MK484 Radio

This AM radio receiver uses the MK484 integrated circuit (IC) in a TO-92 package. This radio on a chip is not a unique component and its modern equivalent is the TA7642 by the Toshiba Corporation. This project is a simple crystal radio circuit with a series of high-gain transistor amplifier stages. It is a two transistor radio, which amplifies the signal to the point so that it can drive a loudspeaker. These types of radios behave and sound very differently from conventional heterodyne designs, because you can hear the hum of the carrier and any noise distorting the signal.

Usually, transistors have difficulty amplifying high frequency radio signals; however, they perform much better amplifying audio frequencies, which are much lower in frequency. In a heterodyne receiver, invented by Edwin Armstrong, a local oscillator generates a sine wave that is close in frequency to the carrier sine wave. When these two waves are mixed, they add in some places and subtract in others, essentially forming harmonics of lower frequencies and higher frequencies. The lower frequency harmonics are of interest to radio engineers because transistors can amplify them easily. This lower frequency is usually the basis for the signal known as inter-frequency (IF).

In this radio design, there is no local oscillator, and we are simply amplifying the raw signal that one would usually hear from a crystal radio circuit. The internal schematic of the MK484 suggests that this IC has multiple stages, which simply amplifies the signal to a large enough level so that any general-purpose audio transistor could use it to drive an earpiece or a loudspeaker. The great thing about this IC is that it can work with radio signals as low as 150 kHz and as high as 3 MHz, consequently it makes an extremely good shortwave and DX radio receiver. It also provides radio frequency (RF) detection, and automatic gain control (AGC). The AGC normalises the output signals because if you can imagine, they originate from different places with different strengths. The internal detector provides a further benefit in that the circuit will not require a germanium diode. These types of radios are interesting, because you are listening to exactly what is in the airwaves including any noise.

If you have not had much luck with regenerative radios, then this one is much simpler and works extremely well, because it uses straightforward amplification. The final output stage uses a Darlington transistor and therefore it is ear splitting loud. The transistors that I am using are low-cost and readily available from eBay. The MK484 IC is not essential and you could use the TA7642. I was able to test the circuit with both, and they both performed well.

Design Considerations

The circuit and build are my designs that are unique and will not be found anywhere else. Initially, I was going to build another screw-cup type circuit, however, it is not appropriate if you want to experiment with the circuit, and therefore I used a low-cost breadboard that I managed to find on eBay for £1.20. This saves you from buying screws, their cups, drilling guide holes, and fiddling with wires. The build is semi-permanent, because I love to use my hot glue gun to stick everything. It is extremely robust and saves me from having to buy metal brackets and screws, which would only serve to add to the final cost, and complexity of the build.

For the speaker driver, I was considering using the LM386 audio amplifier IC, but in the end decided to use a Darlington transistor. Provided you get a good strong signal, the sound can be very loud.

Using a small piece of low-cost breadboard was a good idea because it enables me to test out new circuit ideas and transistors. I had many hours of fun changing the component values and circuit configuration until I found the best design that worked. I wanted a circuit that would work for almost anyone, even after making slight changes to the ferrite rod and variable capacitor, and this one seems very promising.

Buy / Price / eBay

You will almost certainly find the MK484 on eBay, as there are usually many sellers with old stock. The current equivalent IC is the TA7642, which is pin equivalent and works fine as a direct replacement. The price is usually less than two pounds.

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