Table of Contents
Advertisement
Screen Voltage Control
In other TV sets the picture tube's G2 screen control was either:
1. Part of the FBT assembly where a single lead was brought to the pic-
ture tube's C board; or
2. There was a special high resistance, high voltage potentiometer on
the C board that permitted adjustment of the screen voltage.
In this set the screen control utilizes three transistors and a small ¼ watt
standard potentiometer for control of the final 400 to 600 volt G2 voltage.
The configuration is difficult to understand the way it is drawn in the ser-
vice manual so it has been redrawn here for clarification.
Circuit Operation
This stage has several major parts:
Screen Control Major Components
Parts
R9055, R9085,
Reduce FBT source voltage and current for
R9067, R9084
control (series dropping resistors). Many
resistors are used to prevent HV arcing.
Q9014
Active resistor (biased with +12V at its base).
Q9008
Prebiases the emitter of Q9012 at 4.8 volts.
Q9012
Active resistor and regulator. Q9012
conduction is set by screen control RV9002
and a sample of the screen voltage.
Q9002
Turns ON mute. Remains ON for three
seconds each time the TV is turned ON.
G2 Voltage Source
The pulse from the horizontal output transistor's collector is rectified by
D4022 so it appears at CN9002/pin 1 on the C board as approximately
1000 volts DC. This voltage is applied to a voltage divider consisting of
resistors and transistors to reduce and control the voltage before it is ap-
plied to the picture tube's G2 grid.
Purpose
53
G2 Path
The current path from CN9002/pin 1 to ground is:
1. R9055, R9085, R9067, R9084
2. Q9014
3. Q9012
4. R9081
Both Q9014 and Q9012 transistors are biased on, completing the current
path to ground.
The picture tube's G2 voltage is taken at the junction of the resistors and
Q9014. Consequently, a change in the transistor's conductance will
change the G2 voltage, permitting control.
Transistor Q9012 is used for G2 voltage control and regulation. Q9012's
conduction is set by the screen control RV9002 at its base. This sets the
picture tube's screen voltage. If Q9012 conducts harder, the G2 voltage is
reduced and the picture is darker.
Q9012 is also used for regulation. Some of the G2 voltage is sampled by
R9067 and applied to the base of Q9012. An increase in G2 voltage will
cause Q9012 to conduct harder, decreasing the G2 voltage to its nominal
value.
Video Mute
At TV turn ON, the G2 voltage is lowered so the picture is blanked. This is
controlled by the main micro IC3251 and performed by Q9002. Q9002 is
also in parallel with Q9008. Therefore, when Q9002 is turned ON for three
seconds at turn ON, the G2 voltage drops to darken the screen.
G2 Control Range
The normal effects of a misadjusted G2 control are:
CW from the back = Picture reduces in brightness, but does not black out.
CCW from the back = In the 4:3 aspect ratio picture, left and right borders
appear as the screen gets brighter. Retrace lines then appear and the TV
shuts down.
Advertisement
Table of Contents
