Tektronix 7623, Instruction Manual

Page: 67 / 266

Circuit Description-7623/R7623 Service
Mode
Switch Logic
The VERT MODE switch determines the operating
mode of the Vertical Interface circuit. The levels
established by this switch are also used in various other
circuits throughout this instrument. This switch is designed
so it is self-cancelling (i.e., only one button can be pressed
at a time). Specific operation of this switch is described in
connection with the circuits that it controls.
neously, the sudden change in voltage at the emitter of
Q1874 pulls the emitter of Q1869 positive also, to reverse
bias it. The current through Q1874 produces a voltage drop
at its collector to produce the negative portion of the
square wave.
The TRIG SOURCE switch controls the operation of the
Trigger Selector circuit. This switch is also self-cancelling so
only one of the buttons can be pressed at a time. Operation
of this switch is discussed in connection with the Trigger
Selector circuit.
Calibrator
General. The Calibrator circuit provides accurate voltage
output at the front-panel Calibrator pin-jacks. Repetition
rate of the output signal is about one kilohertz.
Oscillator. Q1869 and Q1874 are connected as a
square-wave oscillator to determine the repetition rate of
the Calibrator circuit. Oscillation occurs as follows: Assume
that 01869 is conducting and Q1874 is off. The collector
current of Q1869 through R1869 produces a voltage level
which holds the base of Q1874 low. This keeps Q1874
turned off, and since there is no current through it, its
collector goes positive to produce the positive portion of
the square wave. At the same time, C1871 begins to charge
toward —15 volts through R1872. The emitter of Q1874
goes negative also as C1871 charges, until it reaches a level
about 0.6 volt more negative than the level at its base.
Then, Q1874 is forward biased and its emitter rapidly rises
positive. Since C1871 cannot change its charge instanta­
Now, conditions are reversed. Since Q1869 is reverse
biased, there is no current through it. Therefore, C1871 can
begin to discharge through R1867. The emitter level of
Q1869 follows the discharge of C1871 until it reaches
about -0.6 volt. Then, Q1869 is forward biased and its
collector drops negative to reverse bias Q1874. This
interrupts the current through Q1874, and its collector goes
positive again to complete the square wave. Once again,
C1871 begins to charge through R1872 to start the second
cycle. The signal produced at the collector of QI 874 has a
repetition rate of about one kilohertz.
The Calibrator output can be changed with the AC-DC
jumper. When this jumper is installed in the DC position, it
produces a positive DC voltage output to the front-panel
Calibrator pin-jacks.
Output Amplifier. Transistors Q1876 and Q1879 form
the output amplifier. The 4 Volts adjustment R1884, is set
to provide accurate output voltage at the 4 V Calibrator
pin-jack.
Output Voltage Divider. The collector current of Q1879
in the Output Amplifier stage is applied across the voltage
divider made up of resistors R1888 through R1894. This
divider is designed to provide a low output resistance in the
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