Tektronix 3B5, Инструкция по эксплуатации

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Circuit Description— Type 3B5
overall sawtooth level and the sawtooth must run up farther
to forward bias D228. The effect is a longer sweep of
between 12 and 15 divisions for magnified operation.
When the sweep rises positive enough to forward bias
D228, it turns Q235 off and Q245 comes on. The Sweep
Reset Multivibrator remains in this condition until reset.
With Q235 shut off, its collector voltage goes negative.
This pulls the anode of the Sweep Gate tunnel diode nega­
tive also and D145 reverts to its low-voltage state. D1 45
is held reverse biased so it cannot accept incoming trigger
pulses until the Sweep Reset Multivibrator is reset. The sweep
gate output pulse to Q1 53 and Q253 ends and Q1 53 and
the Disconnect Diode are again forward biased. The Timing
Capacitor discharges rapidly through D1 55 and Q 1 53 until
the grid of the Sawtooth Sweep Generator stage is returned
to its original level. Now, the Sweep Generator circuit is
ready to produce another sweep when the Sweep Reset
Multivibrator is reset by the Holdoff Circuit and another
trigger is received.
For Seek Mode operation, a positive-going reset pulse is
coupled to the base of Q235 each time an advance pulse
is produced (see Seek Circuit discussion). This pulse resets
the Sweep Generator circuit so it starts over at the next
faster sweep rate (faster sweep rate determined by advance
pulse to Counter Circuit). A reset pulse can also be applied
to this circuit through terminal 25 of J30. This pulse resets
the Sweep Generator circuit for single-sweep operation.
Holdoff Circuit
The Holdoff Circuit allows all circuits to return to their
original condition before the next sweep is started. The
sweep output at the emitter of Q 174 is coupled to Q1 83
through a 2:1 voltage divider, R181-R182. The sawtooth
voltage at the emitter of Q1 83 is connected to the holdoff
capacitors through D1 83 to charge them to one-half the
sweep voltage level. C 1 83 is the holdoff capacitor for the
fastest sweep ranges and is connected in the circuit at all
times. Holdoff capacitors C207, C217 or C227 are connected
into the circuit as Q204, Q214 or Q224 are biased into
operation by the decade logic level from the Readout Logic
circuit. Only one of these transistors is on at a time. When
the sweep retraces, D1 83 is reverse biased since its cathode
is held at the charge level of the holdoff capacitor and its
anode rapidly goes negative with the sweep retrace voltage.
This interrupts the charging current to the holdoff capacitor
and it begins to discharge through R183 toward — 100 volts
(C207, C217 or C227 discharge path includes the associated
transistor and related components). When the charge on
the holdoff capacitor drops to about zero volts, D229 is
forward biased and the base of Q235 goes negative. Q235
comes into conduction and Q245 shuts off. The bias on the
Sweep Gate tunnel diode returns to a level that allows it
to accept the next trigger pulse.
For fast sweep rates, the network D241-R241-R242 forces
the Sweep Reset Multivibrator to reset up to about 10%
before the normal holdoff period ends. Positive trigger pulses
from the Sweep Trigger circuit are coupled to D241 through
T140. These trigger pulses are applied to the base of Q245
and attempt to turn it off and turn Q235 back on. However,
Q235 will not come back into conduction unless its base is
near the level where it would normally turn on. Then, the
positive pulse at the base of Q245 raises the common emit­
ters of Q235 and Q245 positive to bias Q235 on. The cir ­
cuit is reset and the Sweep Gate tunnel diode returns to
a bias level that allows it to switch on the next trigger pulse.
A slight current is also added to the Holdoff Circuit from
the LEVEL control, R25A, through R185 to change the charge
and discharge rate for fast sweep rates and aid in this
forced reset. The forced holdoff recovery is only effective
at the fastest sweep rates and allows sweep synchroniza ­
tion for less trigger jitter at these faster sweep rates.
For single-sweep operation, a positive voltage can be
connected to the Holdoff Circuit through terminal 7 of J30
and R184.
This
positive voltage prevents
the
holdoff capaci ­
tor from discharging in the normal manner at the end of
each sweep. Therefore, the circuit is locked out after each
sweep. The next sweep can be produced by resetting the
Reset Multivibrator with a positive pulse through terminal
25 of J30. See Single-Sweep Operation in the Operating
Instructions section for more information.
Unblanking Driver
The negative-going sweep gate pulse at the emitter of
the Sweep Gate emitter follower, Q253, is connected to the
base of the Unblanking Driver, Q264 through R257 and
C257. C257 improves the response of the circuit to the fast
rising and falling portions of the pulse. The emitter level of
Q264 is set at about — 0.4 volts by D261. Therefore, Q264
is reverse-biased when its base is pulled negative from
ground. Quiescently before the sweep is triggered, the base
of Q264 rests slightly positive from ground. As the sweep
is triggered, the sweep gate signal at the emitter of Q253
drops to about — 6 volts. Q264 is reverse biased and its
collector rises positive. The DC level change at the collector
of Q264 is coupled to Q273 through D265 and R265. Zener
diode D265 provides DC voltage matching without a cor­
responding loss in signal amplitude.
An external unblanking control voltage can be connected
to the base of Q264 through terminal 13 of the PROGRAM
connector. —12 volts connected to this terminal reverse
biases Q264 and unblanks the CRT. This method of unblank ­
ing is required for use of the external horizontal input feature.
Unblanking Output
Q273 is connected as an emitter follower. The positive ­
going level at the collector of Q264 produces a corres ­
ponding change at the emitter of Q273. When Q264 is
turned off, its collector attempts to rise to +300 volts but
is clamped at about +125 volts by D269. This level unblanks
the CRT. When the sweep gate signal ends, the emitter
of Q273 must be returned to its quiescent level of about +50
volts very rapidly to produce a fast falling edge on the
unblanking pulse. However, when the base of Q273 drops
negative, Q273 is reverse biased and it does not conduct
any of this falling-edge current. D272 is then forward biased
and allows the emitter level of Q273 to be pulled back to
its original level through C266. This provides a sharp turn
off of the trace intensity at the end of the displayed sweep.
4-82-Volt Source
Q193 provides +82 volts for operation of Q174, Q 1 83
and D342. The divider R192-R193 from +125 volts to ground
sets the base level
of
Q1 93 near
+82
volts. Since the emitter
3-18