Service manual

CB-18
CB CIRCUIT DESCRIPTIONS
Before this processed video is pre-driven by transistors Q2205, 2206, and 2207, the video signal can be
dimmed by the half-tone circuit. This circuit draws a limited amount of current when Q2209 is switched on
by the half-tone pin 40 of IC6000. The half-tone circuit generates a rectangular area surrounding OSD in
which video is present but shaded.
1 2 3 4 5 6 7
+12VSW
+9VSW
R2233
R2236
D2214
C2251
Q2205
Q2206
Q2207
R2237M
R2234M
R2240M
R2257M
R2258M
R2259M
D2206 D2207 D2208
R2260
Q2209
C2243M
PROV
Halftone
VID B+
Q2201
PROV
D2204
PROV
R2241M
PROV
ZD2201
R2231M
Q2202
J2201M
PROV
R2245M
R2247M
R2244M
R2245M
R2243M
1: +12VSW
2: Setup
3: AKB-Sense
4: GND
5: R-Out
6: G-Out
7: B-Out
2C5 Key
IKIN
Pin 21
B-Out
Pin 24
G-Out
Pin 23
R-Out
Pin 22
2C5
<<Figure CB-16>>
Video Amplification
After the RGB signal is pre-driven, it is sent to the CRT socket via connector 2C5. This socket board
delivers the filament, focus and G2, and RGB video signals to the picture tube. Besides the AKB circuit, the
CRT socket has an afterglow suppression circuit.
Each color is amplified in its own circuit section on the CRT board. The red amplifier section consists of
those parts designated with 5120 through 5130. Green uses part designations 5140 through 5150 and blue
uses 5160 through 5170. The video signal is cascode-amplified by a dual transistor arrangement for each
color. +235 volts from pin 1 of the 5F2 connector supplies power to the cascode amplifiers.
For example, Q5141 and 5142 amplify green signal. Frequency response is the major concern with regard
to Q5142. By itself, Q5142 would tend to lose gain at higher frequencies. To remedy this problem,
C5141M is added to the series-parallel network on Q5142s emitter. This capacitor along with R5144M
provide an improved response to higher frequency signals, while the resistor, R5143M, improves lower
frequency response in the cascode amplifier. The resistor network formed here by R5145M and R5143M
affect the black level of the green. Q5141 also helps bandwidth of the cascode amplifier. It reduces
feedback caused by parasitic capacitance in Q5142. Q5141 has a very low feedback capacitance, less
than 3pF, and a nearly 300V collector to emitter breakdown voltage. Its cut-off frequency is about 70
MHz. Its collector passes green signal on to the push-pull coupling amplifier formed by Q5143 and Q5144
and finally to the cathode through CRT1, the actual socket on the picture tube. This example applies to all
three color signals, with their respective components substituted.