I have a very curious radio designer. It was released in 1977 and cost 10 rubles at that time. 35 structures can be assembled from this constructor, moreover, without soldering. A “Krona” type battery is used as a battery.
The design is based on a cassette, where “electronic cubes” are installed in a certain order – modules with four contacts on the sides.
The cassette housing contains a variable capacitor, a variable resistor as a volume control and a battery compartment with an electrolytic capacitor connected in parallel to it. I must say right away that I replaced all the electrolytic capacitors in the designer with new ones, and the volume control with a less worn one.
Modules contain jumpers or radio components: transistors, diodes, resistors, capacitors. There is a module with a ferrite magnetic antenna, there is a module with a headset (earphone), and there is a module with a primitive telegraph key.
Below is a top and bottom view of modules with transistors, diodes, resistors and capacitors.
Transistors used are germanium pnp. Type of transistors GT309. Diodes are also germanium – D9. Resistors are used by MLT-0.25. Capacitors – K10-7.
In terms of methodology, the constructor is just perfect. First, a wiring diagram of the device is given. Then there is a description of the purpose of the device, and only then the electrical schematic diagram.
The presentation of the material is classic. First, circuits of audio frequency amplifiers (AF) are given. Then the diagrams of radio receivers are given. And only then – generator circuits. Moreover, not only schemes of AF generators are given, but also schemes of radio frequency (RF) generators
The entry threshold is minimal. I assembled the device according to the wiring diagram, it worked. It did not work, I checked the correctness of the assembly. It didn’t work again, I replaced the battery.
Later, if it’s interesting, you can try to figure out the circuit. True, the diagrams are drawn a little not according to the canons of the Radio magazine, but they are simple, for a dozen components.
As the first design of the amplifier, a “classical” cascade circuit is given on a single transistor connected according to a common emitter (OE) circuit. Then comes the circuit of the OE cascade on a composite transistor (Darlington circuit), it is indicated that the amplification of such a cascade is higher.
Then an example of an amplifier with an emitter follower (OK circuit) is given. The description says that the OK circuit has a high input impedance and a gain of less than unity. Accordingly, the amplifier circuit with a high input impedance is given in two stages: an emitter follower is used as the first stage, and an OE circuit is used as the second stage.
The amplifier stage according to the common base (OB) scheme is considered using the example of an amplifier with a low input impedance. An interesting feature of the circuit is that the bias at the base of the transistor is set by the voltage drop across two germanium diodes connected in series.
Among the amplifier circuits in the description of the radio designer, my favorite is undoubtedly the AF amplifier design with stabilization:
The amplifier is two-stage with direct connection between the stages. Due to negative feedback (OOS), stabilization of the amplifier operation mode is provided.
Before the advent of inexpensive and high-quality operational amplifiers, such circuits were successfully used in direct conversion receivers, because had a gain from 1000 to 3000. Amplifier circuits with direct coupling and OOS on three transistors already had a gain from 10,000 to 30,000.
The assembled amplifier design looks like this:
Radio receiver designs
The most striking thing is that the design of the classic detector receiver is not here. But not everything is so simple: it is there, but the only transistor in the circuit is used as a detector.
It’s all about the bias voltage at the base of the transistor. In the above diagram, the resistor value in the base circuit is 4.3 MΩ. With such a bias at the base, the transistor works as a detector. In amplifying stages, the value of such a resistor is 1 megohm or less.
Below is a diagram of a 1-V-0 receiver, where the left transistor acts as an RF amplifier and the right transistor acts as a detector:
Further in the section there are constructions with different exotic schemes. For example, a reflex receiver circuit, when the same stage is used for both RF amplification and AF amplification. Or a receiver with an aperiodic input, when the oscillating circuit is not at the input of the first stage, but at its output. Or a receiver with an emitter follower (OK) in the first stage, which gives an increase in the quality factor of the input oscillatory circuit.
After experimenting with receiver circuits, all children usually settled on a 1-V-1 circuit. A similar designation is for radio receiver circuits with one RF amplifier stage, a detector and one AF amplifier stage.
“In the center of the composition” is a diode detector, assembled according to the voltage doubler circuit. The magnitude of the forward voltage drop across the germanium diodes is about 0.3 V. To ensure the operation of the detector, the signal amplitude of the radio station must be greater than this value. For this, the signal of the radio station, isolated on the input oscillatory circuit tuned to resonance, is amplified by the OE cascade on the left transistor according to the scheme. The AF signal highlighted by the detector is amplified by the OE cascade on the right transistor according to the circuit.
On an external antenna, such a receiver receives several radio stations in the CB range.
Photo of the assembled structure of the radio receiver according to the “1-V-1” scheme:
For the share of children of generation X, radio broadcasting on the DV and SV bands was enough. The children of generation Y have not seen radio broadcasting in the Far East. Children of generation Z did not get a single one powerful enough to be received by the “detector” of a local radio station in the LW or MW range.
But children of all generations love to knock with the key.
Generators of three types can be assembled on the basis of the constructor. First, a generator circuit is given, obtained from an amplifier with stabilization by closing the input (left according to the scheme, the output of the capacitor 0.01 μF) to the output (collector of the right according to the transistor circuit):
Then an example is given with a circuit of an almost symmetrical multivibrator, made, as it should be, on the basis of a two-stage amplifier according to the OE scheme:
With such schemes, you can practice working on the key, especially since the Morse code is in the application.
The third type of generators is one-transistor inductive feedback generators. They generate not only AF signals, but also RF signals. And with such equipment it is already possible to go on the air.
My favorite design, as usual, is the last one on the list. This is the construction number 35 “Morse”. The long wave (LW) range for broadcasting has not been used for a long time, but in this range the radio receiver can receive the Morse code signal. True, the signal is very weak, it can be received at a distance of 1-2 meters, but this also causes wild delight among young radio amateurs. Tested on Generations X, Y and Z children.
Outwardly, “Morse code” looks like this:
Its circuit is very simple, the generation frequency is set by the settings of the oscillatory circuit, positive feedback is carried out through the coupling coil of the magnetic antenna. A telegraph key is included after the electrolytic capacitors in the power circuit to prevent chirping (CHIRP).
Video of the work of “Morzianka” on the air: