Quote:
Originally Posted by cpacamper
On these old tube FM/AM radios, why would anyone choose plain FM over FM AFC? Why would manufacturers even have seperate FM and FM AFC choices on the band switch?
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AFC stands for automatic frequency control. This locks in an FM station so it won't drift after you tune it in. Normal procedure for tuning in an FM station on a radio with an AFC position on the band switch is to place the switch in the FM position, tune in your station, then place the switch on FM AFC and leave it there until you change stations again.
Very old FM radios did not have AFC circuitry, so they are very prone to drifting while the set warms up. I don't know when AFC was first introduced in FM sets, although my best guess would be in the early-mid-1950s.
Most FM radios today have what is known as "full-time" AFC which cannot be defeated; the band switch only has AM and FM positions. I personally am not fond of this arrangement because it will cause the AFC to pull the radio to the strongest signal it receives; this can be a problem if you are trying to receive a weak station which is very close to a stronger station on the dial. An example here in my own area, 35 miles east of Cleveland, is a classical music station on 104.9 MHz and a country-western station on 104.7. The latter station is so strong here (the other station is much weaker) that the AFC circuit in any FM radio will lock the receiver onto the stronger signal.