Toshiba GR-207E Refrigerator Fails to Defrost Automatically

Fault Description:
The refrigerator does not enter defrost mode when the defrost button is pressed.

Analysis and Repair:
First, operational errors must be ruled out. For instance, if the freezer temperature is above 0°C, the defrost function will not activate even if the button is pressed.

Under normal cooling conditions, when the internal temperature is higher than the set value, the output of the operational amplifier (pin 3) in the defrost control circuit remains at a low level. This keeps transistor Q812 in a cut-off state and the defrost relay RY02 disconnected. Even if the defrost button is pressed at this point, the op-amp output does not flip, and Q812 remains off, preventing defrosting.

As the freezer temperature gradually decreases, the voltage at pin {11} of the op-amp changes. As long as this voltage remains lower than the voltage at pin {10}, the op-amp output (pin {13}) stays low, and the defrost relay remains disengaged. However, the changing voltage at pin {11} prepares the circuit for a state transition.

When the defrost button is pressed, pin {10} is grounded. At this point, the voltage at pin {11} exceeds that at pin {10}, causing the op-amp to flip. The output at pin {13} switches to a high level, which is applied to the base of Q812 via R829 and D810. This turns Q812 on, energizing the defrost relay RY02 and activating the electric heating element to initiate defrosting.

After confirming that the freezer temperature was below 0°C, the defrost button was pressed. However, the defrost relay RY02 still did not engage. Voltage measurement at the base of transistor Q812 showed a normal value of 0.75V. Suspecting a faulty Q812, the transistor was removed for inspection. A cold solder joint was found at the emitter pin, though the transistor itself was undamaged.

Resolution:
The emitter pin of Q812 was re-soldered to restore a proper connection. After reassembly, the defrost function operated normally, and the fault was resolved.