Ramon. Tuner theory of operation is extremely complex and difficult to understand. Therefore, I would say technical tuner discussion is probably beyond the scope of this forum and better suited to the more geeky forums populated by tech-heads.
But in anwer to your question about AFC, AFC (automatic frequency control) is a circuit frequently used to counteract a phenomenon called oscillator drift. It is used only on FM and not AM circuitry. The reason is due to the fact that oscillator drift is a greater problem with higher frequencies and FM operates at far higher frequencies than AM.
Almost all modern FM tuners are superheterodyne receivers. That means the antenna signal is mixed (heterodyned) with another signal to obtain a third frequency whose frequency equals the difference between the two original frequencies. The antenna signal mixes with the second frequency provided by the local oscillator to produce the IF signal. By arranging the local oscillator to be tuned to a higher frequency by a constant amount than the antenna frequency, the net difference should also be constant. The way both the antenna frequency and oscillator frequency is kept consistently constant (for a net difference) is by using a common tuning control to simultaneously vary both the antenna and local oscillator frequency and therefore track so that the same IF frequency is always produced. The FM IF frequency is almost always 10.7mhz. On AM the IF frequency is typically 455khz.
The reason for converting the FM signal to the IF signal is that since the IF amplifier operates at only one frequency, it can be economically designed to produce a high gain and narrow bandwidth.
Anyhow, hopefully you now understand the importance of the local oscillator in generating a consistent and stable signal to feed to the mixer. However, inductance and capacitance values will increase in value due to thermal expansion as the operating temperature rises. Various methods are used to compensate for this instability. However, in addition to those countermethods, AFC circuitry is frequently used. No sense going into the details of AFC since we'll just bore everyone to death. But in a nutshell, the AFC circuit controls the oscillator frequency and therefore provide stability of the receiver tuning.
This is a very basic description of tuner operation. As you can see, the tuner is a complicated animal and haphazard adjustment of tuner adjustment locations can easily make things very messy. Making one adjustment often affects another. That's why I cringe when I read suggestions to play with (4) adjustments. Every tuner is different and there is NO magical 4 adjustments that will fix your tuner. In fact, although this is the basic theory behind how the tuner operates, every design is different and the adjustment locations and expected output is measured at different locations depending upon the engineer and designers of that particular tuner. That's why documentation is so important. Although you can have a basic understanding of the theory, you will still require servicing documentation for an effective and practical tuner adjustment/alignment.
Anyone interested in educating themselves further is encouraged to obtain a copy of THE THEORY AND SERVICING OF AM, FM AND FM STEREO RECEIVERS BY GREEN/BOURQUE. I feel this is probably the best publication available for anyone interested in the field of servicing radio tuners. Be advised that this book is not cheap, even used. Much of the information above was summarized from this book.
