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Model 7754A/7414A
Section IV
Principles of Operation
SECTION IV
PRINCIPLES OF OPERATION
4-1.
INTRODUCTION.
4-2. This section describes the principles of operation of
the Thermal Tip Recorder. The Recorder includes signal,
power control, and power circuits, and a chart drive mecha-
nism. The signal circuits are described for one channel only,
as all channels are identical. The power circuits and chart
drive are common to
all
channels. This section provides
functional block and schematic diagrams of the Recorder;
for complete schematics, refer to Section VI at the rear of
the manual.
4-3. As shown in the Block Diagram, Figure 4-1, the pre-
amplifier signal is routed through a jumper cable on the rear
panel to the driver amplifier, where it is amplified, limited,
and combined with position feedback from the stylus posi-
tion transducer. It is then amplified further and applied to
the galvanometer, which drives the heated signal stylus, pro-
ducing a trace on the heat-sensitive Permapaper. The Perma-
paper is pulled through the recorder by a drive roller, which
is driven through a variable speed gearbox that is electri-
cally controlled from the front panel. Four solenoids and
gear sets are used in various combinations to produce the
full range of chart speeds. In conjunction with speed selec-
tion, the selector switch varies the stylus heat so it is pro-
portional to the chart speed. Heat adjustment controls on
the front panel may be used to further vary the stylus heat
to compensate for greater stylus travel caused by increased
signal frequency. Other circuits in the recorder include
marker, timing, rectifier, regulator, and oscillator circuits. A
regulated oscillator provides a 200 kHz excitation signal for
the position transducer. The block diagram shows the
assembly location of these functional component groups as
a troubleshooting aid.
4-4.
PREAMPLIFIER POWER SUPPLY CIRCUITS.
4-5. Figure 4-2, a simplified schematic diagram, shows the
preamplifier power and signal connections at the top, and,
at the bottom, the recorder and preamplifier power supply
circuits, including oscillators. The Preamplifier Power Sup-
ply board, A2A3, in addition to containing an oscillator
fuse and power supply resistors, provides power amplifi-
cation for the 440 Hz oscillator output, and connects the
front panel circuit boards to the transfer board. Refer to
the description of the recorder power supply for a descrip-
tion of these circuits.
4-6. Signals from the individual preamplifiers are routed
into the power supply, on Transfer Board A3Al, and pass
through the rear panel, a jumper cable, and back into the
07754-1
power supply. From there they are applied to the inputs of
the four driver amplifiers, which are discussed next.
4-7.
RECORDER SIGNAL CIRCUITS.
4-8. The signal processing circuits consist of the driver
amplifiers, one for each channel, and the galvanometers. A
simplified schematic diagram of these circuits is presented
in Figure 4-3.
4-9. The signal from the preamplifier (or other source that
can be connected to the RECORDER INPUT jack) enters
the driver amplifier and is applied to a summing junction
with the gain control signal and a transistor switch that
shorts the junction to ground when the Recorder is in
standby mode. The signal is applied through R5 to voltage
amplifier Ul input with position control voltage (±10mm
on chart) and amplifier feedback being applied at the same
point. After amplification, the signal and its control com-
ponents encounter the limiter diodes, which cut the signal
off at adjustable high and low levels. The diodes are
powered by emitter follower voltage sources (not shown).
High, low, left, right, upscale and downscale are defined on
the diagram. The signal, applied through R15 to amplifier
driver U2, is then current-limited to 600 mA by transistors
Q5 and Q8 and further amplified by power amplifier Q4,
Q7 (not shown), Q6, Q9. A feedback circuit around the
power amplifier and its driver acts as an output voltage
limiter (±12.5V).
4-10. The galvanometer circuit receives the driver amplifier
output signal, and the drive coil rotates a crank attached to
the signal stylus, which marks the recording chart. The
galvanometer shaft is also attached to a capacitive position
transducer, described in Paragraph 4-14.
4-11.
Stylus Linkage.
4-12. The linkage system shown in Figure 4-4 provides a
linear motion at the stylus tip from the rotary motion of
the galvanometer. The galvanometer moves the end of the
crank arm through the arc of a circle, and the end of the
crank arm moves the center of the stylus through the same
arc. But because the inner end of the stylus frame is con-
strained by a bearing that can move only toward or away
from the chart, the stylus tip moves in a straight line.
4-13.
Galvanometer Position Feedback.
4-14. The position transducer consists of a capacitor with a
split bottom plate, one top plate that excites both bottom
4-1
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