Zenith ZVM-121 Service Manual page 17

RB35
RB49
=
Q806
VIDEO
PREAMP
RB12
CBOB
B+12.7V
RB14
CRB04
RB15
Page
5-3
RB06
r
CB05
- .........
--_~--_TORB01
~
RB20
RB13
R203
CB11
Figure 5-3
VIDEO PREAMP (Figure 5-3)
The video input is configured to accept a standard
RS170 composite video with a 75-ohm terminating
input. The video is coupled to the emitter of QB06
through CB14 and RB35. QB06 is a common-base
amplifier which amplifies the one-volt peak-to-peak
video to three volts without a phase inversion. The
output of QB06 is coupled through CBOB to the base of
the video driver (QB05). In the base circuit of QB05, a
sync tip clamp; consisting of CRB04, RB14, RB15, and
RB20; is used to clamp the video signal so there are no
black level shifts with variation in input signal.
OB05
2
RB04 RB03
220 2.2K
B+ 12.4V
SYNC AMP (Figure 5-4)
The video signal at the emitter of video driver QB05 is
sent to the video output through the contrast control.
It is also sent to the base of sync amplifier QB01. The
sync amplifier is used to stretch the sync portion of
the video signal before the signal is sent to the sync
separator, QB02. QB02 is a standard dual-time-
constant sync separator. Its output is sent to the hori-
zontal and vertical oscillators to keep the deflection
in sync.
HORIZ.
PROCESSOR
B+12.4V
NEG. SYNC.
TO C601
VERT. OSC.
CB04
I
220PF
Figure 5-4