Back in the 60s, communications were not as widespread as they are today. People had the radio and some early form of the television, but this is a one-way information link. If they wanted to send a message across the globe, they had to use the expensive and unreliable telephone. This is why the hobby of Amateur Radio was then, what the Internet is today.
Nowadays, an AM radio receiver is cheap, reliable and useless (let’s admit it, no one hears AM radio any more). Most of the times, it is incorporated along with an FM receiver, but that was not the case back then. Fifty years ago a radio receiver was not cheap or reliable and it was certainly not useless.
There are many ways you can receive and demodulate a radio signal. One of the earliest methods is called Tuned Radio Frequency receiver. Basically, it is an array of amplifiers that amplify the signal to a satisfiable level and then handle it to the demodulator to extract the information. Finally, the information signal is getting amplified by a low frequency (audio frequency) amplifier. There is one big advantage and many disadvantages associated with this method.
The advantage is the simplicity. All you have to do is take some amplifier blocks and connect them in series. One disadvantage is the poor selectivity, because the selectivity of the receiver depends on the Q of the tuned circuit. Another disadvantage is the number of active elements required. In addition, when you have a lot of transistors (or tubes) connected in series, noise is getting amplified and propagates through the receiver.
With the advent of semiconductor technology in the 60s, the TRF concept was revived. The number of the active elements is not an issue in integrated circuits and people want to build cheap and simple AM receivers. Companies like Ferranti introduced fully integrated TRF blocks in three-pin packages (TO-92). A good example is the TA7642.
As an amateur radio operator myself, I couldn’t resist the urge to build an AM receiver based on this little piece of technology. Looking at the datasheet I realised it wouldn’t be that hard. Most of the job is done for me and all I have to do is tune the early stage (find the correct AGC resistor) and install the audio frequency amplifier.
Finding the correct AGC resistor turned out to be tricky. Even if you use a potentiometer to find the correct resistance, most of the times that would be an absurd value with no standard resistance equivalent. In addition, the TA7642 output quality is very sensitive to the AGC resistor. The end stage is a typical AF amplifier using the LM386 power IC. You can see the final circuit below.
The L1 inductor and the C1 capacitor form the tuned circuit. The C1 capacitor is variable and covers the entire AM frequency band and the L1 inductor uses a ferrite core. The AGC resistor is denoted as R2. Here’s a video with my final circuit assembled on a piece of wood with nails acting as connecting points (Greek folk music!).
One last note, below is shown an example of the superior and more complicated superheterodyne receiver, just so we can appreciate a little bit of the obsolete TRF design.