Hitachi CM801U Service Manual - Page 5

DISCRIPTION OF CIRCUIT 1. Power Supply Circuit This model incorporates a wide range universal power supply utilizing a switching regulator (see block diagram in Figure 1). 1.1 AC input AC input consists of AC inlet, EMI filter (C903-C906) and rectifier D901, C908. Rectifier circuits adapt to fullwave method. Inrush current limiting circuit (R902) protects from excessive inrush current at initial stage of power on. 1.2 Switching Regulator circuit Switching Regulator circuit is designed to handle variations of two conditions to ensure constant +B voltage to secondary circuit : (Circuit 1) varying AC input voltage 90-240 and varying load conditions of video, (Circuit 2) varying horizontal frequencies. 1.2.1 Circuit #1 : Circuit 1 consists of chopper transformer (T940), chopper component (Q940) and control IC (1940). T940 detects input voltage and provides signal to 1940 pin 5 which adjusts the pulse width based on the pin 5 voltage level to provide constant voltage output. If the secondary circuit becomes overloaded, primary current through T940 is detected at R94A, R948 and stops the switching operation. Once the circuit has overloaded, the power switch must be turned off for a short period and then turned on to re-establish power. Switching frequency is determined by time constant of R95E- R95G, C952. 1.2.2 Circuit #2 : Circuit 2 consists of chopper transformer (T910), chopper component (Q910) mainly providing +8 voltage for horizontal deflection and high voltage circuits. Regulator method is to detect secondary voltage at 19C1 and feed through photocoupler I9A2, providing constant voltage by controlling 19A1 pin 12 at primary circuit. Switching operation is performed by frequency synchronization with circuit #1, to control 19A1 pin 10 from circuit #1. As mentioned above, while detecting secondary voltage at I9C1. 19C1 simultaneously receives voltage information from microprocessor which is proportional to the horizontal frequency. Therefore, output of,•' ' circuit #2 is additionally regulated by the horizontal frequency. In this circuit, gate circuit (Q9C4, Q9C8) controls the switching operation at power control (19A1) through photocoupler (19A3). This circuit is designed to receive power off command from the microprocessor, resulting in stopping chopper operation at the power supply. 1.3 Degaussing circuit When the power is switched on, the CRT is degaussed automatically by current flowing through the degauss circuit while the relay (591R) is closed. Degaussing current flows for approximately 20 seconds and stops after secondary circuit operation is stabilized causing relay (591R) to be opened. The circuit also allows for manual degauss by user switch at front panel which closes S91R through Q999, to allow current to flow through the degauss coil. Figure 1 : Block diagram for power supply circuit Main SW In rush limit AC inlet - Circuit #1 Chopper Control - +80V +27V + 12V + 6V -12V Outputs Degauss Circuit 1n Manual Degauss Chopper +B output Detect/ Amp Control Circuit #2 On/Off control Standby power supply Control Chopper Outputs +5V