Understanding AFC/AGC

Description:  You need to be able to understand what is going on here, so if you have problems you can be able to solve them.
               
When you tune a station plus or minus off frequency, a set of events happen:

When a station is tuned on frequency and when the detector is tuned correctly, both of the diodes charge the 10uf capacitor and the audio out behind the 1uf capacitor is neutral.

When a station is mistuned slightly above the target frequency, the diodes charge the capacitor, but the audio out behind the 1uf capacitor becomes positive.

When a station is mistuned slightly below the target frequency, the diodes charge the capacitor, but the audio out behind the 1uf capacitor becomes negative.

When no station is tuned, the 10uf capacitor is drained and the audio output behind the 1uf capacitor is neutral.

Note:  If this is backwards, most likely you have the output of the bottom transformer (behind the two diodes) switched around backwards.

Automatic Frequency Control / Automatic Gain Control

Description:  AFC locks on a frequency and automatically fine tunes it;  AGC lowers the signal level when a powerful signal is hit.

AFC Circuit:

Note:  Your AFC should grab from both sides (and not push) if not, there is something wrong with your detector.

Note:  If you detector pushes instead of pulling, switch the two geranium diodes on the IF transformer side.

Note: The AFC's BIAS is on the detectors “backside”, you can remove the “AFC zero adjust” Potentiometer, 47K resistor, 22uf capacitor and put the two 10K resistors to negative (That is, if you do not want afc).

Note: You can make this circuit work without the VHF variac diode by disconnecting the 47K resistor off the oscillators BIAS from positive, and connecting it to the 100uh and 1uf capacitor (remove everything else); be careful to not let the voltage get so low that it will kill the oscillator; or the AFC will modulate when a powerful station is hit (put another resistor to positive).

1.  Build the AFC circuit.

2.  Set the "AFC Zero adjust" so the LED is at medium brightness (You might want to verify that your tuning diode is adjusting the frequency while doing this).

3.  Realign the Local Oscillator.

Optional AGC circuit:

(Note: The radio actually works better without it, this is just here to prove it can be done.)

Note: You may not need this circuit (it works very well without it), but I live by two FM radio station towers that are literally down the street, and I don't need a radio to receive them; just a audio amp.

It should be noted that only one of the stations slightly distorted the detector when tuned to it's frequency and the antenna was extended and it was annoying,

It should be also noted that this station interferes with some other radios much more severely, most have the problem above with varying severity when in the area; the problem went away after I made a soldered version of it to double check my work.

1.  Build the circuit as shown above (without disconnecting the 22K resistor on the mixer)
2.  Adjust the AGC Level Adjustment.
3.  The light should go out when a strong station is tuned (if not change the 22k resistor plus or minus).
4.  Connect the AGC to the 22K on the mixer as shown above.

Note:  If you wanted to do this right you would connect the AGC stage to the RF and AF amp, but this was hard to do on the prototype board; it was done however on the soldered AM/FM combination.

Realign everything for best performance.

Thermally Stabilized AFC / AGC

If you want to improve the radio, you can build this thermally stabilized version:

This version has tighter AFC and requires no AFC BIAS adjustment, and is thermally stable; however it requires more transistors.

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