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Section IV
Principles of Operation
Model 7754A/7414A
drive to Q17 momentarily, so the marker coil is energized
and the marker makes a jog on the paper. The diode at the
collector of Q17 removes the large transient that results
when the marker solenoid stops conducting.
4-53. MARKER HEAT DRIVER. The marker heat ele-
ments are powered from the -20 volt unregulated line.
Heat regulation originates at the heat potentiometer sub-
assembly of the front panel, and is applied to the base of
Q10, which drives Q12, a series regulator that is similar to
the voltage regulator circuits. Circuit feedback is applied for
stability, and the circuit is protected against a shorted out-
put by current limiter Qll (shown in Figure 6-15).
~54.
440 Hz
Oscillator.
4-55. The 440 Hz oscillator circuit (Figure 4-11) consists of
oscillator printed circuit assembly A2 and associated power
regulation ·circuits on the power supply chassis. Power for
the oscillator is regulated by Q2 and Q3 from the -20 volt
oscillator supply (refer to Figures 24 and 6-7) on the Pre-
amplifier Power Supply Assembly. The reference voltage is
obtained from the -12 volt supply. Voltage divider Rll
and R13 senses any change in the output of regulator Q3.
The change is sensed by regulator driver QI and is applied
to the base of Q3 to return the output to its former level.
4-56. Oscillations generated by push-pull oscillator QI2,
Q13 are frequency stabilized by capacitor CIS and the pri-
mary windings of transformer T4, which together form a
tank circuit in the oscillator output. Output from the
secondary of T4 is controlled in amplitude by diodes CR21
and CR22 at a level determined by breakdown diode CR23.
Each side of the T4 secondary is connected to half of push-
pull power amplifier Q4, Q5 to drive output transformer
TI. The output of T1 is distributed to the preamplifier
connectors and the auxiliary power output connector as
shown in Figure 4-2.
~57.
2400 Hz
Oscillator.
4-58. The 2400 Hz oscillator furnishes a transducer excita-
tion signal to pressure amplifiers such as the Model 8805A
or 8805B. The preamplifier further amplifies the excitation
voltage for use with a transducer. Power for the oscillator
(Figure 4-12) is provided by the -20 volt oscillator supply
through series regulator Q19. The base of QI9 is held at a
regulated -12 volts from the preamplifier regulated supply,
providing collector voltage to Q20 and Q21 through the
primary of T7. This voltage is fJltered by capacitor C21.
4-59. Oscillations generated by push-pull oscillator Q20,
Q21 are frequency stabilized at 2400
Hz by the primary of
oscillation transformer T7, which, with the capacitance of
CI8 and C19, forms a tank circuit across the oscillator
output. Thermistors RTI and RT8 provide temperature
stability. The output of T7 is a 2400
Hz sine wave balanced
with respect to ground, with diodes CR25 and CR26 con-
trolling the oscillation amplitude at a level determined by
CR24. The differential signal is distributed to all channels
and auxiliary power output connector J8.
j
1-----------;;7-----
1
E
R=
019
6800
I
I
RT7
I
F I T
I
J
I
•
R421
If-
(7)
470
I
If-
R~I
II
470
I
If-
I
C
I
I
L -
e21
R39
I
I -
.0056
6800
1
---------_-1
868-500A-CI3
2400", OSCILLATOR P.C. BOARD ASS'y
If-
I
II
f-
CR22
I
l
I
lS:-1
10
+
I
50V
'-'-----"\Mr-~
L
~
~
868-500A-CI4
440'" OSCILLATOR P.C. BOARD ASS'Y
1- - - - - - - - - - - - - - - -
~
cr(='
01
R26
1
8
I
'= -16V
1800
I
R23
1
I
1800
J
CR21
I
If-
R27
I
IK
Figure
4-11.
440 Hz Oscillator Circuit
Figure
4-12.
2400 Hz Oscillator Circuit
4-12
07754-1
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