Service manual

CB-16
CB CIRCUIT DESCRIPTIONS
Introduction
The CB chassis video features include two-tuner PiP, comb-filtered video, auto kine bias, half-tone video,
and direct component video input to the video processor. Certain receiver models also include 2H digital
comb filtering, Gemstars
®
Guide Plus, front A/V inputs, and scan velocity modulation.
At the heart of the video circuitry is the NTSC video processor, IC2200. This video IC is powered by the
+9VSWF at pins 33 and 44. Its primary means of communication with the microprocessor is via the serial
clock and data (I
2
C) bus at pins 34 and 35.
Video Development
The NTSC video IC handles multiple types of video input and processes the output as RGB (red, green,
and blue). Specifically, IC2200 receives luma and chroma (Y/C) information on pins 4 and 2 respectively.
This input generally serves as main screen video. However, the video processors main video input can be
switched via pin 36 to use component video (YUV) that is input to pins 37 through 39. Secondary video
(PiP) is input to the video processor as RGB through pins 26, 27, and 28. The PiP daughter board receives
vertical and horizontal sync information and thereby sends fast-blanking (FB) pulses to pin 25 of IC2200
when main video needs to be blanked for PiP. In this sense, PiP has priority over main video. The micro-
processor and, where used, the Gemstar
®
board also send OSD input as RGB to the video processor into
pins 29 through 31 with FB signals from either source entering the video IC at pin 28.
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10
TP5VBK
R6090
PROV
R6090M
+5VGEM +5VSBM
C6090
R6091M
Q6090
Q6091
R6092M
R6093M
R6094M
PDD
+5VPiP
+9VSWF
+5VGEM
W6001
W6002
W6003
W6004
C2290
PROV
L2205
PROV
R2297M
PROV
R2296M R2295M
R2294M
R2290
R2291M
R2292M
R2293M
SCL
SDA
VRamp
Y+C/CV-PiP
R-PiP
G-PiP
B-PiP
FB-PiP
FB-Micro
B-Micro
G-Micro
R-Micro
B-OSD+GEM
G-OSD+GEM
R-OSD+GEM
FB-OSD+GEM
Power-Fail
Flyback
V-Sync
IF2-Video
Reset
SCL2
SDA2
IRBLST2
IRBLST1
Jumpers Used in
Non-GemStar
Module Receivers
Only
IQB27B44W
IQB32B44W
IQB36B44W
ONLY
GemStar Connectors
& PDD Circuitry
PiPB1
PiPA1
GEM2
GEM1
<<Figure CB-14>>
Before video can be completely processed, it must also receive ABL (automatic brightness limiter) and ACL
(automatic contrast limiter) information. IC2200 receives this at pin 3. Unlike many receivers, the CB uses
a single input to control both ABL and ACL. For simplicity, pin 3 may be referred to as the ABL pin. ABL
is first taken from pin 8 of the flyback transformer, TX3204. The signal is made positive across C2206 by
the voltage divider formed by R2201, R2212, and R2213. If current level in the picture tube penetrates the