Dynamic Focus/Astigmatism Circuitry; Parabolic Correction Voltage; Differential Input Voltage - HP 1317A Operating And Service Manual

Theory
Model 1317A
_____ VOLTAGE AT BASE OF A6Q7
w
<.!J
«
I-
~
o
>
." II
VOLTAGE AT WHICH
~
CR3 OR CR4 BECOME
7:;'
FORWARD BIASED
~--
VOLTAGE ATWHICH
CR7 OR CR8 BECOME
FORWARD BIASED
RIGHT EDGE
CRT
DISTANCE _ _ _ _ _ _ _ _ _ _ _ _ _
.~
1317A-103-07-76
Figure 4-1. Parabolic Correction Voltage
4-21. When a slow-moving or static beam is detected
and the phosphor protect control is activated, the
positive output from the voltage sensors is applied to
the input of the phosphor protection circuit on the
Z-axis assembly (schematic 7). A positive voltage at
this point causes A5VR4 to clamp the Z-axis output
at approximately 14 V.
4-22. During periods when the beam is being deflect-
ed at a safe rate, the X-axis phosphor protect control
grounds input to the phosphor protection circuit, the
voltage clamp is removed, and the Z-axis output
operates over its full range.
4-23. Some applications make it necessary to obtain
a bright display of a slow-moving or static beam.
Setting the phosphor protection switch to the OFF
position disables the phosphor protection circuit by
grounding all inputs to the circuit.
4-24. DYNAMIC FOCUS/ASTIGMATISM CIRCUITRY.
The circuits that change focus and astigmatism volt-
ages with reference to beam position are quite similar.
Only dynamic focus circuits are described here, and
reference to dynamic astigmatism is made only where
circuit variation is involved.
4-25. Circuit action of dynamic focus is identical for
the X axis and Y axis so only the X axis is discussed
here. The differential voltage proportional to beam
position is taken from the input of the X-axis ampli-
fier emitter follower stage (schematic 2) and applied
to the X-input amplifier stage on the dynamic focus
assembly (schematic 8). In the dynamic focus as-
sembly, the output of the X-input amplifier is applied
to a diode shaping network that generates a parabolic
voltage as a function of beam position (figure 4-1).
4-26. With A6FXl and A6FX2 adjustments in full
ccw position (schematic 8), the diode shaping network
is reverse biased and the X-input amplifier collector
voltages will vary linearly as the differential voltages
change on their bases (figure 4-2). Adjusting A6FXl
and A6FX2 as described in Section V establishes a
reference voltage level on X-input amplifiers at which
linearity of the voltage ramp is altered.
4-2/ ( 4-3 blank)
4-27. The output voltage of the diode shaping net-
work is applied to a current amplifier and converted to
a current signal. These current signals from X- and
Y-input amplifiers are summed and applied to a com-
plementary output amplifier. The complementary-
output amplifier output is a correction voltage applied
to the floating focus supply in the high voltage power
supply (schematic 11). These circuit effects serve to
adjust beam focus automatically in relationship to
beam position on the CRT face.
Glli§B
If a pattern consisting of only a few dots is
present on the CRT, the phosphor pro-
tection circuit will detect deflection and
disable the phosphor protection circuit.
If the beam current is high and the duty
cycle is near 100%, the average current
at each dot may be sufficient to damage
the CRT. Therefore, caution must be used
in setting the INTENSITY control.
4-28. Dynamics focus and dynamic astigmatism vary
only in that the dynamic focus circuit is designed to
provide a relatively low output impedance to suppress
noise feedback from the normal focus circuit.
_ _ _ _ BASE VOLTAGE AT A6Q7
WITH FX1, FX2 CCW
1317A-104-07-76
Figure 4-2. Differential Input Voltage