Tektronix 475 - Sweep Generators

Circuit Description—475
SWEEP GENERATORS
General
The Sweep Generators produce the sawtooth voltages
that are amplified by the Horizontal Amplifier to provide
horizontal deflection on the CRT. These sawtooth voltages
are produced on command (trigger pulses) from the Trigger
Generator circuits. The Sweep Generator circuits also
produce gate waveforms that are used by the Z Axis Logic
circuit to unblank the CRT during sweep time, and by the
Sweep Logic circuit to terminate sweep generation. There
are three Sweep Generator circuits in the instrument.
Operation of all three is very similar; therefore only the
operation of the Delaying Sweep Generator and the
differences in operation of the three generators is discussed.
Fig. 3-6 shows a detailed block diagram of the Sweep
Generators. A schematic of the Sweep Generators is shown
on Diagram 8 at the rear of this manual.
Delaying Sweep Generator
Basically the Delaying Sweep Generator is composed of
Q904, Q920, Q922# and Q914. This generator only runs in
the A INTEN, M IX, and B DLY'D positions of the HORIZ
DISPLAY switch and generates the A portion of the
display. The sweep rate is selected by the A TIM E/D IV
switch.
Main Gate Amplifier
Q902 is the Main Gate Amplifier stage. The negative
going Main Gate waveform from the Sweep Logic circuit is
applied to the base of Q902. The amplified and inverted
waveform at the collector of Q902 is applied to the
Delaying Sweep Generator (through CR903), or to both of
the Non-Delaying Sweep Generators (through Q992). This
initiates sweep generation.
Sweep Disconnect Amplifier
Q904 is the Sweep Disconnect Amplifier. The biasing on
the base of this stage determines whether the Delaying
Sweep Generator will run. In the A position of the HORIZ
DISPLAY switch, the anode of CR907 is connected to +5
volts through R908 and the HORIZ DISPLAY switch. This
biases the base of Q904 far enough positive that the
positive-going gates on the emitter cannot turn off Q904.
Q904 therefore conducts all of the timing current through
timing resistor Rt, keeping timing capacitance Ct dis
charged. When the HORIZ DISPLAY switch is in any
position other than A, the anode of CR907 is connected to
—8 volts. CR907 is now reverse-biased, which lets the base
of Q904 be biased at a level that will allow the positive
going gates on the emitter of Q904 to interrupt the current
flow through Q904. When Q904 turns off, the timing
current starts to charge timing capacitance Ct-
DELAY PIGKOFF
3-12
®