Increasing the reliability of the PTT. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Increasing the reliability of the PTT. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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With intensive use of radio stations, their PTTs begin to falter. There are cods, noises, sometimes communication becomes impossible. The article analyzes this phenomenon and proposes measures to improve the reliability of the tangent.

The experience of operating CB equipment in cars shows that often the cause of communication failure is the failure of the PTT.

In harsh operating conditions, the PTT must have a housing protection rating of IP54 and comply with the requirements of the international military standard MIL-STD 810 in sections C, D, E. However, it is no secret that simplified technologies, cheap materials and components are used in "civilian" communications. This is easy to verify by comparing the tangents from any CB radio station and a professional mobile transceiver (MOTOROLA, VERTEX, ICOM, KENWOOD, etc.).

The main causes of PTT failure are: wire breaks in the cord, connector and inside the PTT, failure of switching products, microphone, corrosion phenomena in solders and metallized holes, microcracks in printed conductors, breakage of plastic parts. Of particular danger are short circuits in the PTT circuits, which can lead to burnout of chokes, printed conductors and damage to the input ports of the microprocessor. Other malfunctions of the PTT are less dangerous, but can make it difficult or completely disrupt communication: excitation, hum, crackling, rustling, no sound, modulation, transmission, spontaneous channel scanning.

The PTT can be treated in different ways, for example, consider it as a "consumable" and buy a new one every time ($15), constantly repair it, wasting time and money, or ... make it more reliable.

Knowledge of the factors that determine the reliability of radio-electronic equipment makes it possible to carry out a number of measures aimed at increasing the "survivability" of the tangent. For example, it is obvious that devices that are simpler in principle of operation and have a smaller number of elements and controls break down less often.

Guided by this principle, the author made a PTT, which contains only two radio components: a microphone and a PTT button. The unreliable twisted cord, consisting of 5-6 conductors, brittle in bending and stretching, was replaced by a durable two-wire cable. Extended control of the transceiver from the PTT is not provided, since in this situation, reliability and maintainability are more important than functional "bells and whistles".

The PTT has a six-pin Micro-6 microphone connector (NC-518), which is used in many models of ALAN, YOSAN, MAYCOM, MEGAJET, DRAGON transceivers. The operation of the PTT is provided by a simple control module (adapter), which is mounted inside the transceiver.

The module diagram is shown in the figure. The principle of operation is similar to that used in headsets of wearable radio stations. In receive mode, the transistor switch VT1 is closed, relay K1 is de-energized, the SPK loudspeaker circuit is connected to a common wire. When the PTT key is pressed, a bias is applied to the base of the transistor VT1 and it opens. In this case, relay K1 is activated, turning off the loudspeaker and turning on the transmission mode. The base current of the transistor VT1 is also the power supply current of the BM1 electret microphone, so the audio signal from the load resistor R1 enters the MIC circuit and then to the modulation generation stages. Elements R2 and C3 form a power filter for the microphone. Capacitor C1 blocks RF interference from the connecting cable.

Increasing the reliability of the PTT

If the tangent is supposed to be used in a car with a transmitter power amplifier, then additional measures should be taken to protect against a high-frequency field - install chokes with an inductance of 20 ... 100 μH at the adapter input, and shunt the microphone with a 1000 pF ceramic capacitor.

To guarantee a long and trouble-free operation of the PTT, you should use a high-quality wear-resistant button SB1 (PTT). On a product of domestic production, it is desirable to have the stigma "OS" in its marking. It is permissible to use miniature and tact buttons, microswitches of various types. If the switch has several contact groups, then for reliability they should be connected in parallel. The most advanced switching element is a reed switch controlled by a small permanent magnet mounted on the PTT key.

When choosing a connecting cord, the author was guided by the criteria for its flexibility and tensile strength, so I used a high-quality microphone cable from WORLDWIDE. Special care must be taken when terminating the cable in the PTT and microphone jack.

Electromagnetic relay K1 - RES49, it can be replaced by any small-sized sealed relay with a response voltage of 6 ... 9 V, for example, REK23. The BM1 electret microphone can be of almost any type, as long as it provides good modulation quality. For reliable electrical contact of the aluminum housing of the microphone with a negative terminal, it is desirable to seal the area of their compression by embossing at several points along the circumference of the rolling.

The body of the PTT has a simple shape and is made by milling from impact-resistant material (PA-6 polyamide). When using a regular body, it is advisable to increase its structural strength. To do this, epoxy resin reinforces the mounting racks and other fragments that are potentially susceptible to cracking and chipping.

All soldering must be done with a neutral flux and must be washed with alcohol. Long-term corrosion resistance of solder joints is achieved by coating them with a double protective layer of polyurethane or nitrocellulose varnish.

Please note that a transceiver equipped with an adapter should only be used in conjunction with a modified PTT.

Author: A.Sokolov, Moscow

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Comments on the article:

Eugene
Help to deal with the problem, which is the following. The Kenwood TK 7108 radio station, programmed the scanner button, when the tangent is disconnected from the radio itself, the scanner button is not active, while the scanner button turns on when the tangent is connected to the station and inserted into the tangent holder, which is fixed in the car, and on the tangent itself, where there is a lamb attachment a voltage of 5 volts is suitable and only when they are shorted to the mount, the scan function is activated.

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