Changhong cabinet-type air conditioner, the outdoor unit only occasionally works. Beijing Changhong Air Conditioner After-sales Service Center Technical Information on December 13, 2011.
Fault Phenomenon: According to the user's introduction, when this cabinet-type air conditioner is powered on and started, the outdoor unit only occasionally works, and in most cases, the outdoor unit cannot enter the working state. When the outdoor unit does not work, the fault indicator light can be lit.
Fault Analysis: After powering on and starting up, the outdoor unit occasionally enters the working state, indicating that there is no obvious short circuit in the outdoor unit circuit. It is estimated that the fault may be caused by poor contact, such as the virtual soldering of the lead of a (or some) component or part, poor contact of the connector, or breakage in the connecting line. Of course, it is also possible that the characteristics of a component have deteriorated, causing intermittent problems leading to this fault. The input control circuit of the main board of the outdoor unit of the cabinet-type air conditioner includes temperature detection circuits, compressor current detection circuits, etc.; the output control circuit includes drive circuits for the compressor, four-way reversing valve, crankcase electric heater, one-way valve, outdoor fan, etc., and fault signal indication circuits. The core of the entire circuit is a single-chip microprocessor IC14 (PC75066).
Maintenance Method: Under the condition of power off, first check the appearance and soldering conditions of each component in the microprocessor control system circuit of the outdoor unit, and no obvious damage or breakage was found.
1. Determine the approximate location of the fault
(1) Check the power supply of microprocessor IC14. When the fault occurs, use the 10VDC range of a multimeter to measure the voltage on pins 19-21 of the outdoor unit microprocessor IC14, which is about 5.2V, indicating that the power supply voltage of the microprocessor is normal.
(2) Test the reset signal of microprocessor IC14. Measure the reset signal on pin 12 of microprocessor IC14, which is normal, but there is no reset signal output on pin 3. This suggests that IC14 microprocessor may not be working.
(3) Test the clock signal of microprocessor IC14. When measuring the waveform on pins 4 and 5 of IC14 with an oscilloscope, touching pin 4 did not detect any oscillation waveform, but suddenly a waveform appeared when touching pin 5. At this time, the outdoor unit can enter the working state. Therefore, suspect that the X4 quartz crystal oscillator connected externally to pins 4 and 5 or the two capacitors are faulty.
2. Fault Handling
(1) Remove the X4 quartz crystal oscillator and the two 30pF capacitors C63 and C64 for testing, no obvious leakage was found. Based on experience, estimate that the X4 quartz crystal oscillator is faulty.
(2) X4 is a quartz crystal oscillator with an oscillation frequency of 4.18 MHz. Since there are no spare parts of the original model available temporarily, try baking the removed X4 quartz crystal oscillator under a 60W incandescent lamp for about 15 minutes. After X4 naturally cools down and is reinstalled in its original position, power on for testing, the outdoor unit works normally. After long-term testing, the above fault did not reappear, and the fault was eliminated.