Video Preamplifier - Tatung OMNISCAN SERIES Service Manual

MICRO BOARD POWER SUPPLY CIRCUIT
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Fig-38
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3.39 VIDEO PREAMPLIFIER
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(Refer to circuit diagram Fig-39)
The video amplifier consists of preamplifier M51399P,
output stage and cutoff adjustment circuit.
PREAMPLIFIER
There are three identical video preamplifier (0002,
0062, 0122) being used to generate the video signal.
The RED channel preamplifier for example:
The input video signal (0.7V peak) is buffered by
emitter follower 0001. The output of which is AC
coupled (C012-R013) to pin 1 of the preamplifier
0002.
RED VIDEO PREAMPLIFIER CIRCUIT
The positive clamp signal
is
fed to pin 12 of 0002 to
restore the DC-level at the preamplifier output (pin 8).
Only the pedestal electric potential undergoes
sampling by the pin 12 clamp pulse.
Pin 9 is the DC-level input for the clamp circuit. The
DC-level is restored via external capacitor (Hold
capacitor) C006.
Therefore as a result of this, the pedestal electric
potential on pin 8 is held, according to the pin 9
electric potential.
The video signal which is output from pin 8 is
amplified and turned around by the attached circuit
and having been attenuated by the resistor divider
R016-R012, it is fed back to pin 11 again.
The signal voltage on pin 8 is typically 3.5V peak at
maximum channel gain. It is fed to the output stage
(0003-0006) via buffer 0005.
C008
is AC coupling capacitor, 0001-0002-0003 are
dc-bias diodes_
Pin 4 is the gain control of the preamplifier 0002.
The supply voltage on pin 14 is 12V.
Fig-39
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VIDEO
OUTPUT
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