Theory of Operation
5-1. B Board
1. Input selector
IC201 switches the input signals of either CN201 input (VIDEO1, 2, Y/C) or CN302 input (COM/RGB,
2. Clamp circuit (1)
Pedestal level of the Y/G signal is clamped to the pedestal voltage by the Y-CL-P pulse at IC202 (3/3) (analog
switch) using the sample-and-hold circuit. In IC203 (1/2), the pedestal voltage is compared with the reference
voltage (Y/G black voltage) so that the error voltage control the bias current of the Y/G signal clamp circuit
(Q201 to Q203) until the pedestal level is kept constant always. Pedestal levels of the PB/B signal and the PR/
R signal are also controlled in the same way, but their clamp pulses are clamped by the C-CLP-P pulse.
3. Black level insertion circuit
The Y/G signal is sampled-and-held at IC202 (2/3) by the H. BLK1 pulse of the deflection system. The
Y/G BLACK is adjusted so that the pulse-insertion period has the same level as the pedestal level. The
pulse-insertion period of the PB/B signal and the PR/R signal are also controlled in the same way.
4. Matrix circuit
The Y, R-Y, and B-Y signals are converted to the R, G and B signal by the matrix circuit when the Y, PB
and PR signals are input.
IC204 is the Y level adjustment amplifier. IC304 and IC404 are the chroma signal amplifier.
The R signal is generated by adding the Y signal and the R-Y signal supplied from IC1401 (PR gain
control) at Q1403.
The G signal is generated by adding the R-Y signal and the B-Y signal to the Y signal at Q463. The R-Y
signal that is supplied through IC1401 (PR gain control) is added to the B-Y signal that is supplied
through IC1301 (PB gain control), at Q1203. Mixing ratio of these signals is determined by R1205,
R1206 and R1207.
The B signal is generated by adding the Y signal and the B-Y signal supplied from IC1301 (PB gain
control) at Q1402.
5. RGB selector switch
IC1451 selects either the RGB signal or the YPBPR signal (matrix circuit).
6. Clamp circuit (2)
The R signal is sampled-and-held at IC1453 (3/3) by the timing pulse of deflection system. IC1452
compares this voltage with the reference voltage so that the error signal control the DC bias of the R
signal amplifier (Q1451 to Q1453) until the pedestal level is kept to a constant level all the time. The G
and B signals are controlled of their pedestal level in the same way.
7. OSD insertion circuit
The R signal OSD is inserted at IC1453 (2/3) by inserting the OSD blanking at IC1453 (1/3). IC106
(OSD IC) also uses this signal line because it generates the window signal for AUTO WHITE BAL-
ANCE adjustment. The G signal OSD and the B signal OSD receives the same processing.
8. CXA139S peripheral circuit
The RGB input signal is used during normal operation. The color difference input signals are used only
during the blue-only mode.