Spot Burn Protection; Brightness; Focus; Dynamic Focus - Zenith ZVM-1240 Service Manual

IC301 also contains a flyback generator which retraces
the vertical scan current quickly without excessive
power penalty. It also provides vertical retrace blanking
from pin 3.
Spot Burn Protection
When the monitor is turned off, the filament of the CRT
is still hot and capable of emitting electrons. With a
high potential still on the face of the CRT, it is possible
that a beam of electrons could be attracted to one
particular area of the screen. If this occurs, the phos-
phor on the screen may be burned, leaving a light spot
which is visible even after the filament has cooled and
no current flow exists.
To prevent spot burn, capacitor CX124 charges to ap-
proximately 60 volts while the monitor is in use. When
power is switched off, CX124 does not have a dis-
charge path because the collector of Q202 represents
a high impedance. The cathode of the CRT, pin 2,
is therefore held positive, attracting the electrons from
the hot filament and preventing them from striking the
CRT. Diode CR108 is reverse biased, preventing
CX124 from discharging through it.
Circuit Description
Brightness
Adjustment of master brightness R142 changes the
amount of voltage across the external brightness con-
trol, R143. R143 is used to vary the bias voltage on
the grid of the CRT, pins 1 and 5. This voltage controls
the acceleration of the electron beam and therefore
controls the intensity of illumination.
Focus
Potentiometer R141 is used to adjust the bias voltage
on the final grid of the CRT, pin 7. Changing this volt-
age will change the focus (clarity) of the raster dis-
played on the CRT. The
+
500-volt power supply pro-
vides voltage to pin 6 of the CRT and to focus control
R141.
Dynamic Focus
A parabolic voltage is taken from C113 (yoke "S"-
shaping capacitor), amplified by Q103, and combined
with the DC focus voltage at the arm of focus control
R141. This parabolic voltage compensates for the flat
CRT by raising the focus voltage at the sides of the
CRT, thus keeping the beam in focus as it sweeps
across the entire face of the CRT.
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