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Overhaul of common faults of car radios
Jan 17, 2019

Overhaul of common faults of car radios

      In the long-term practice of radio management, gradually accumulated some experience of common walkie-talkie maintenance, such as C150, HX260, TM231 and other high failure rate, but the maintenance is relatively simple, provided below, for reference only.
      Fault phenomenon  
      The liquid crystal display and reception of the C150 walkie-talkie are normal, and the operation launching light is normally bright, but the signal cannot be transmitted.
      Analytical maintenance  
      The intercom transmitter's indicator light is normally lit, indicating that the 5V voltage is normal and the PTT button is also fault free. The problem may occur in power amplifier circuits and voice amplifier circuits. Connect the walkie-talkie to the 2955 integrated tester and measure the power output to zero. Open the cover, press the PTT button, and measure the transmission power amplifier tube V423 (2SC1971) with a UHF millivoltmeter. The c-pole voltage is 0.6V (normal value is 6.9V), and the b-pole voltage is 0.4V (normal value is 0.47V). ), the value of the positive and negative resistance between the poles of V423 is very small, indicating that it has been broken down. After replacing the power amplifier tube according to the original model, the re-measurement output power is about 1.5W after the walkie-talkie is connected with the comprehensive tester, but the walkie-talkie Still can't speak normally, plug the plug into the external microphone test machine, the fault remains, and the voice modulation circuit is suspected to be faulty. The voltage of pins 3, 5, and 7 of the test integrated circuit A203 (NJM4558E) is normal, and the voltage of pin 1 is 0V (normal value is 2.7V). When the load of 1 pin is disconnected, the forward and reverse resistances of the ground are measured to be close to 0Ω. Note A203 is damaged, and the fault is eliminated after replacing the original model.
      Fault phenomenon  
      C150 walkie-talkie, the upper part of the boot display shows half a character and disappears immediately.
      Analytical maintenance  
      Check that the display circuit has no abnormality, the voltage of the 33rd pin of the microprocessor μPD7514G is 3.95V normal, and the voltage of the 70 pin is 0V (normally 3.40V) abnormal. This voltage is obtained by R123 of 4.5V of the cable W101, R123 The voltage is normal. Check that the R123 to 70th cable is disconnected and reconnect the fault.
      Fault phenomenon  
      C150 walkie-talkie, the screen characters flash and cannot be sent and received after power on.
      Analytical maintenance  
      The character flashes after power on. Under normal circumstances, the power supply line has no fault, but the transceiver does not work normally. The problem may occur in the phase-locked loop. This fault can be determined by detecting the cable J201UL voltage. It is lost lock. First check whether the PLL circuit Q401 works normally. It is tested that the voltage of pins 1 and 2 is 1.6V is low level, and it is in the state of loss of lock. Further inspection reveals that the external capacitor C434 is leaking, replace the capacitor, and the fault is eliminated.
      Fault phenomenon  
      After the Kenwood TK-208 walkie-talkie is turned on, the LCD display is normal. Pressing the PTT button does not normally transmit, and the emission indicator does not light.
      Analytical maintenance  
      The LCD display of the walkie-talkie is normal, indicating that the reception may work normally. The signal is transmitted by another co-frequency machine and can be listened to normally. It can be judged that the receiving circuit has no fault and the battery voltage is normal. The fault may occur in the relevant parts such as the launch button. When testing, open the cover and check that the launch button is elastic. Observe a small amount of dust on the button switch S301. After wiping with anhydrous alcohol, the fault remains. Carefully observed with a magnifying glass, the printed circuit board connecting the soldering piece of S301 was connected to the other side of the circuit board through two conductive holes. The resistance of one of the conductive holes was found to be ∞ by a multimeter, indicating that the conductive hole was broken, and two wires were used. The terminal is connected, the intercom is normally transmitted, and the fault is eliminated.
      Fault phenomenon  
      When the HX260 is turned on, the LCD display or programming mode does not start properly.
      Analytical maintenance 
      There are two situations that will cause the full display of the display to occur: First, the channel selection knob stops at the middle of the two channels. Just rotate the channel knob to eliminate the fault; second, the number keypad is damaged, remove the dial or change it again. With a one-digit keypad, this fault disappears. Sometimes the encoding mode does not start properly when the HX260 is re-transmitted, which is caused by too much dust on the front panel. Remove the front panel and wipe with absolute alcohol. After the alcohol has completely evaporated, install the front panel and restart to enter the programming mode.
      Fault phenomenon  
      After the TH-26A walkie-talkie is turned on, the display and lighting are dim. Press the launch button to transmit the signal, but after a few seconds, it will automatically go to the receiving state.
      Analytical maintenance  
      According to the failure analysis, the fault may occur in the display and CPU control circuits. Open the cover, first check the microprocessor's clock oscillation 3.58M crystal is not good, use the oscilloscope to observe the normal waveform, further analysis of the suspected power supply for the liquid crystal display is not good, try to adjust the power supply circuit Q216 (2SB798) Replace, troubleshoot.
      Fault phenomenon  
      TH-28A walkie-talkie press POWER button to start without any reaction, the machine does not work.
      Analytical maintenance   
      There is no reaction when the machine is turned on. The fault usually occurs in the power supply and control circuit. Since the machine has a time switch function, the CPU is always in the power supply state. Open the cover, use a multimeter to measure the CPU (97) pin voltage is 2.8V (normally 4.7V), test IC9 (2) pin input terminal voltage is 7.2V normal, (3) foot output terminal 0V (normally 5V), The internal short circuit of the three-terminal regulator SC17710Y is damaged, and the fault is eliminated after replacement.
      Fault phenomenon  
      The Kenwood TK-808 car battery is connected to the electric battery for ignition, or no power is displayed, and there is no action in the machine.
      Analytical maintenance   
      The radio station cannot be powered normally, and the phenomenon of ignition occurs when power is applied. This is mainly because the positive and negative power lines are reversed when the machine is installed, causing the reverse current protection diode to be broken down, so that the power cannot be normally applied. Open the rear cover of the car, check that the reverse current protection diode has been burnt, replace the same type of diode, and the fault can be eliminated.
      The other case occurs mainly in the power section. Add power supply, measure 13.6V voltage normal, measure MC7808 three-terminal regulator without output voltage, input voltage is also very low, test V17 (2SB1302) base and emitter voltage are 13.6V, indicating that switch V17 does not work, V5 (DTC14WK) output 5V is normal, indicating that V18 is turned on, check V17 bias circuit is normal, indicating that V17 is damaged, replace V17, and troubleshoot.
      Fault phenomenon 
      The TM-231 type radio station does not work according to the launch.
      Analytical maintenance 
      First check the transmitter microphone without any malfunction. Use 2955 comprehensive tester to measure no power output, try to change the power amplifier block, although there is power output, but the power supply control terminal of the power amplifier block always has voltage, and the output power is not adjustable. It is normal to measure Q17, and thus it is judged that the fault may be in the automatic gain control circuit. Check the switch tube Q3 (2SA1162) and find that its emitter base is short-circuited with the collector. The switch tube has been broken down, causing Q17 to conduct. The controlled end of the power amplifier power supply always has voltage, so that the output power cannot be adjusted. Replace Q3 troubleshooting.

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