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My learning process began with tube testing. Once I got good at working my tester I began testing tubes on a favorite nonfunctional AA5 set and found some bad ones.
I took the set to a local shop that had restored tube radios for sale and procured some new tubes. The owner and talked about the set some, and I was told that given that the set hummed it needed new electrolytics. I was sold some, and instructed on the installation procedure. That night I replaced the lytics and it came to life! That was a thrill far better than just collecting radios, that I was instantly hooked on. Soon I was doing lots of recappings which brought most all of my sets back to life(except for a few dogs). I got more interested in how stuff worked and began reading plenty of books on radio and TV (which I eventually started working on for more of a challenge).
A Superhetrodyne AM radio is basically a string of amplifiers (with a couple of other simple supporting circuits). The signal starts at the antenna and goes to an RF amp (some sets don't use an RF amp stage) from there it goes to one of two circuits that both do the same thing. The signal is now at the mixer or converter stage the mixer is an amp that combines two signals...The station and the oscilator signal. The station is tuned with a band pass filter consisting of a fixed coil and the larger half of the variable capacitor before it enters the mixer. The oscilator signal either originates in a seperate tube in sets that use a mixer-osc stage or is generated within a converter tube (which is clever way of making a single tube act as both a mixer and an osc.). An oscilator is a circuit that generates an AC wave form (which in an AM radio is an RF frequency sine wave). The oscilator frequency is tuned in step with the channel tuning such that when the osc frequency is subtracted form the station frequency a fixed lower frequency is obtained. When the station and osc are signals mixed the osc frequency DOES subtract from the station to form a fixed frequency that contains the modulation of the station. This fixed frequency output is called the IF signal and is usually at 455KHz for AM radios. This signal is then sent to 1, 2, or more stages of IF amplification, and then to the detector. The detector in an AM radio is very simple...all it consists of is a diode which rectifies the IF signal to recover the audio signal. From then on it is an audio signal who's processing you are already familiar with.
The Superhetrodyne has two major benefits over other older designs. The mixer/converter hetrodyning process makes the set more selective (meaning that closely spaced stations that might otherwise run together making it impossible to tune only one can be seperated and tuned individually).
The same process also makes the set more sensitive to weak stations. This occurs because if we design a tube RF amp to function over the entire AM band and put a variable band pass filter at it's input it's gain would be far less than an amp designed to function over the bandwith of a single station. Basically the converter/mixer system grabs and converts any selected station and converts it to a new single RF frequency which we call IF (standing for Intermediate Frequency) this frequency can than be amplified by that single station width maximum gain amp I mentioned a few sentances back.
AM stands for amplitude modulation. One can simulate the action of an AM station in the audio frequency spectrum with a audio sine wave generator and an audio amp. If you play the generator (at the higher end of frequencies than you can hear) through the audio amp and you vary the volume control you modulate the amplitude of the output. This modulation waveform when viewed on an oscilloscope appears as a sine wave that expands out from center and contracts back towards center. If your hands could move fast enough you could generate a deep bass note by varying the volume control, but it would likely be unnoticed as this modulation cancels/averages it's self out when heard on the speaker and the generator is predominantly noticed. Now if we halfwave rectify the output and filter the generator signal from it we will only hear the deep bass volume control fluctuations. And then if we turn off the generator the deep bass sound also disapears despite none of the generator signal reaching the speaker. This occurs because the high frequency generator is acting as a carrier for the deep bass generated by rotating the volume control back and forth at the rate of a low audio freqency. This is exactly how an AM radio transmitter and reciever work only in a radio transmitter the audio generator from the above illustration is replaced by an RF source and the hand working the volume control is replaced by a circuit that varies the "volume" of the RF in step with the audio program content that the station sends. The diode in the above example represents the most basic radio that can be made to work.
I need to go to bed now, but can describe the operation principals of radio better or answer questions/clarify later on.
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