Raytheon DIDS-400 Series Technical Manual page 204

DIDS-402-2AMI3
4-29 HORIZONTAL AND VERTICAL DEFLECTION AMPLIFIER A2
The horizontal and vertical deflection amplifier circuitry (Raytheon
Drawing No. 389300) develops skew-corrected deflection voltages for deyeloping
the CRT raster. These voltage s are applied to a deflection yoke, LI, on the CRT
neck to electromagnetically deflect the CRT scanning beam in a horizontal and
vertical direction.
4-29. I Horizontal Deflection Amplifier
The horizontal deflection amplifier develops a horizontal deflection voltage
from H Drive digital levels coupled from the character counter on Timing and
Discrete Board Al4. The H Drive signal, which is positive for 80 character'
times (active scan) and negative for eight character times (retrace), is converted
to a sawtooth voltage by a ramp generator formed by transistors QlO and Qll.
The sawtooth output of Qll is applied to an operational amplifier circuit
formed by transistor Ql2. The amplified output of Ql2 is then coupled to a
push-pull amplifier formed by transistors Q13 and Q14. Amplifier Q13 and Q14
is arranged in a complementary symmetrical circuit configuration to produce
positive going and negative going outputs •. These outputs are connected to
current driver transistors located external to the circuit for additional amplifica-
tion. The amplified sawtooth current flowing through the driver transistors is
applied to the horizontal deflection coil and causes the CRT beam to scan across
the face of the CRT.
The negative going signal at the emitter of Ql4 is also fed back to the input
of amplifier Ql7 via the feedback network that includes R47, RSO, and R37.
Adjustment of the value of gain potentiometer R50 in this network controls the
amplitude of the voltage fed back and, therefore, the gain of the overall opera-
tional amplifier circuit. Adjustment of the value of horizontal position potentio-
meter R3S, which connects to the operational amplifier circuit input via resistor
R38, establishes the dc level of the amplifier input. Varying this dc level varies
the positioning of the horizontal sweep on the CRT face.
Connections made from the horizontal deflection coils to resistor R47
provide for a sampling of the current flow through the coils. The summing of this
current with the normal feedback current flowing through the resistor provides a
compensating effect that improves linearity and stability of the operational ampli-
fier circuit. A portion of the feedback signal that is compensated in this fashion
is taken at the intersection of gain potentiometer R50 and resistor R37 for
applica tion to the vertical de fle ction amplifier as the skew output voltage. The
use of the skew voltage is described later on in the text.
The voltage developed across resistor R49 by the current sample taken from
the deflection coils is applied to a CRT protection circuit. This circuit consists of
diodes CR8 through CRIO, resistor RS2 and capacitor CIS. In the presence of a
horizontal deflection voltage, a negative potential exists across resistor R49.
Capacitor CIS is discharged to this potential via diodes CR8, CRlO, and CRS2.
Diode CR 11 blocks the negative potential from the video amplifier circuits at the
output.
If the deflection voltage fails, the voltage across R52 goes positive and
backbiases diodes CR8, CRIO, and CRS2. Capacitor CIS then charges toward
+22 volts through R52. Diode CRII now conducts, producing a signal which indi-
cates that the deflection voltage is absent.
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