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Rill adds series feedback which makes the terminal-to-terminal voltage gain
relatively independent of transistor variations as well as stabilizes the device
against voltage and current changes caused by ambient te^mperature variations.
The negative going signal at the collector of QlOl is DC-coupled to the cathode
of the CRT. The class B biasing of the video driver allows a larger video out
put signal to modulate the CRT's cathode and results in a maxumum available
contrast ratio.
The overall brightness at the screen of the CRT is determined by the negative
potential at the grid and is varied by the brightness control.
2.2
VERTICAL DEFLECTION
Transistor Q102 is a programmable unijunction transistor, and together with
its external circuitry, forms a relaxation oscillator operating at the vertical
rate. Resistor R115, variable resistor R116 and capacitors C105 and C106 form
an RC network providing proper timing.
When power is applied, C105 and C106 charge exponentially through R115 and
R116 until the voltage at the junction of R116 and C105 equals the anode "A"
firing voltage. At this time, one of the unijunction's diodes that is connected
between the anode and anode gate 'G" becomes forward biased allowing the
capacitors to discharge through another diode junction between the anode gate
and the cathode "K" and on through R120.
R117 and R118 control the voltage at which the diode (anode-to-anode gate)
becomes forward biased. This feature "programs" the firing of Q102 and
prevents the unijunction from controlling this parameter. Therefore, the changing
of firing points from one device to another, together with the temperature depend
ency of this parameter, is no longer a problem as it can be with conventional
unijunction transistors.
The vertical oscillator is synchronized externally to the vertical interval from
the vertical drive pulse at R113. At the time of the vertical interval, an
A-2
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