Tektronix 221 Instruction Manual page 16

Circuit Description—221 Service
The output of the differential amplifier (Q226-Q234) is
coupled through emitter follower Q236 to the Schmitt
Trigger, composed of transistors Q244 and Q246. This
circuit controls the action of the Sweep Multi. When Q246
turns on, TriggerSlopetransistorsQ216 (— Slope) orQ218
(+Slope) will conduct. This couples the output through
T312, changes CR317 to a high state, and generates a
trigger pulse for the Sweep Multi. Diode CR317 returns to
the low state when Hold-off transistor Q312 becomes
turned off
The Sweep Multi circuit composed of transistors Q322,
Q324, and Q326 drives the Sweep Start transistors Q330
and Q334, which produce an unblanking signal for the crt
and sets the sweep start voltage for the Miller Integrator
When the sweep is triggered, transistor Q322 is turned off
and Q324 turns on. Transistor Q332 then charges up
sweep hold-off capacitors C325, C326, and C327 Tran­
sistor Q334 turns off Q328 and Q330. The Miller Integrator
is composed of Q336A and B, Q338, Q340, and Q342.
Transistor Q316 is turned off and Q312 is turned on. This
resets CR317 to its low state, inhibiting any further trigger
pulses. The sweep is now set for the run-down. As the
sweep runs down, Q324 becomes turned off by the current
through CR328 and R328. When Q324 turns off, Q322
turns on. Transistor Q328 also turns on and supplies
4-4
retrace current through R333. The sweep runs back up
until Q330 turns on and sets the sweep start voltage. The
retrace current from the collector of Q330 then discharges ,
the sweep start hold-off capacitors. When transistor Q316
turns on and Q312 off, the trigger pulses are no longer
inhibited.
The Latch circuit composed of transistors Q252, Q254,
and emitter follower Q256, controls the Sweep Multi
operation in the Auto Preset position. The output of Q244
turns on the Latch transistors and discharges C256, the
free-running hold-off capacitor, when S213B is in the
open position (Auto Preset mode). In the closed position,
S213B keeps C256 from being charged through resistor
R256, permitting trigger pulses to initiate the circuit
action. Emitter follower Q256 changes CR317 to its high
state (in the absence of trigger pulses) through R258,
R316, and the secondary of T312.
In the EXT Sweep mode of operation, CR228 is
connected to — 5 volts by S350, which turns off the trigger
amplifier. Transistor Q322 is turned off by R318 and
CR318, turning on Q324 to unblank the crt. Transistor
Q328 is turned off, which turns off the retrace current,
permitting the Miller Integrator to be connected as an
inverting operational amplifier
REV. B, AUG 1974