Tektronix 7623, Инструкция по эксплуатации

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Circuit Description — 7623/R7623 Service
signais. Transistors Q1148, Q1152, Q1154, and Q1156
form a current driven operational amplifier. The input and
output transistors are complementary to provide a fast
rise-time and a fast fall-time response. The amplifier input is
through resistor R1108. Resistor R1152 establishes a low
current in the series connected output transistors. Tran­
sistor Q1148 supplies additional current through C1151 for
the positive transients, and transistor Q1156 supplies
additional current for negative transients. Capacitor C1158
is adjusted for optimum square-wave output, resistors
R1158 and R1159 along with capacitor C1158 form the
feedback network. Zener diode VR1142 provides the
necessary change of voltage from the collector of Q1107 to
the base
of Q1 156.
Auto Focus Amplifier
The voltage developed across R1108 by the Z-Axis
amplifier driving current is inverted and amplified non-
linearly by Q1110 and Q1118, to conform to the require­
ments of the CRT focus electrode. As the base of Q1110 is
driven negative CR1115 is forward biased, producing a knee
in the amplifier response. The Level where the knee occurs
is determined by the adjustment R1121. The operation of
the remaining amplifier is identical to the Z-Axis amplifier.
High-Voltage Oscillator
Power for operation of the high-voltage supply is
provided from the + 15-Volt Supply. At the time of turn on,
CR1215 is reversed biased holding the collector of Q1214
positive. This allows the starting base bias current for the
High-Voltage Oscillator to be supplied from the +5-Volt
Supply through R1214, Q1214, and the base feedback
windings of T1225 while the emitter potential of
Q1216-Q1218 is established by the negative side of the
+15-Volt Supply. As the output of the high-voltage supply
increases to its required output level, the collector of
Q1214 goes negative until CR1215 is forward biased. Then
the collector level of Q1214 is clamped about 0.6 volt more
negative than the negative side of the + 15-Volt Supply. This
configuration provides a controlled starting current for the
High-Voltage Oscillator at turn-on, and at the same time
allows the High-Voltage Regulator stage to control the
current
for the High-Voltage Oscillator after the stage
reaches operating potentials to provide a regulated high-
voltage output.
Q1216-Q1218 and the associated circuitry comprise an
oscillator to drive high-voltage transformer T1225. When
the instrument is turned on, assume that Q1216 comes into
conduction first. The collector current of Q1216 produces
a corresponding current increase in the base-feedback
winding of T1225 to further increase the conducivity of
Q1216. At the same time, the voltage developed across the
base-feedback winding connected to Q1216 holds Q1218
reverse biased.
As long as the collector current of Q1216 continues to
increase, voltage is induced into the base-feedback windings
of T1225 which holds Q1216 forward biased and Q1218
reverse biased. However, when the collector current of
Q1216 stabilizes, the magnetic field built up in T1225
begins to collapse. This induces an opposite current into the
base windings which reverse biases Q1216, but forward
biases Q1218. When the induced voltage at the base of
Q1218 exceeds the bias set by the High-Voltage Regulator,
Q1218 conducts and the amplified current at its collector
adds to the current flowing through T1225 due to the
collapsing field. Then, as the current through T1225
stabilizes again, the magnetic field around it once more
begins to collapse. This reverses the conditions to start
another cycle.
The signal produced across the primary of T1225 is a
sine wave at a frequency of 35 to 45 kilohertz. The
amplitude of the oscillations in the primary of T1225 is
controlled by the High-Voltage Regulator to set the total
accelerating potential for the CRT. Filter network
C1222-L1222 decouples high peak operating current from
the +15-Volt Supply.
High-Voltage Regulator
A sample of the secondary voltage from T1225 is
connected to the High-Voltage Regulator stage through
divider R1245A-R1245B. Q1201 and Q1206 are connected
as an error amplifier to sense any change in the voltage level
at the base of Q1201. The ground connected to the emitter
of Q1201 through R1202, provides the reference level for
this stage. The output voltage is set by the fixed values of
the components in this circuit.
Regulation occurs as follows: If the output voltage at
the -1475 V test point starts to go positive (less negative),
a sample of this positive-going change is connected to the
base of Q1201 through R1245B. Both Q1201 and Q1206
are forward biased by this positive change, which in turn
increases the conduction of Q1214. This results in a greater
bias current delivered to the bases of Q1216-Q1218
through Q1214. Now, the bases of both Q1216 and Q1218
are biased closer to their conduction level so the feedback
voltage induced into their base-feedback windings produces
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