TA7642 AM Radio - Project Tom

AM Radio with MW and SW

OK, Attention! The first duty of any prisoner of war (POW) is to escape. Project Tom, is the first of a three part AM Radio construction project. This construction uses the TA7642 radio IC, and the amplifier IC is LM386. I hope that this project will help little whippersnappers escape from the clutches of poverty, because it is designed to be affordable.

Although heterodyne radios were widely available after 1924, their benefits were unknown until after the Second World War. Before the war, and during, many radios that were in use were simple TRF, and regenerative types.

In the film The Great Escape, they showed a crystal radio which prisoners made to obtain intelligence information. There is historical evidence to suggest that secret crystal radios were used by POWs to gather intelligence.

I remember seeing the film when I was a little whippersnapper. They constructed three tunnels, called Tom, Dick, and Harry. The discovery of one tunnel would simply mean continuing work on the next. They would have escaped as well, if only Sir Richard Attenborough had not been so polite to the ticket inspector :-)


Radio Circuit with Build

If you have been following The Little Whippersnapper's Crystal Radio project, then this is just an extension from that. After building the crystal radio and experimenting with some simple transistor radio circuits, you will feel more confident to build a slightly more complex radio.

The Little Whippersnappers Crystal Radio project leads to Project Ultra, which is an 8-band crystal radio. This is just a simple ferrite coil with taps and a selector switch.

Project Ultra then leads to Project Tom, which is an AM Radio with integrated circuits.

The Poor Transistor Radio Circuit Architect

AM Radio IC

After designing a BC549C transistor radio, I reconsidered when I did the costing. If The Architect in The Matrix had to explain, then this is how I believe he would say it:

"The first transistor radio circuit I designed was quite naturally perfect; it was a work of art. Flawless, and sublime. I even calculated the values of the bias resistors taught to me by Agent Johnson. However, after calculating the cost of all the components it occurred to me that it was not acceptable. The High Street price for a resistor is 36 pence at the Maplin store, and all the biasing resistors were adding to the total bill.

Hence, this triumph was equalled by its monumental failure. The inevitability of its doom is apparent to me now as a consequence of the imperfection inherent in every transistor circuit design coinciding with inflation. Thus, I redesigned it based on the LM386 amplifier IC to more accurately reflect the grotesqueries of the BTEC, and GCSE Design & Technology curriculum."

Speaker

Although the circuit is low cost, you will still need to shop around for many of the parts. I had to shop at eBay, Maplin, local hardware stores, and thrift stores. I also used recycled components such as the loudspeaker, ferrite rod, tuning capacitor, and tools. I hope that you will have something that you will be able to use or recycle for this build.


Advantages of using TA7362 IC

My whippersnappers, who are also members of the scout organisation, wanted to build a simple three transistor radio, and were considering building the Ladybird Book Transistor Radio.

Back in its time, it was a great radio design as germanium transistors were readily and cheaply available. The OC45 and OC71 were like the general purpose BC series of silicon transistors that we widely use today.

After looking at many transistor radio circuits, I decided to settle on the TA7642 AM Radio IC. The great thing about it is that it is an IC with only three pins. Hence, little whippersnappers will find it easy to connect it. You could almost mistake it for a transistor; however, it is much more powerful than a transistor.

If you have ever built a transistor radio, then you will know that this IC solves many problems simultaneously.

1. There is no need to use regeneration. This was a form of feedback method to increase gain, but often resulted in creating awful howling and whistling noises.

2. There is no need to look for a germanium detector diode, as it is not required in this circuit. Germanium diodes have been getting expensive over the years, and some sellers have been sneaky selling ordinary silicon Schottky diodes that do not perform as well, and sometimes not at all.

Moreover, here is the real kicker; the price of a germanium diode is comparable to the price of the TA7642 IC!

3. There is no need to buy trimmers. I always hated those nasty things because you had to constantly turn the screw, and eventually, either the screw breaks, or the brass plates remain stuck! Moreover, today, trimmers are expensive as well.

4. The best advantage of all is that there is no need to use a Radio Frequency Choke (RFC). I really hate those things as well. Honestly, the person who created those things ought to be taken outside and summarily shot. They create a magnetic field, which interacts with the ferrite coil producing feedback. Moreover, if you were using the regeneration technique it was a nightmare to tune into a radio station, because you ended up listening to whistling, howling, and all sorts of feedback noises... You were never sure if your trimmer plates or the RFC interaction caused the feedback.

5. Who remembers the twisted wire capacitor? This is where you had a pair of insulated wires closely coupled by twisting them together. That was just awful! Most people will remember twisting it and getting a farting sound from the loudspeaker.

So there you were trying to tune into a radio station; 1. You twist the wire pair until the farting sound disappears. 2. You adjusted the trimmer until the howling sound was just on the verge of disappearing. 3. Then you tuned into a station. 4. Then you had to turn the screw of the trimmer plates to increase the regeneration feedback

Of course, one of the wires in the twisted pair capacitor always breaks off because you twisted it too much...

I always say that those persons that were making radios in those days ought to get a Nobel Prize, because they had very little to go with, and yet they always came up with an interesting solution.

Therefore, after considering all the options, I decided to settle on the most logical solution, for my whippersnappers who are just starting out in electronics.

This circuit could also be used for their GCSE Design and Technology Project, or perhaps BTEC Electronics, or whatever the current party political torture happens to be going.

This Article Continues...

TA7642 AM Radio - Project Tom
TA7642 AM Radio Baseboard Construction

Ferrite Coil
TA7642 Ferrite Coil Connection

Radio Stage
TA7642 AM Radio IC
TA7642 Power Resistor Rp
TA7642 Circuit Build

Amplifier Stage
LM386 Amplifier - TA7642 AM Radio
LM386 Circuit Build

Misc
TA7642 Circuit
TA7642 AM Radio Parts List
TA7642 AM Radio Parting Shots