Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The proposed article deals with how to make switching grids in MAYCOM CB radios more convenient, as well as powering radios in a car.

On the domestic market, the model range of the MAYCOM company is represented by four developments (AN-27, SH-27, EM-27 and HM-27), which successfully compete with the equipment of other companies. The secret of success is quite simple: MAYCOM transceivers combine good technical characteristics, rich functionality and reasonable price.

The mobile-base radio station EM-27D enjoys well-deserved popularity. The transceiver expands to 15 grids (600 channels), which covers the frequency range of 23,815...30,555 MHz. The first mixer of the receiver is made on field-effect transistors in a balanced circuit, there is a paging based on DTMF encoder / decoder, 7 memory channels, 2 types of scanning, repeater mode with arbitrary frequency spacing, programmable PTT and many other useful functions. For our radio amateurs and users, the function of prompt switching of the "European" and "Russian" frequency standards is extremely important. It is especially valuable that this does not require a reset and restart of the microprocessor (as in ALAN 48+/78+ and YOSAN JC2204/YT07 radios).

Unfortunately, the manufacturer did not quite take into account the national specifics of the Russian CBS air, where they are equally active in both "zeros" and "fives". When developing the transceiver, we proceeded from the concept that the "0 kHz" grid will be used, at best, sporadically and gave this mode the status of a non-main one. This is probably why access to switching frequency standards is organized extremely inconveniently. At first glance, the procedure looks elementary simple: the operator must, while holding down the M1 button ("FUN"), press the MB button ("BEEP / BAND").

In reality, things don't go so smoothly. The mentioned buttons are positionally spaced, and you can press them either with both hands, or with the help of a "finger-tip", popularly known as a "horned goat". The button pushers themselves are located on the same level with the front panel and copy its configuration with their wave-like shape. With such a stylish, but, alas, non-ergonomic design, the buttons are almost not felt tactilely, fingers are not fixed on them and slip off. Insufficient control over the buttons is not only tiring, but also leads to annoying situations. For example, when the M1 + M5 button combination is pressed by mistake, the microprocessor is reset (RESET CPU).

After suffering for some time, the author of these lines decided that things would not go on like this. Therefore, as a continuation of the topic started in [1], a control module was developed that allows you to switch the "European" and "Russian" standards in an ideal way - without touching the transceiver and almost instantly. After refinement, the F button, located on the tangent, starts working according to the following algorithm: when pressed for a short time, the standard function is implemented, when pressed and held for 0,3 seconds, the frequency standards are switched.

The control module diagram is shown in fig. one.

The device is powered (pins 1, 2) with a stabilized voltage of +8 V, available in the transceiver, and consumes a current of 80 μA. In the receive mode, pin 3 may have a voltage in the range of +1 ... 2,5 V (depending on whether one of the programmable buttons P1-P1 is pressed or not). Transistor VT0,4 is open, the voltage on its collector is less than 2 V, so transistors VT3 and VTXNUMX are closed.

With a short press of the F button, output 3 is switched to a common wire, the transistor VT1 closes and a voltage of +8 V appears on its collector. In this case, the transistor VT2 opens and "closes" the M1 button. If the F button is held down, the capacitor C1 is charged through the resistor R3 and after a delay of 0,3 s, the transistor VT3 opens, which "closes" the Mb button. Thus, automatic switching of frequency standards is realized. The process is accompanied by a change in the frequency readings on the LCD display in units of kilohertz (0 or 5), and is also confirmed by a double beeper signal (if the BEEP function is activated).

After releasing the F button, the capacitor C1 quickly discharges through the diode VD1 and the open emitter-collector junction of the transistor VT1, as a result, the device is immediately ready for further use. It should be noted that the transistor VT1 must have a guaranteed high static current transfer coefficient in a circuit with a common emitter (400 ... 1000), so it is undesirable to use other types of transistors.

Structurally, the module is a rectangular tile measuring 40x20x3,5 mm. The side planes are made of thin fiberglass, between which radioelements are located. The resulting "sandwich" is pressed with epoxy resin. Flexible terminals of the module are made of a flat ribbon cable (loop). A traditional PCB mounting option is also possible. Installing the module in the transceiver requires care and the obligatory use of measures to protect against static electricity.

The proposed refinement can also be implemented in the NM-27 model, of course, provided that its options are configured for the "Russian" frequency grid.

Now let's move on to the features of using the MAYCOM EM-27D CB transceiver in a car. If the transceiver is turned off using the PWR button, it does not turn off at all, but goes into the "hot" standby mode. At the same time, many functional units remain energized and the current consumption is 100 ... 130 mA. Leaving such a tangible consumer in a car overnight is at least uneconomical. Moreover, it is also unsafe, since spontaneous operation of the electronic unit for turning on the transceiver is possible (for example, from interference).

Experienced users prefer to undock the transceiver from the car's on-board network just in case at the end of the working day. However, another problem immediately arises. In an autonomous state, the internal memory of the microprocessor is powered by a backup current source - an ionistor, which has an electric capacity of 0,047 F. In about 30 minutes, the voltage across the ionistor decreases from 5 to 0,8 V, after which all information from the memory is erased. For many users, this is extremely inconvenient, because each time you have to program from 2 to 10 or more parameters (channels, grids, DTMF codes, etc.).

Of course, this drawback can be eliminated by using a higher capacity ionistor or even a battery, but this will require qualified intervention in the transceiver. Instead, a simple and affordable method is proposed that does not require opening the device.

On fig. 2 shows an alternative scheme for connecting the EM-27D to the car's on-board network.

When the SA1 toggle switch is in the "WORK" position, the transceiver receives power and can be turned on with the PWR button. If the toggle switch is moved to the "SLEEP" position, a current of 1 mA will flow through the R1HL8 circuit, and the LED will indicate the enabled mode. With such a small current, not a single transceiver node functions and it cannot be turned on with the PWR button. Nevertheless, in this mode, a voltage of 1,2 ... 1,3 V is maintained on the ionistor, which is sufficient to store information in the RAM of the microprocessor for an indefinitely long time.

Literature

1. Sokolov A. "Russian" grid in C-B radio station. - Radio, 2000, No. 9, p. 64.
2. MAYCOM Co. Ltd. Service manual. Model EM-27 (Electrical Diagram).

Author: A.Sokolov, Moscow

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