(1) One of the four rectifier diodes has a problem
(2) There may be carbon powder or carbon brush defects between the motor rotor and the rectifier. Following the principle of easy repair first and difficult repair later, the adjustable resistor was first measured, and the resistance value changed regularly with adjustment, proving that the potentiometer was good. Then, the thyristor was measured and no problems were found. As the diode is a high current metal sealed tube, one end is fastened to the circuit board with a screw, and the other end is soldered to the circuit board with a wire. When removed from the circuit board, it was found that the positive and negative poles were loose. The diode was then measured and shaken to confirm that it was sometimes good or bad.
(3) There are also many problems with thyristors, usually caused by breakdown short circuits, open circuits, inability to trigger, or poor internal contacts.
(4) Adjustable resistor, cabinet centrifugal fans have a very high frequency of use due to the installation of adjustable resistors on the control panel. Over time, poor contact is common.