Circuit Description—Type 324
The Q329 emitter voltage is applied to the upper plate
of the Timing capacitor, C330. The upper plate of C330
goes positive at almost the same rate as the negative charge
accumulates on the lower plate, keeping the lower plate at a
relatively constant voltage with respect to ground. This
results in an extremely small change across R330 during
sweep time, keeping the R330 current constant. The con
stant current charges C330 at a constant rate, creating an
extremely linear sweep voltage.
During generation of calibrated sweeps, the selected
timing resistor (R330) is connected directly to —5 V. When
the V A R IA B L E control (R334A ) is moved from the CAL
position, the voltage applied to R330 is decreased. The
resulting increased time required to charge C330 causes a
decrease in sweep rate (larger time/div value) from that
indicated by the T IM E /D IV switch.
Sweep Retrace. The positive-going Q329 emitter voltage
causes an increasing amount of current to flow through
R346 and SWEEP LENGTH potentiometer R347. This
causes the current through D303 to decrease, because R303
current is relatively constant. D303 switches back to its low
state as soon as its current drops below the amount re
quired to hold it in the high state. See Fig. 3-3(C). The
setting of R347 determines the output voltage (and there
fore the sweep length) required to cause D303 to switch to
its low state. D305 then goes into conduction, turning
Q305 off. The positive signal at the Q305 collector enables
D309 and D350. The positive potential coupled through
D309 to the gate of Q311 causes the output at the Q329
emitter to drop until the voltage at the emitter of Q343 is
low enough to permit Q343 to go back into saturation. See
Fig. 3-3(D ), (E), (F), (G). The output voltage feedback
through the base-emitter junction of Q343 and through
D309 causes the output voltage to stabilize at its quiescent
value.
Holdoff Time. When Q305 cuts off and retrace is
completed, the current from R343 again passes through
Q343, saturating it. D342 and D343 stop conducting and
the C342-R342-C340 junction discharges sufficiently for
D301 to conduct and hold D303 in its low voltage state.
The RC time of R342, C342 and C340 determines the time
required before D301 can again conduct triggers. This delay
is referred to as holdoff time. It allows the circuit to
stabilize between sweeps, thereby minimizing sweep hori
zontal jitter. See Fig. 3-3(G ), (H) and (B).
Oscillation of the Sweep Generator circuit is prevented
by the addition of the C313, C316, R316 and C321.
Unblanking. The C RT is blanked during quiescence by
the fo llo w in g conditions: Current from
R355 flows
through the Q373 base-emitter junction, saturating Q373.
The voltage at the bottom of R355 sets the emitter of
Q356 at about +0.6 V. The cathode of D353 is held at
about 0 V by the potential on the base of Q343. The 0.6 V
across D353 and Q356 base-emitter junction is not suffi
cient to cause forward conduction, and Q356 remains cut
off. The voltage at the emitter of Q363 is also set by the
Q373 emitter-base junction and is not sufficiently negative
to cause Q363 to conduct. This causes both the base and
the emitter of Q 370 to be at +100 V , so that Q 370 is also
cut off. With Q 370 cut off and Q373 saturated, the voltage
at the Q 370 collector is very near 0 V. This is connected to
one of two opposing unblanking plates, the other of which
is at +100 V whenever the oscilloscope is on. The electro
static effect of the plates during the unbalanced condition
prevents the beam from striking the face of the cathode-ray
tube.
When a trigger signal causes Q301 to conduct, the nega
tive gate which is coupled through D350 causes D 353 and
Q356 to go into conduction. The emitter voltage of Q356
decreases, turning Q373 off and D363 on. Current through
R363 lowers the Q363 collector voltage by about 0.6 V,
saturating Q 370. This effectively connects the cathode-ray
tube unblanking plate to +100 V , permitting the beam to
strike the face of the CRT.
Application of a positive signal of 5 V or more at the
E X T BLANK connector J350 will turn off Q 356, again
causing blanking to occur. Protection from large external
blanking signals is provided by R351 and D 351. Signals at
the E X T BLANK connector in excess of approximately +8
V will cause D351 to go into conduction, limiting the signal
at D353 to +5.6 V .
When the end of the sweep initiates retrace, Q 305 turns
off and D 350 goes into conduction. This turns D 353 and
Q356 off and the circuit returns to its quiescent condition.
C361, C362 and C364 improve circuit response sufficiently
to cause the electron beam to be blanked before an appre
ciable amount of retrace occurs.
Residual voltage exists in the high voltage power supply
for a brief period after the oscilloscope is turned off. As
long as some voltage remains in the +100 V power supply,
it appears on the non-driven unblanking deflection plate.
Residual voltage in the +11 V supply is applied to the base
of Q373 through contacts of the POWER switch and R369.
This keeps Q373 in conduction, grounding the driven
unblanking deflection plate, thereby keeping the CRT
blanked while the high voltage power supply discharges and
the CRT filament cools.
E X T H O R IZ Operation.
When E X T H O R IZ operation
is selected by the T IM E /D IV switch, R340A is connected in
parallel with R342. R330 and C330 (the timing compo
nents) become disconnected from the circuit. The parallel
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