Heathkit IO-4550 - Page 25

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HORIZONTAL
The horizontal signal is coupled from the sweep generator to
the base of Q210 in the first horizontal deflection
differential amplifier (Q210/Q211). A reference voltage
from the horizontal position control provides a coarse
(R263A) and fine (R263B) offset adjustment at the
differential amplifier (base of Q211). Amplifier gain is
approximately 10. Control R268 adjusts the gain of this
amplifier, and hence the overall gain of the horizontal
deflection circuit. This is to compensate for the individual
deflection characteristics of the CRT. Transistor Q216 is the
constant current source for this amplifier section.
Final amplification occurs in the second deflection
amplifier. It is a cascade differential amplifier with a gain of
approximately 25. The output is coupled to the horizontal
deflection plates of the CRT for beam control. Horizontal
beam deflection requires between 19 and 25 volts/cm,
depending on individual CRT characteristics. Control R278
is used to adjust the emitter resistance of Q212/0213 so
that when the X5 switch (SW204) is closed, the gain of the
second amplifier is multiplied by exactly 5.
When X-Y operation is desired, the trigger signal from the
Channel Y1 trigger amplifier is coupled to the base of Q210
through the TIME/CM switch. The TIME/CM switch also
grounds the clear input to the display control (1C211). This
forces the Q output low to insure that the diode switch
couples the Channel Y2 signal to the vertical deflection
amplifier. The low from the TIME/CM switch also presets
the blanking control to insure that the CRT remains
continuously unblanked. X-Y CAL control R258 is used to
calibrate overall horizontal deflection gain for the X-Y
mode; while capacitor C233 reduces high frequency
phase-shift.
External X operation allows you to look at two signals in
relation to a third signal, sort of a two channel X-Y. With
the TIME/CM switch in the Ext X position, a low is coupled
to 1C203A (through 1C209D) to preset the blanking control
and unblank the CRT. The chop oscillator is unlatched and
allowed to run continuously (sweep circuit disabled). The
external sweep signal is coupled to the horizontal deflection
amplifier through the external horizontal input buffer. The
buffer functions as a dual-inverted emitter follower. This
provides a high input impedance while maintaining a gain of
approximately 1 with low offset voltage. (The emitter-base
drops of Q207 and Q208 cancel.) Transistors ZD212 and
ZD213 function as reverse-biased zener diodes to limit the
signal level to the base of Q207.
POWER SUPPLIES
+170 AND +150 VOLTS
Full-wave rectifier diodes D301, D302, D303, and D304
produce the +170-volt supply used in the horizontal
deflection amplifier, the chop blanking amplifier, and the
Astigmatism control. A second filter divides the 170-volt
source down to +150 volts for the vertical deflection
amplifier.
+15 VOLTS
Diodes D305, D306, and D307, D308 comprise two
full-wave rectifiers that produce positive and negative 21
volts DC from the power transformer. These are filtered and
then coupled through pass transistors Q301 and Q302.
Transistor base voltage is set by the dual-polarity tracking
regulator 1C301. Resistors R306 and R307 set the maximum
current that the regulator will supply. This limiting, in turn,
limits the supply current for short circuit protection.
Voltage adjust control R309 calibrates the —15 volt line
and, through the regulator, the +15 volt line. Capacitors
C307 and C308 reduce load transients.
+5 VOLTS
Full-wave rectifier diodes D309, D310, D311, and D312
produce a positive and negative 8-volts DC from the power
transformer. Capacitors C309 and C311 filter the raw DC,
while transistors Q303 and Q305 serve as pass elements.
Their outputs are limited to 5 volts by zener diodes ZD313
and ZD314, which are connected to the base of each
transistor. In the positive 5-volt supply, resistor R315 sets
the base-emitter current of Q304 to shut down the supply
above approximtely 400mA. Resistor R313 and transistor
Q306 limit the negative 5-volt supply to approximately
100mA. Capacitors C313 and C312 reduce load transients.
The line sync trigger signal is coupled from the 8-volt
transformer secondary winding through resistor R206 to the
Trig switch. The pilot lamp and resistor R7 are also
connected across the 8-volt transformer secondary winding.