DIY single transistor reflex AM radio with regeneration and AGC

 

This is a piece of circuitry magic; it's a great pleasure to assemble and configure it! The receiver sounds pretty good if a medium-wave station is nearby or an AM transmitter is at home.

In the previous post, we've assembled an eight-transistor AM MW superheterodyne. This is an excellent, straightforward solution for reliable, high-quality radio reception. Still, the scheme is a bit too complex for a beginner.

A few decades ago, it was also not budget-friendly; it was three times more expensive than a tube All American Five (midget, peewee) radio. Simpler amateur DIY receivers were more common, using one or two and sometimes up to three transistors. Today, we will assemble and look into one of those schemes.

By the way, if you experiment with vintage tube radios and TVs, remember that many are powered directly from the mains without a transformer. That means the chassis and all components could be under high voltage relative to the ground!

Peewee radios often came with a special power cord called a "line cord resistor." It was a cord with a certain resistance, so the voltage drop across it was a pre-defined value. This cord heated up during operation; it was introduced just to move the heat source outside the small radio housing.

And finally, vintage transformers after long-time sitting may have a winding breakdown, primary to secondary or primary to the magnetic core, which is connected electrically to the chassis.

Therefore, before powering on an old electronic device, start by examining its schematics and checking for insulation breakdowns. Make sure you use the correct power cord and the correct arrangement of contacts in the wall plug (chassis to ground); use fuses and breakers. It wouldn't hurt to have an isolation transformer, either.

All that being said, let's begin. RF-tuned radios are usually described with the formula number-V-number. The letter V denotes the detector; the first number is the number of RF gain stages, and the second is the number of AF gain stages.

Let's take a crystal radio and connect a single-transistor AF amplifier after the detector. The result is a radio receiver 0V1.

Or you can connect the transistor before the detector. Then, the device will amplify the radio frequency and will be called 1V0. It works like this: the amplified RF voltage drops across choke L3, which has a high impedance at high frequency. It is detected (rectified) by a diode, integrated by a capacitor C4, and sent to the earbud.

And here, we have a 0V0 receiver because the transistor in this circuit serves not as an amplifier but as a detector. It operates in such a mode as to amplify only one half-cycle of the input signal, the positive one. The bias resistor R1 is responsible for the transistor mode. L3 and C4 are integrators that convert the energy of high-frequency oscillations into low-frequency ones.

So, we've remembered that capacitors have low impedance for high frequencies and high impedance for low frequencies and do not pass direct current through at all.

With inductors, it’s the other way around: they easily pass direct current, have little resistance to low-frequency current, and have strong resistance to high-frequency current.

What if we amplify a high-frequency signal from a radio station with a transistor, feed it to a detector, integrate it, and feed it to the input of the same transistor again, this time in the form of a low-frequency audio signal?

Surprisingly, it is possible. On a single transistor, we will get ourselves a 1V1 receiver like the 1962 Japanese Lark TR-107.

In those years, imported radios with less than three transistors were eligible for customs exemptions when imported into the United States. It was an intelligent decision. On the one hand, it stimulated the domestic production of more complex and advanced receivers. On the other, it made a pocket receiver accessible to everyone so that in the case of an emergency, everyone would receive public warning alerts in no time.

Therefore, two seemingly mutually exclusive trends occurred. Manufacturers from Japan were making some of their receivers as advanced as possible using one or two transistors and a pair of diodes to successfully compete in the lower price segment of the US market.

On the contrary, other devices used a third transistor simply as a diode to create the cheapest mid-range radios. The same thing was happening in the premium segment. In the last post, we assembled an 8-transistor superheterodyne, where one of the transistors acts as a diode. For the end customer, 8 transistors sounded more prestigious than 7.

Replacing a cheap diode with an expensive transistor today seems unreasonable. But let me remind you that poor-quality transistors in a batch were easy to find in those days. Those were picked and used instead of diodes, which was also a saving!

I should add that the same marketing increase in active elements occurred in the tube era. Take All American Five, with a sixth lamp added as a ballast resistor. It did not improve the quality of radio reception or sound. But a six-tube radio is more prestigious than a five-tube radio.

The substantial benefit of having the sixth lamp was that it made it possible to ditch the line cord resistor, thereby eliminating the suboptimal outcome of setting a curtain on fire. For the line cord resistor to cool down properly, it should not be twisted or covered. Fires were a real threat when this safety requirement was neglected.

So, we have a diagram of the 1962 Lark TR-107. The lower end of the secondary winding L1 is grounded at high frequency by capacitor C2. From its upper end, the audio signal enters the base of the transistor and is amplified.

L2 is a high-frequency transformer that is often wound on a ferrite ring. Capacitor C3 and two diodes form a voltage-doubling rectifier, and C2 is its integrator, forming a detector.

The low-frequency signal of the detector passes freely through the secondary winding L1 and is amplified by the transistor, then passes freely through the primary winding L2 and enters the primary winding of the matching transformer T, the output of which has an earbud connected to it.

The inductance of the winding L2 and the capacitance C3 are small, so the audio frequency signal basically does not reach the detector diodes. Therefore, self-excitation does not occur.

Thus, one transistor amplifies both radio and audio frequencies! Such a radio receiver is called reflex, like in "reflection": the signal after the detector is reflected back and again enters the transistor amplifier stage.

The receiver I've put together is based on the iconic Chinese radio "636" design from 1963. Thanks to its clever design, it was a bestseller and is still considered one of the best single-transistor radios. Let's compare it to the 1962 Lark TR-107.

The custom "reflex" transformer was replaced with a simple mass-market RF choke L. This brought down the cost and simplified the assembly without sacrificing quality. The device was sold as a finished product or an assembly kit and was widely available.

Over 60 years, the scheme has undergone several improvements. There was a two-transistor version for low-impedance headphones and a three-transistor version with a loudspeaker. Still, these variations altered only the audio path and are insignificant for our study. Let's take a look at improvements to the radio part.

First, a positive feedback winding is added between the transistor emitter and the ground. This is a tickler winding, which can significantly increase the receiver's sensitivity.

It adds some energy to the weak magnetic antenna signal at the same frequency that the antenna resonant tank is tuned to. This way, a weak signal becomes a strong one.

The feedback winding contains no more than two turns, so it does not interfere with amplifying high and low frequencies.

The end result was not just a reflexive but a regenerative receiver. If we do not need regeneration, the additional winding can be short-circuited with the jumper JP1.

To prevent regeneration from turning into self-excitation, a trimming resistor W is added for gain adjustment.

To ensure that a weak signal is amplified more and a strong signal less, there's auto gain control: an integrator consisting of capacitor C4 and resistor R1 selects the amplitude envelope of the detector signal and supplies it to the base of the transistor.

Then, LED D1 is not a power-on indicator but a transistor base bias stabilizer. The LED works like a stabilator: the voltage across it is almost independent of the current flowing through it.

Thanks to this LED, the gain will not go down as the batteries are used up. And when swapping molten-salt batteries with alkaline ones, the receiver, despite being configured for the former, will not self-excite.

Jumper JP2 may be needed to adjust the collector current of the transistor. If there is no input signal, it should equal 1 mA.

Open jumper JP3 and close JP4 if we connect not an audio amplifier but an earbud to the radio output. If this is a mono headphone, then open JP5.

The video below shows this receiver's assembly and operation.