Tektronix 1S1 Instruction Manual page 50

transistors turned off during the retrace. Near the end
of the retrace period, as the holdoff waveform (at the junc-
tion of C210 and R210) drops, the voltage at the base of
Q225 returns toward the triggerable level of Q225, slightly
above ground. With the DISPLAY MODE switch set to NOR-
MAL, negative pulses are continuously being applied from the
comparator circuit through C205 and D205. The first nega-
tive pulse that is able to forward bias the base-emitter junc-
tion of Q225 turns on the transistor, switching the multivi-
brator to initiate a staircase sweep. If no trigger arrives,
the voltage at the base of Q225 is held at the triggerable
level by current through D237 and R208.
As Q225 turns on, its collector goes positive, turning on
Q235. The collector of Q235 in turn goes negative and the
regenerative action of the circuit causes both transistors to
saturate. The negative voltage at the collector of Q235
reverse biases D246 and D247, disconnecting the Miller cir-
cuit from its discharge path through the diodes, and the
staircase capacitors are allowed to begin charging.
One of the two return paths from the staircase output to
the sweep-gating multivibrator is through R290 (SWEEP
LENGTH), D216 and D215. As the staircase voltage rises,
more current is drawn through R215 and D216, diverting
current from the base of Q225. SWEEP LENGTH adjust-
ment R290 is set so that when the staircase voltage reaches
approximately 10.2 volts, Q225 will turn off and reset the
sweep-gating multivibrator. As the multivibrator resets, the
positive-going voltage step at the collector of Q235 is
Fig. 3-8. Block diagram of the Staircase circuit.
Circuit Description – Type 1S1
applied to the retrace blanking amplifier and to the dis-
connect diodes, D246 and D247.
Diode D246 is forward biased by the positive voltage
applied to its anode, permitting the staircase capacitor to
begin to discharge. The resulting rise on the grid of V263
operates through the Miller circuit, causing the staircase
output voltage to drop quickly to near ground voltage.
When the negative-going output forward biases D247, the
diode conducts and a state of clamped equilibrium exists.
Diode D246 remains forward biased, diverting current from
the grid circuit of V263 until the sweep-gating multivibrator
is triggered again.
A second return path from the staircase output to the
sweep-gating multivibrator is through R212, D210 and R210.
As the staircase capacitor begins to discharge and the stair-
case output voltage begins to drop, D210 becomes reverse
biased, disconnecting the output from the sweep-gating
multivibrator. Sweep holdoff capacitor C210, which had
charged up to the staircase voltage, discharges through
R210 and D237, setting the holdoff time of the staircase.
As the voltage at the base of Q225 again returns to the
triggerable level, Q225 becomes armed and ready to be
triggered by the next negative-going pulse from the com-
parator to initiate a new sweep.
With the DISPLAY MODE switch set to SINGLE SWEEP,
sweep-unlock pulses from the comparator are no longer
connected to the sweep-gating multivibrator. Capacitor
C205 is charged to +19 volts through R203 when the
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