I've done that before and had
success.......

[morzh]

Sometimes when a cap has wire around it, it works like a choke. Ck Schematics.!!

Nope, it doesn't.

[TerryMSU]

So from a theoretical viewpoint, a winding around a conductor (which the capacitor is) serves as a current transformer. The current thru the wire is proportional to the current thru the capacitor. In this case, the effect can only be a very high frequency effect, since this is basically an air core transformer. As a transformer, there is also an inherent inductance. As this is a single turn the inductance is VERY small. I suppose there may be a common mode chocked effect here since the wire has a few twists around itself. I suspect that any improvement is wishful thinking on the part of the designer, but I did compromise and leave the loop, but around a modern capacitor, it is a much looser loop. The radio seems to work with this change. At this point the recap is about 75% completed, and the radio seems to be fully functional thus far.

Thanks for everyone's input,
Merry Christmas,
Terry

[Mondial]

The wire wrapped around the capacitor (inductance) and the capacitor itself form a series resonant circuit, broadly tuned to the IF frequency. Because circuit ground is isolated from chassis ground, it is important that the impedance between them is kept to a minimum. Otherwise IF oscillations may occur. At the series resonant frequency of the wire coil and capacitor, the AC impedance effectively approaches zero, thereby efficiently bypassing circuit ground to chassis.

This technique was more important with the old type paper-foil capacitors which had a lot of parasitic inductance, making an effective bypass difficult. With modern film capacitors, series inductance much less of a problem.

I would guess the designers of the radio had a problem with IF stage oscillation, and went to the wire coil-capacitor as a solution. Can't see them doing this otherwise, as added production time and cost.