**Case of Haier KFR-2518GW Air Conditioner Repair: No response from the unit when turned on with the remote control.**
**Analysis and Testing:** After testing, it was found that the computer board TM87C846 was faulty.
**Repair Method:** Replace the computer board TM87C846 to eliminate the fault.
**Warm Reminder:** The control circuit of the indoor unit mainly relies on the computer board TM87C846, which includes several key parts such as the power supply circuit, zero-crossing detection circuit, power-on reset circuit, oscillation circuit, storage circuit, fan motor speed control circuit, drive circuit, and temperature sensor circuit.
(1) **Power Supply Circuit:** 220V AC is stepped down by a transformer, then fully rectified by D1-D4. After filtering by C08 and C11, it serves as the input for the linear integrated voltage regulator 7805. The output 5V DC is filtered by C09 and C12 and used as the power supply for the indoor unit control circuit. To prevent the zero-crossing detection circuit from impacting the power supply circuit, D05's unidirectional conduction characteristic is used for isolation.
(2) **Zero-Crossing Detection Circuit:** Its main function is to detect the indoor unit's supply voltage. The working principle involves stepping down 220V AC to 10V AC via transformer T1, which is then rectified by a silicon bridge into a 100Hz pulsating voltage. The voltage is divided by R12 and R16 to serve as the base bias for ooi, controlling the transistor's conduction and cutoff. This generates a zero-crossing trigger signal at the 32nd pin of the indoor unit CPU.
(3) **Power-On Reset Circuit:** To initialize the data in the CPU upon startup and ensure the stability of the air conditioner's operation, a reset circuit composed of R13 and C13 is used. During the initial power-on phase, the charging and discharging process of the capacitor generates a reset signal that is input to IC08.
(4) **Oscillation Circuit:** An oscillation circuit composed of XT01 and R37 serves as the clock signal for the CPU's operation.
(5) **Fan Motor Speed Control Circuit:** When the CPU sends a low-level pulse to IC05, the internal bidirectional thyristor conducts, supplying power to the indoor fan. By changing the pulse width sent by the CPU, the effective working voltage of the fan motor is altered, thereby achieving fan speed regulation.
(6) **Drive Circuit:** To enhance load-carrying capacity, a reverse amplifier U1N2803A is used as the core. When the CPU sends a control signal to IC09, the corresponding pin of the reverse amplifier will receive a low-level signal, which is then output to the corresponding components. This controls the operation of the stepper motor, the beeping of the buzzer, and the engagement of relays (including the main relay for outdoor unit power supply, four-way valve, outdoor fan motor, and compressor control relay).
(7) **Temperature Sensor Circuit:** Two thermistors are used in conjunction with R28 and R26 to divide the voltage and detect the indoor environmental temperature and the indoor coil temperature.
**Source of Information:** Beijing Haier Air Conditioner Repair Center
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