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Notes from the October 21, 2006 Meeting

Program director Mike Grimes conducted the meeting of about 25 people. Eric Kirst asked everyone that might be going to the forthcoming Christmas party to bring their "Find of the Year" radio or related item to the party, where they can tell everyone about their experience with that special "find". Past president Chevie Crandell’s granddaughter was a guest at the meeting.

Mike introduced the meeting topic, CAPACITORS, and discussed the issues of converting from different numerical designations to others, such as nanofarads to microfarads and picofarads. We have all noticed that we are seeing the nanofarad (nF) designation nowadays on new parts. It seems to have originated outside the US and has shown up on imported parts, primarily. Of course none of our antique radios use this designation on the parts themselves or in any parts list in the service information. We are all going to have to get used to it so we don’t install a replacement capacitor of 1000 times too much or too little capacitance.

Here are some humorous conversion factors:

1 kilomillimegamicrofarad = 1 farad

1 million phones = 1 megaphone

1,000,000,000,000 microphones = 1 megaphone

One way to be sure about a capacitor’s value is to use a meter designed to measure capacitance directly. (Notably, the readout ranges are shown in nanofarads on some meters). Some of the electronic multi-meters have the capacitance function built in, and they are available from Fluke Instruments and Radio Shack, for example. The old capacitance checkers (Heath Company and others) can also measure the capacitance values.

Cleo Cherryholmes presented an excellent discussion of capacitors and their various applications within a radio. He defined basically four applications:

    1. Filtering – these are sometimes in a can, but are also found in cardboard boxes. Before the electrolytic condenser was developed, these were large paper/foil units, usually in steel cans filled with tar. They had capacitance values of 1 to 3 microfarads. Because they had such low capacitance, the radios they were in needed large, heavy inductors (chokes) to provide adequate filtering. This is why early AC sets are so heavy! When electrolytic capacitors came out, they were usually 8 microfarads, and a small choke or speaker field were all that was needed for the remaining filtering. Later, 40 microfarad capacitors eliminated the need for any choke.
    2. Coupling- these are almost always paper/foil, in old radios or (rarely) mica. These are used to block DC voltages from being passed on to another circuit element. The most prevalent one prevents a bias voltage on the audio output tube, e.g. 43, 6F6, 50L6, from the first-audio stage plate voltage.
    3. Bypass- these are paper/foil in the old radios and ceramic disc or sometimes mica in later ones. They eliminate unwanted RF or audio frequency signals from radio circuits. The automatic volume control circuit (AVC) uses them to prevent oscillations and control the speed of reaction to changes in received signal strength. Others make sure that feedback does not cause trouble in the screen-grid circuits of intermediate frequency (IF) and radio frequency (RF) amplifying stages.
    4. Tuning – these are the intermeshing plate capacitors of many different physical configurations. The plates move when you tune to different stations. Others affecting tuning can be fixed-mica or adjustable mica trimmer capacitors.

There are basically four problems with capacitors – shorts, opens, changed values and leakage. Shorts and opens almost always cause a serious performance problem. Changed values and leakage may leave the radio playing poorly or prone to fail entirely if played for awhile.

Leakage and shorts in the power supply filtering circuits can cause hum, or cause poor performance due to lowered supply voltages. There is also the risk of burning out or shortening the life of tubes or power transformers if leaky filter capacitors are not replaced.

The coupling capacitor to the audio output tube is probably the most important paper/foil capacitor in a radio. If it leaks, the voltage on the grid of the output tube will be raised, causing the tube to draw more current, reduce its life, or even to distort the sound of the radio because of too much current through the output transformer. Mica capacitors rarely leak, but one thing to be aware of is that there is a misleading trade name "Mica-Mold" that physically looks like the usual mica capacitor, but it’s really paper/foil (and they leak). Modern metalized film capacitors have unlimited life and essentially zero leakage.

A little leakage of the screen voltage supply paper bypass capacitors can be allowed without hurting performance. But because the circuits for AVC have very high resistance values, leakage can upset the performance of the radio and cause overloading for large received signal strengths. Open screen bypass caps can result in spurious oscillations that manifest themselves as squealing sounds coming from the speaker.

The best philosophy is to replace all of the electrolytic and paper capacitors as a routine measure. Whether or not you do may depend on your plans for the radio – to pass it on to another collector or keep it in your personal collection.

Chevie Crandell discussed a very unusual capacitor checker which uses a 20 MHz signal and the transmission-line properties of the test leads to find out if a capacitor is open or closed – even while it’s installed in a circuit with both leads still connected.

Jon Butz-Fiscina suggested methods for saving tubes that have lost their top cap connection. Sometimes, with luck, by chipping away a little glass around the wire, enough can be exposed to allow re-soldering the cap (or one from a dud tube). Then the cap can be glued on with General Cement product GC-10-302. This can be obtained from Radio Daze, Unicorn Electronics or www.greenbrookelectronics.com (It seems their website is under construction right now). You may want to wrap the tube in a rag so that if it breaks, you won’t get cut by the implosion. Stuffing the rag and tube in an empty food can may also work.

(The author is working on replicating the same material used by the tube and lamp manufactures for attaching the caps and bases – will advise)