Circuit Description—475
DC Restorer Circuit
C l372, C 1371, CR1377, CR1379, and R1378 form a
DC. restorer circuit. All DC levels in this circuit are
referenced to the negative potential of the CRT cathode.
The voltage difference across R1378 approximately equals
the voltage swing present at the junction of CR1371 and
CR1373. The control grid end of R1378 is more negative
than the end connected to CR1379. The amplitude of the
voltage swings present at the junction of CR1371 and
CR1373 is determined by the voltage levels established by
the Z-Axis Amplifier and the CRT Bias adjust circuit. The
output level of the Z-Axis Amplifier sets the limit of the
negative excursion (through CR1371) and the Grid Bias
adjust circuit sets the limit of the positive excursion
(through CR1373).
CALIBRATOR
General
The Calibrator circuit produces a square-wave output
signal with accurate voltage and current amplitudes. This
output is available as a voltage or current at the CALI
BRATOR current loop on the instrument front panel. Fig.
3-10 shows a detailed block diagram of the Calibrator
circuit. A schematic of this circuit is shown on Diagram 13
at the back of this manual.
Multivibrator
Q1502 and Q1504 along with their associated circuitry
compose an astable multivibrator. The basic frequency of
the multivibrator is approximately one kilohertz and is
essentially determined by the RC combination of C1505
and R1502. The multivibrator symmetry is set by R1502
and R1504. Q1502 and Q1504 alternately conduct, pro
ducing a square-wave signal that is taken from the collector
of Q1504.
Output Amplifier
The output signal from the Multivibrator overdrives
Output Amplifier Q1512 to produce a square wave at the
output. When the base of Q1512 goes positive, Q1512 is
cut off and the collector level drops down to ground. When
the base goes negative, Q1512 is biased into saturation and
the collector of Q1512 rises positive to about +5 volts.
Amplitude adjustment R1515 adjusts the resistance be
tween the collector of Q1512 and ground to determine the
amount of current allowed to flow, which in turn deter
mines the voltage developed across R 1517.
FAN MOTOR CIRCUIT
General
The fan motor used in the 475 is a brushless DC fan
motor using Hall Effect devices. The fan motor circuitry
varies the rotational speed of the fan with variations in
operating temperature. When the ambient temperature
increases, the value of thermistor RT1696 reduces. This
biases Q1698 on harder to conduct more current through
the Hall devices. Higher currents through the Hall devices
causes the potential difference across them (for instance
between pins 6 and 8 of the fan) to increase. This potential
difference biases one of a pair of transistors on and the
other off. For instance, if pin 8 is more positive than pin 6
of the fan, Q1690A will be on and Q1690B will be off. The
higher the potential difference between pin 8 and pin 6 the
harder the on transistor will be conducting. The harder the
transistor is conducting, the faster the fan rotates.
3-20
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General
