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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Typical defects of CB radio station Alan. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications By purchasing a civil radio station (27 MHz), each user hopes for its long-term trouble-free operation. However, sooner or later, any equipment fails and requires repair. This article discusses the causes of failures and ways to improve the reliability of radio stations. There are many specific reasons for failures, but according to statistics, some defects are repeated much more often than others, so they can be called typical. They are mainly due to design flaws laid down at the development stage. Knowledge of typical defects reduces the time for diagnostics and restoration of equipment performance, and also allows you to take preventive measures to prevent the occurrence of these defects. Often the reason for the failure of radio stations Alan 78 plus, Alan 48 plus, Alan 48 Excel is the massive destruction of soldering on the printed circuit board in the area of \u6b\u18bmounting radio elements related to the VCO. As a rule, the defect develops gradually: suddenly appearing, it can eliminate itself for a long time, and it manifests itself steadily after XNUMX ... XNUMX months of operation of the transceiver on board the vehicle. When operating the transceiver in a closed heated room with stable climatic conditions, the defect appears much later or does not occur at all. An external symptom of a malfunction is the absence of transmission (TX) and/or reception (RX). When removing the screen covering the printed wiring of the VCO, one can detect concentric solder cracks around the terminals of those radio elements that have relatively large dimensions (L401 coil, transistors, capacitors). Since there are many other similar components on the board, and this defect is exclusively local in nature, there is an assumption that paraffin, which is generously filled with the entire volume of this functional unit, is to blame. The use of a viscous damping fill is caused by the need to eliminate the acoustic (from the loudspeaker) and vibration (from shaking) effects on the VCO elements, leading to the "microphone effect" and even self-excitation of the entire radio path. The defect mechanism is as follows. Paraffin (or other applied wax-like casting composition) is an organic substance having a high temperature coefficient of volume expansion. This substance has good adhesion. Enveloping the radioelement, the casting mass deforms with temperature changes, resulting in a force in the direction of the longitudinal axis of the leads. This force is proportional to the dimensions of the radio element. In other words, when heated, the paraffin expands and tends to "pull" the radio component out of the board, and when cooled, it contracts and "presses" the radio component into the board. The cyclic alternating loads arising in this case destroy the crystal structure of the solder, leading to the appearance of centers of fatigue cracks. When operating the transceiver in a car, the process of solder destruction is significantly accelerated due to significant temperature fluctuations (-30 ... +60 ° C) and shock-vibration loads. In order to prevent the development of the described defect, it is recommended to render up to 90% of paraffin. To do this, use an electric hair dryer equipped with a narrow nozzle. The temperature of the hot air flow and the duration of its exposure must exclude thermal damage to the radioelements. The melting point of paraffin is in the range of 50...70 °C. For reliable detection of printed wiring defects, it is convenient to use a special light source with a circular shadowless lamp and a magnifying glass. Identified defective solderings must be carefully restored and be sure to rinse with alcohol or acetone. The weak point of Alan 78 plus transceivers is the NP-250-2 dynamic head. This SAMMI driver with a nominal power of 2 W and a voice coil impedance of 8 ohms has good acoustic properties, but has a low reliability of the cone leads, so it often fails. It is not easy to repair it, it is even more difficult to find a full-fledged replacement due to severe restrictions on dimensions (height - 20 mm, diameter - 65 mm). You can significantly extend the life of a dynamic head by connecting a resistor with a resistance of 5 ... 10 Ohm, 1 W in series with it. The resulting slight loss of volume is hardly noticeable due to the peculiarities of our hearing. And another simple tip: when installing a radio station in a car, try to position it in such a way that the sound passes freely to the driver. This will allow, without extreme need, not to force the UMZCH of the transceiver and not to overload the head. For the same purpose, it is recommended to use a dedicated external speaker by connecting it to the "EXT" jack. The next defect occurs in Alan 48 Excel radios and concerns a constant resistor R803, or rather, a very recklessly put on PVC insulating tube. During operation, the resistor heats up to temperatures above 100 ° C, and this synthetic polymer begins to decompose, releasing hydrogen chloride and other corrosive agents that corrode even ... anodized steel of the transceiver cover! However, the main trouble is that the aggressive gaseous environment affects closely spaced mechanical elements - the channel selector ("CH"), buttons ("FUN", "M2 / DW", etc.) and the pointer S-meter. As a result of chemical reactions, all available metal parts of these elements are covered with a build-up of crystallized salts. The contact resistance of switching products increases sharply. The listed elements begin to function with failures, and then completely lose their working capacity. When operating the transceiver in a car where there are conditions for the formation of water condensate, hydrogen chloride turns into hydrochloric acid and the process is aggravated. As a result, there is a complete electrochemical corrosion of the printed conductors located on the front board of the transceiver within a radius of up to 5 cm from the aforementioned resistor. To prevent the development of such a serious defect, it is advisable to use an insulating tube made of a more heat-resistant material or do without it if a gasket made of thin fiberglass is placed between the resistor and the front board. A large number of failures are caused by the poor quality of the trimmers, which are marked RV on the diagrams (for example, RV101). In this case, the external manifestations of malfunctions depend on which of the following parameters is used to adjust the resistor:
The reasons for the failure of the resistors are the violation of contacts in the places of riveted connections of the leads to the metallized tips of the carbon resistive layer, a crack in the getinax base, a breakdown in the movable contact circuit. A faulty resistor cannot be repaired and requires replacement, followed by mode adjustment. Author: A.Sokolov, Moscow
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