6-6
6. High Voltage Block
6-1. High Voltage Regulator Circuit
The high voltage regulator of this unit uses a DC converter
type power supply circuit to reduce the power consumption.
The following is an outline of the operations of the high volt-
age regulator.
The detection voltage which is obtained by resistance-divid-
ing the HV voltage with the high voltage detection resist-
ance HVR inside the FBT is passed through the IC501 (1/2)
buffer and input to IC503. IC503 compares the reference
voltage of IC502 and this detection voltage (difference am-
plification) and performs PWM modulation. Q551 is PWM-
modulated and driven by the output of IC503. The voltage
supplied to the FBT drive circuit (Q109, C108, C104, and
FBT) is controlled by the ON/OFF of Q551.
Next, the current is supplied to the CRT, and if the HV volt-
age drops, the HV detection voltage also drops. As a result,
the PWM output of IC503 works to expand the ON period of
the Q551 switching FET.
The voltage switched by Q551 is passed through the combi-
nation choke L552 (LOT) and supplied to the converter cir-
cuit for driving FBT. As the PWM modulator is synchro-
nized by the HV DRV pulse, the size of the drain current of
the FET from Q555 of the FBT drive circuit depends on the
ON period of Q551. Consequently, when the ON period of
Q551 increases, the Q555 drain current increases and the
C559 potential increases.
When Q555 turns OFF, the flyback pulse generated by the
combined inductance of the LOT and FBT and the resonance
of C108 and transmitted to the secondary side of the FBT to
generate the HV voltage.
6-2. High Voltage Protector Circuit
HV is detected using the voltage of the HV.PROT winding,
the tertiary winding of FBT.
The HV.PROT is connected to the ‘ input terminal of IC531
(2/2) via D901 and R902 of the P board and the rectification
circuit composed of C541.
When HV increases due to some error, fault, etc., the
HV.PROT voltage drops. When the voltage of the ‘ input
terminal decreases below the ’ input terminal voltage, the
protector operation reference voltage, the comparator output
becomes “Low”, and turns OFF IC503via D545.
Consequently, the drive pulse of the high voltage converter
is shut down and the high voltage output circuit stopped.
6-3. High Voltage Current Protector, ABL Circuit
The high voltage current protector holds down the high volt-
age regulator when the current Ik flowing through the CRT
exceeds the setting value in errors and malfunctions.
The voltage obtained by resistance-dividing at R531 and R532
the difference between REF (IC502) and the VABL1 obtained
by voltage-converting the current flowing through the FBT
secondary winding of the P board at R906 is supplied to the
‘ terminal of the comparator, and the protector operating
point voltage is supplied to the ’ pin of the comparator after
resistance-separating VREF by R533 and R534.
The ‘ terminal voltage of the comparator is normally higher
than the ’ terminal voltage. When the CRT beam current
increases, the VABL1 voltage decreases and consequently the
‘ terminal voltage of the comparator also decreases. There-
fore when the beam current, which makes the ‘ terminal
voltage drop below the ’ terminal voltage, flows through
the CRT, the protector operates and shuts down the PWM
control IC DRIVE, and holds down the high voltage regula-
tor.
The ABL circuit serves to protect the CRT by preventing the
beam current from exceeding the reference value.
The beam current flowing through the CRT flows to R903 of
the P board. VABL2 is obtained by converting this current to
voltage. VABL2 is supplied to the ‘ terminal of IC901, and
when it drops below the reference voltage of the ’ terminal,
ABL operates and makes the luminance consistent.
6-4. DF Drive Circuit
The DFX and DFY signal from the IC301 is amplified by
Q301 to Q305 and T301 (DFX), and DFY is amplified by
Q321 and Q322 to modulated the G4 and GM voltage of the
CRT via the focus pack integrated with the FBT of the P
board.
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