Tektronix 422, Instruction Manual

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Circuit Description—Type 422 AC-DC
When a trigger signal is present, a negative-going pulse is
connected to the base o f Q345 through T377 and R341 as
Tunnel Diode D375 switches to its high-voltage state. This
negative-going pulse momentarily interrupts the conduction
o f Q345 and its emitter goes negative. The emitter o f Q347
goes negative also. A t the same time, the positive-going
change at the collector o f Q345 is connected to the base o f
Q347 through RC network C343-R344. Q347 is forward
biased and its emitter level rises, forward biasing D346 to
provide more emitter current fo r Q347. The collector o f
Q347 drops negative to charge capacitor C400 (through
R400) to the level at the collector o f Q347. The voltage at
the base o f Q400 as determined by divider R402-R407 be­
tween the collector level o f Q347 and —12 volts is negative
enough to reverse bias Q400, since its emitter is clamped at
about —0.3 volt by D407. The 0.6-volt forward drop across
D408 sets an output level to the Sweep Generator circuit of
about +0.3 volt. This level allows a triggered sweep to be
produced. As C343 charges, the base current of Q347 is
reduced and its emitter voltage level goes negative. Diode
D346 is reverse biased and Q345 turns on as the emitter
level o f Q347 drops to about zero volts. As Q345 turns on,
its collector goes negative and this negative-going change is
connected to the base o f Q347 through C343. Q347 is
turned o ff completely and its collector goes positive to
allow C400 to charge. Diode D344 clamps the base o f Q347
at about —0.6 volt to prevent base-emitter breakdown as
C343 discharges. When C343 is discharged, D344 is reverse
biased and the base of Q347 returns to about zero volts.
If the A uto M ultivibrator does not receive another trig ­
ger pulse, C400 charges to a level which allows Q400 to
return to conduction in about 50 to 100 milliseconds.
Then, Q400 is biased on again to produce a positive auto
gate level fo r a free-running sweep. This also occurs if the
trigger signal repetition rate is less than the recharge time o f
C400. However, if a repetitive trigger signal is received be­
fore C400 is recharged to the level that allows Q400 to turn
on, Q347 turns on and discharges C400 completely so it
starts its recharge cycle again. Since Q400 remains reverse
biased, the auto gate level remains negative to allow a trig ­
gered sweep to be produced as long as a trigger signal is
available to the Sweep Generator circuit. More information
on AUTO Mode triggering is given under A uto Trigger Mode
Operation in the Sweep Generator discussion.
When TRIGGERING Mode switch SW355 is in the
NORM position, the collector o f Q347 is held at ground
potential regardless o f the trigger signal or the condition of
the A uto M ultivibrator stage. This places a bias level of
about —3 volts on the base o f Q400. Since its emitter is
clamped at about —0.3 volt by D407, Q400 is reverse
biased. The 0.6 volt drop across D408 when it is forward
biased after sweep holdoff time sets an output level to the
Sweep Generator circuit o f about +0.3 volt.
SWEEP G ENER ATO R
General
The Sweep Generator circuit produces a sawtooth volt­
age which is amplified by the Horizontal A m plifier circuit
to provide horizontal sweep deflection on the CRT. This
o utput signal is generated on command (trigger pulse) from
the Sweep Trigger circuit. The Sweep Generator circuit also
produces an unblanking gate to unblank the CRT during
sweep time. In addition, this circuit produces an alternate
sync pulse fo r the vertical deflection system and a gate out
signal at the fro n t panel. Fig. 3-8 shows a detailed block
diagram o f the Sweep Generator circuit. A schematic o f this
circuit is shown on diagram 5 at the back o f the manual.
The TRIGGERING Mode switch allows tw o modes of
operation. In the NORM position (LEVEL control pulled
out), a sweep is produced only when a trigger pulse is
received from the Sweep Trigger circuit. Operation in the
AUTO position (LEVEL control pushed in) is much the
same as fo r NORM except that a free-running trace is dis­
played when a trigger signal is not present, when the trigger
repetition rate is too low or when the amplitude o f the
trigger signal is not adequate. The following circuit descrip­
tion is given fo r operation in the NORM mode. Differences
in operation fo r the AUTO mode are discussed later.
Normal Trigger Mode Operation
Sweep Gate. The positive-going trigger pulse generated
by the Sweep Trigger circuit is applied to the Sweep Gate
stage. Tunnel diode D405 is quiescently biased in its low-
voltage state. When the positive-going trigger pulse is
applied to its anode, the current through D405 increases
and it rapidly switches to its high-voltage state, where it
remains until reset at the end o f the sweep. The positive­
going change at the anode o f D405 is connected to the base
o f Q414 through C405-R405 and Q414 goes into saturation
(collector drops to ground). Collector current fo r Q414 is
supplied from R415 (through R414), which also supplies
emitter current fo r Q424 and Q434. Capacitor C418
couples the fast changes at the collector o f Q414 directly to
the collector circuit o f Q424 to insure that the sweep starts
quickly when triggered. The negative-going voltage at the
collector o f Q414 is also connected to the Unblanking Cir­
cuit.
Unbfanking Circuit. The negative-going gate pulse from
the Sweep Gate stage is connected to the base o f Q473
through divider R471-R472. Capacitor C471 provides fre­
quency compensation fo r the divider so the signal applied
to the base o f Q473 maintains a fast rise. The collector level
o f Q473 is set at about —6.2 volts by zener diode D474.
The negative-going gate pulse forward biases Q473 and its
em itter goes negative. This signal is connected to the CRT
Circuit through C477-R477 as an unblanking gate signal to
unblank the CRT during sweep time so a sweep can be
displayed. It is also connected to the GATE OUT connector
on the fro n t panel through D479, R479 and R478 to pro­
vide a gate signal coincident w ith the sweep. External sig­
nals can be connected to the front-panel EXT BLAN KIN G
connector to blank portions of the displayed waveform.
Diode D476 allows only the positive signals to pass to the
CRT Circuit. Diode D479 blocks the EXT BLAN KING sig­
nal from the GATE OUT connector.
3-13