Kortek KT-1903NA Service Manual page 16

KORTEK Corporation
KT-XX03NA
CRT cathode is composed of three electrodes of R, G and B and GRID 1 electrode is also
composed of three electrodes. Three electrodes maintain different separations from each
other and the voltage proportional to each separation should be supplied to obtain the
constant color. Because operational principle of circuit is same for R, G and B, RED circuit
will be described instead of describing all circuits. When heater voltage is supplied to CRT
and surface temperature of the nearest CRT cathode exceeds 900℃, CRT cathode emits
heat electrons. At this time, despite of that there are a few differences due to the conditions
of voltage supplied to GRID 1, GRID 2 and GRID 3 and 5, heat electrons generally move to
CRT face and current is generates to the opposite direction. The current appears on the
surface of CRT RED cathode. SUB PCB Pin 4 is connected to RED cathode and current and
voltage passing through R962 change in Q953 emitter. This changed current appears in the
form of wave signal on Q953 collector, and sent to IC950 to be amplified. IC950 amplifies the
signal at rate of R956, R957 and R955 and sends it to IC950 Pin 3 for the second
amplification. Second amplification of IC950 is made at rate of R952 and R964 and pulse is
converted to DC level by ZD950 and C952. The converted DC voltage is sent to SUB PCB
Pin 6 by controlling Q951, Q952 and Q950 and controls Q953 base. When the voltage of
Q953 base changes, emitter voltage changes and controls emission of heat electrons from
CRT cathode. This operation is made through feedback loop and maintains a constant value
with repeat.
6-6. PULSE GENERATOR CIRCUIT
Pulse generator circuit supplies control power to auto bias circuit. The circuit makes and
supplies pulse in order for auto bias circuit to operate only during blanking period for
preventing error. For detailed timing chart, see waveform of Circuit Diagram.
The circuit is supplied horizontal & vertical blank pulse through R917, and controls the
delay and the width of the waveform for easy operation of auto bias at the pulse oscillator
circuit, which is composed of IC902, C920, R921, R923,
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