Tech Section: Capacitors with Three Leads in Postwar AM-FM Philcos
Applies to Models: 47-1227, 47-1230, 48-475, 48-1264, 48-1266, 48-1286, 49-1613
Some postwar Philcos which have FM capability have an odd capacitor used as a bypass in the FM circuitry. This capacitor, with a value of 0.01 uF, has three leads instead of the usual two and is known as a feedthrough capacitor.
What is a feedthrough capacitor? It has three terminals; a wire which runs through the unit which makes up two of the leads (input and output), connected to one terminal of the capacitor, and a third wire which connects to the other terminal of the capacitor. The third wire is normally grounded. These types of capacitors feature much lower residual inductance compared to normal two-terminal capacitors.
Read more information on feedthrough capacitors. (requires Adobe Acrobat Reader).
Feedthrough capacitors are most often used these days for AC line noise suppression.
Look at the partial pictorial diagram below of a 47-1230. C419 in this drawing is our feedthrough capacitor.
Now look at the partial schematic of the 47-1230 below. Notice how C419 is drawn just like an ordinary capacitor.
In some Philco schematics, this capacitor is drawn with dotted lines surrounding the part.
Nevertheless, if you encounter one of these capacitors in a vintage postwar Philco set, do not worry! It is not necessary to replace it with a new feedthrough capacitor - you can replace it with a conventional two-lead metalized film capacitor, and FM will not be adversely affected by the change.
Remember, since two of the wires are actually connected together inside the old capacitor, before you replace it you should use your DMM or VOM to determine which of the two wires are joined together. You will be replacing these leads with one lead wire, as shown in the partial pictorial diagram below. One end of the new lead wire is also connected to the replacement capacitor, as shown below. The other end of the capacitor is connected to ground as shown. Keep the leads of the new capacitor as short as possible; if there is another ground terminal closer to the new lead, use it (there is on the 47-1230).