Tektronix 422, Instruction Manual

Page: 53 / 213

Circuit Description—Type 422 AC-DC
Time/Div Switch. The timing capacitor and resistor are
selected by the T IM E /D IV switch to provide the various
sweep rates listed on the fro n t panel. Diagram 7 shows a
complete diagram o f the T IM E /D IV switch. V A R IAB LE
T IM E /D IV control R440A (see Timing Switch diagram)
provides continuously variable, uncalibrated sweep rates
between the calibrated settings by varying the charge rate
of the tim ing capacitor.
Sweep Reset Amplifier. The positive-going sawtooth sig­
nal at the collector of Q441 is connected to tunnel diode
D455 through voltage divider R451-R452. D455 is quies­
cently biased in its low-voltage state. As the sawtooth out­
put level reaches about +30 volts, the voltage level con­
nected to the anode o f D455 rises positive enough to switch
D455 to its high-voltage state. The positive voltage change
at the anode of D455 as it switches is connected to Q464
through R456 and Q464 goes into saturation (collector
drops to ground). This results in less current to tunnel
diode D405 and it reverts to its low-voltage state. The base
o f Q414 also goes negative and it turns o ff to allow Q424
to return to conduction. When Q424 turns on, its collector
rises positive and Disconnect Diode D439 is forward biased
to quickly discharge timing capacitor C440 through R442.
This produces the retrace portion o f the sawtooth signal.
Transistor Q464 turns o ff as tunnel diode D455 reverts to
its low-voltage state along w ith the negative-going retrace.
The Sawtooth Sweep Generator stage is now ready to pro­
duce another sweep as soon as tunnel diode D405 is
enabled after the hold-off period and a trigger pulse is
received. The positive-going change at the collector of Q414
is also connected to the Unblanking Circuit so the CRT is
blanked during retrace time.
Sweep Lockout Amplifier. Quiescently before the
sweep is triggered, Q424 and Q434 are both conducting and
their emitter current is supplied through R415. The collec­
to r o f Q434 is held at about +0.3 volt as determined by the
auto gate level from the Sweep Trigger circuit (see Auto
M ultivibrator discussion). The sweep gate from Q414
diverts the current from R415 at the start o f the sweep and
both Q424 and Q434 turn off. In addition, the positive­
going sawtooth output reverse biases D430 to interrupt the
base-current path fo r Q434. The collector of Q434 goes
negative toward —12 volts through R454 and this negative
level at the collector of Q434 holds D401 reverse biased
through R401 and T401 (see Sweep Trigger circuit) to pre­
vent incoming trigger pulses from reaching tunnel diode
D405. The circuit remains in this condition until the sweep
gate from Q414 goes positive at the end o f the sweep.
Then, the current from R415 is again available to Q424 and
Q434. Q424 is forward biased, but Q434 remains o ff since
D430 is held reverse biased by the positive sawtooth level at
the output o f this circuit. Therefore, most o f the current
available from R415 flows through Q424, producing the
retrace portion o f the sawtooth. As the sawtooth at the
o utput o f this circuit drops negative to its quiescent level,
D430 is forward biased as its cathode drops about 1.2 volt
more negative than the level at the em itter o f Q434. Then,
Q434 is forward biased and its collector attempts to rise
positive. However, the RC networks C432-R432 and
C434-R434 along w ith capacitor C401 delay the voltage
change at the collector o f Q434 to allow a hold o ff period
fo r all circuits to return to their original conditions before
the next sweep is produced. As these capacitors charge, the
anode level of D401 rises along with the collector o f Q434
until it again can allow the positive-going trigger pulses to
pass.
A u to Trigger Mode Operation
Operation o f the Sweep Generator circuit in the AUTO
position of the TRIGGERING Mode switch (LE V E L con­
trol pushed in) is the same as fo r NORM when a trigger
pulse is present. However when a trigger pulse is not pre­
sent, a free-running reference trace is produced in the
AUTO mode. This occurs as follows (refer to the Sweep
Trigger diagram fo r reference to the A uto M ultivibrator,
C401, R401 and D402):
The auto gate level from the A uto M ultivibrator stage is
connected to the collector o f Q434. When the instrument is
triggered, the auto gate level from the A uto M ultivibrator
stage is held at about zero volts and the sweep is produced
and reset as just described fo r NORM operation. However,
when a trigger pulse is not available, the auto gate level rises
positive to about +1.5 volts. Now, when the sweep is reset
and Q434 turns on, its collector attempts to rise positive
toward +1.5 volts as the holdoff network charges. Diode
D402 bypasses C401-D401-R401-T401 and connects the
collector level o f Q434 directly to the anode o f D405. The
more positive collector level fo r Q434 set by the auto gate
allows more current to flo w through Sweep Gate tunnel
diode D405, so that it switches to its high-voltage state and
automatically retriggers the sweep. Therefore, the Sweep
Generator circuit is automatically retriggered at the end of
each holdoff period and a free-running sweep is produced.
Since the sweep free runs at the sweep rate o f the Sweep
Generator as selected by the T IM E /D IV switch, a bright
reference trace is produced even at fast sweep rates.
External H orizontal Operation
When the T IM E /D IV switch is in the EXT HORIZ posi­
tion, the Sweep Generator circuit is disabled so it cannot
produce a sweep. In the EXT HORIZ position, the T IM E /
DIV switch connects —12 volts to tunnel diode D405
through D403 and R403. This holds D405 reverse biased so
it cannot accept the applied trigger pulses. A t the same
time, a negative unblanking level is connected to the CRT
circuit through diode D404 so the external horizontal dis­
play can be presented. In all other positions of the T IM E /
DIV switch, both D403 and D404 are held reverse biased
by the positive voltage applied to their cathodes through
R404.
H O R IZ O N T A L A M P LIFIE R
General
The Horizontal A m p lifie r circuit provides the output sig­
nal to the horizontal deflection plates o f the CRT. In all
positions o f the T IM E /D IV switch except EXT HORIZ, the
horizontal deflection signal is a sawtooth from the Sweep
Generator circuit. In the EXT HORIZ position, the hori-
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