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Description Of Circuit - Hitachi H972 Service Manual

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DESCRIPTION OF CIRCUIT

1. Power Supply Circuit
This model incorporates a wide range universal power supply utilizing a switching regulator (see block diagram in Fig.6).
1.1 AC input
AC input consists of AC inlet, EMI filter (C901C905,C907) and rectifier (D901D904). Rectifier circuits adapt to
full-wave method. Inrush current limiting circuit (R90A) 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 #2) varying load conditions of video, (Circuit #3) varying horizontal frequencies and load
conditions.
1.2.1 Circuit #1 :
Circuit #1 consists of chopper inductor (L910), chopper component (Q910), rectifier component (D910, C920) and
control IC (I920). R918 and R919, R91A detect output voltage and provide signal to I920 pin 20 which adjusts the pulse
width based on the pin 20 voltage level to provide constant voltage output.
1.2.2 Circuit #2 :
Circuit #2 consists of chopper transformer (T921), chopper component (Q920) and control IC (I920). T921 detects
output voltage and provides signal to I920 pin 4 which adjusts the pulse width based on the pin 4 voltage level to provide
constant voltage output. If the secondary circuit becomes overloaded, primary current through T921 is detected at R923,
R924 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 R92E and R92F, R92G.
1.2.3 Circuit #3 :
Circuit #3 has two outputs. +5V is used mainly to drive the microprocessor circuit and input the signal selection circuit.
+6.3V is used for the CRT heater.
When the DC voltage from circuit #1 is applied to I940, I940 starts oscillations. Once oscillations start, the switching
transistor is driven by the voltage taken from the tertiary winding of T940. The output voltage of the T940's tertiary
winding is rectified by D942 and C942.
The main power supply (Circuit #2) turns on (off) when a signal High (Low) is applied to the base of Q984 from the
microprocessor circuit.
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 (S99R) is closed. Degaussing current flows for approximately 20 seconds and stops by causing relay
(S99R) to be opened after secondary circuit operation is stabilized.
The circuit also allows for manual degauss by using "DEGAUSS" control on OSD menu which closes S99R through
Q991 to allow current to flow through the degauss coil.
 7 

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