Quote:
Originally posted by millerdog
Bill,
Are those things called air caps? I had a Heathkit antenna tuner that had kidney shaped plates. I noticed something similar in my dead Fisher. Also, that tube you mentioned is a reference tube as to frequency? Sort of a guide tube that resonates at a certain freq? Certain concepts are beginning to return from my uh...youth
thanks,
md
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MD,
The caps are called "air variables". The tube is not a reference tube, it is the actual oscillator that generates the local injection frequency to mix with the incoming signal, developing the 10.7 MHz IF. It may be labelled on the tube placement chart as "oscillator/mixer" or "converter" (on pre-war radios it is called the "first detector"). On post-war FM radios the oscillator operates at 10.7 MHz above the dial frequency, so as the dial tunes from 88 to 108 MHz, the local oscillator tunes from 98.7 to 118.7 MHz. It is the difference beat frequency between the station signal & local oscillator (generated when they are mixed) that provides the 10.7 MHz IF signal.
