Heathkit IO-4550 - Page 20

Page 18
Differential emitter follower Q107-1/108-1 serves as a buffer
between the switched-gain amplifier and second amplifier. It
also provides vertical trace positioning. Y1 Position control
R138-1 controls trace position by shifting the emitter
current between the two emitter circuits. From the follower,
the signal is coupled to the trigger amplifier and second
amplifier. The trigger amplifier will be described after the
“Vertical Deflection” section.
The second amplifier is a differential amplifier with a gain of
approximately 10. Its output is direct-coupled to the diode
bridge. CAL control R164-1 adjusts the gain of this
amplifier and the overall calibration of the vertical circuit.
DIODE SWITCH
Both preamplifier circuits (channels Y1 and Y2) share the
vertical deflection amplifier. This is accomplished with two
high-speed diode switch networks (D107-1 thru D110-1 and
0107-2 thru D110-2) that are actuated by the display
control circuit. When one diode switch is turned on, the
other is turned off so that only one signal can be coupled to
the vertical deflection amplifier. Two-channel operation is
accomplished by turning each diode switch network on and
off at a rapid rate or on alternate display sweeps. Control of
the diode switch will be described in a later section.
VERTICAL DEFLECTION
From the diode switch, the input signal is direct-coupled to
the vertical deflection amplifier. This amplifier (comprising
transistors Q111, Q112, Q113, and Q114) is wired in a
(xe Fe VS
differential cascade configuration, with a gain of
approximately 20. Capacitor C126 across the emitters of
Q111 and Q112 provides high-frequency square wave
compensation. Ferrite beads FB101 and FB102 in
common-base amplifier Q113/Q114 prevent oscillations in
the amplifier. Circuit loading is supplied by resistors R174
and R176, while inductors L101 and L102 serve as peaking
coils. The output of this amplifier is coupled to the vertical
deflection plates of the CRT for beam control. Vertical
beam deflection requires between 12 and 15 volts/cm,
depending on individual CRT charactistics. The vertical CAL
control, R164-1, in the vertical input circuit adjusts overall
vertical gain to match the CRT deflection characteristics.
TRIGGER AMPLIFIER
A differential amplifier and follower comprise the trigger
amplifier circuit. Its output is used to supply a trigger signal
to the horizontal time base, trigger, and sweep circuits. In
addition, the Channel Y1 trigger signal can be switched to
the horizontal deflection circuit for X-Y operation.
A portion of the input signal is coupled from follower
Q107-1/Q108-1 to the input of the differential amplifier in
the trigger amplifier circuit. Emitter follower Q117-1
couples the trigger signal from the inverting leg (Q115-1) of
the differential amplifier (Q115-1/Q116-1) to the horizontal
time base, trigger, and sweep circuits. Transistor Q118-1 is a
temperature-compensated constant current source for this
circuit. The Zero control (R149-1) in the emitter leg adjusts
the current so that the output of the follower will be zero
with no signal to the trigger amplifier. Thus, the circuit
performs as a differential to single-ended converter.
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