Circuit Description—475
Q1318 off by way of the feedback voltage to the base. The
voltage waveform at the collector of Q1318 is a sine wave
at the resonant frequency of T1320. Q1318 remains off
during the negative half cycle while the field collapses in
the primary of T1320. When the field is collapsed suffi
ciently, the base of Q1318 becomes forward biased into
conduction again and the cycle begins anew. The amplitude
of sustained oscillation depends upon the average current
delivered to the base of Q1318. The frequency of oscilla
tion is approximately 50 kilohertz. Fuse F 1318 protects
the +15 volts supply in the event the High-Voltage
Oscillator stage becomes shorted. C1318 and L1318 de
couple the +15 volt supply line and prevent the current
changes present in the circuit from affecting the +15 volt
regulator.
High-Voltage Regulator
Feedback from the CRT cathode -2 4 5 0 volt supply is
applied to the base of Q1306 through R1303A. Any change
in the level at the base of Q1306 produces an error signal at
the collector of Q1306, which is amplified by Q1308 and
Q1316 and applied to the base of Q1318 through the
feedback winding of T1320. Regulation occurs as follows.
If the output voltage at the —2450 volt test point starts
to go positive (less negative), this positive-going change is
applied to the base of Q1306. Q1306 conducts harder,
which in turn causes Q1308 and Q1316 to conduct harder.
This results in greater bias current to the base of Q1318
through the feedback winding of T1320. Now, Q1318 is
biased closer to its conduction level so that it comes into
conduction sooner to produce a larger induced voltage in
the secondary of T1320. This increased voltage appears as a
more negative voltage at the —2450 volt test point to
correct the original positive-going change. By sampling the
output from the CRT cathode supply in this manner, the
total output of the High-Voltage Supply is held relatively
constant.
Q1312 and Q1310 compose an overvoltage protection
circuit. In the event the regulating action of the circuit
should cause the CRT cathode supply to approach approxi
mately —3000 volts, the voltage level at the emitter of
Q1316 will be very close to —6 volts. Normally Q1312 and
Q1310 are biased off and do not conduct. When the voltage
level at the emitter of Q1316 reaches approximately —6
volts, Q1312 is biased into conduction, which in turn biases
Q1310 on. Q1310 now starts to reduce the base drive
( in t e n s it y !
RI335
SWEEP
UNBLINKING
GATE
-----
CHOP
BLANKING
2 AXIS
AMP QI332
''OUTPUT AMP
QI344 QI35H
01352 Q\35&
QI362
CI37I CRI379
RI378
HIGH VOLTAGE
OSCILLATOR
QI3I& TI320
REGULATING
AMP
QI30 G Q13IO
Q1308 Q13I2.
Ql3Ko
CATHODE
SUPPLY
CRI320
CRI32I
R 139*7
Fig. 3-9. Detailed block diagram of the CRT Circuit.
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