Circuit Description—434
current through R742, R796, R794, R692, and R690.
When sweep triggering occurs, the positive-going excursion
at the base of U680C is present in the emitter circuit and is
applied to pin 1 of the Sawtooth Sweep Generator U700
and the base of U680D.
Gate O utput Comparator. The positive-going signal at
the base of U680D turns U680D on, and turns U680E off.
The negative-going excursion at the collector of U680D is
applied to the em itter of H oldoff Driver U760C, and is also
coupled to the Z-Axis circuit through R790, CR794, and
CR795 to unblank the CRT during sweep time. A t the same
tim e that the collector of U680D is going negative, the
collector of U680E is going positive. This positive-going
voltage is applied to the base of the Pulse A m p lifier U760E
and a positive-going Alternate Trace Sync Pulse is coupled
from the em itter to the Channel Switch circuit while a
negative going Enhance Pulse is coupled from the collector
to the Storage circuit.
H o ld o ff D riv e r. The negative-going signal at the
collector of U680D when the sweep begins is connected to
the Holdoff Capacitor through U760C. This negative-going
signal discharges the H oldoff Capacitor completely at the
start of each sweep to provide accurate sweep holdoff time.
CR785 clampsThe collector of U760C so it does not go
more negative than about —0.5 volt.
Sawtooth Sweep Generator. The basic sweep generator
is a Miller Integrator circuit. When pin 1 of U700 is
positive, the tim ing capacitor, Cy, begins to charge through
the timing resistor, R y. The tim ing resistor and capacitor
are selected by the TIM E /D IV switch S700 to provide the
various sweep rates listed on the fro n t panel. Diagram 8
shows a complete diagram of the T IM E /D IV switch. The
Sweep Cal adjustment R727 allows calibration of this
circuit fo r accurate sweep timing. The Variable TIM E/D IV
control R799 provides continuously variable uncalibrated
sweep rates by varying the charge rate of the timing
capacitor. R703 is an Offset adjustment to adjust the
quiescent DC level at pin 9 of U700 to zero volts. This
reduces timing current errors between positions of the
TIM E /D IV switch.
The output sawtooth voltage signal at pin 8 of U700 is
applied to the Horizontal Am plifier input. Voltage divider
R705-R706 determines the amplitude of the sawtooth
ramp. When the ramp reaches this predetermined level, a
reset pulse is generated at pin 4 of U700. This reset pulse is
applied to the base of U760A in the Sweep Reset M u lti
vibrator and to the base of U680E in the Gate O utput
Comparator.
Sweep Reset M ultivibrator. Quiescently, U760A is
turned o ff and U760B is turned on. The sweep reset pulse
(a narrow, positive-going, rectangular pulse) is applied to
the base of U760A through R700 and CR710. U760A is
turned on and its collector goes negative. This negative
movement is coupled to the base of U760B and turns it off,
causing its collector to go positive. The positive movement
at the collector of U760B is coupled to the base of U650C
which, prior to this, had been turned o ff. The increase in
forward bias of the base-emitter junction of U650C drives it
into saturation. The collector of U650C moves negative
until it is only a few tenths of a volt more positive than the
grounded emitter. The negative movement of the collector
of U650C is coupled to the anode of Sweep Gating Tunnel
Diode CR678 through R678, causing CR678 to switch
from its high-voltage state to a low-voltage state where it is
conducting almost no current. As long as U650C is turned
on, CR678 is prevented from switching to its high voltage
state.
H oldoff Emitter Follower. When the sweep reset pulse
causes U760A and U760B to reverse states, the pulse also
causes U680E and U680D in the Gate Output Comparator
to switch states. The reset pulse turns on U680E which
causes U680D to turn off. The positive movement at the
collector o f U680D when it turns o ff causes U760C to turn
o ff and the H oldoff Capacitor, Ch o » starts to charge
positive through R778 and R60A. This positive movement
is coupled through H oldoff Em itter Follower U760D and
em itter resistor R774 to the anode of diode CR765. When
sufficient forward bias is achieved, CR765 w ill turn on and
couple the positive-going movement to the base of U760B
in the Sweep Reset M ultivibrator, causing U760B to turn
on and U760A to turn off. The negative movement at the
collector of U760B turns U650C o ff and CR678 conducts
current in its low-voltage state, ready to receive the next
trigger pulse.
R60A varies the charge rate of the H oldoff Capacitor to
provide a stable display at fast sweep rates. This change in
holdoff allows sweep synchronization for less display jitter
at the faster sweep rates, and has very little effect at slow
sweep rates. R60A is ganged w ith the TRIGGER LEVEL
control and is adjusted by rotating the LE VEL knob. The
H oldoff Capacitor is changed by the T IM E /D IV switch for
the various sweep rates to provide the correct hold off time.
Auto Sweep Mode Operation
Operation of the Sweep Generator circuit in the AUTO
mode is the same as fo r the NORM mode just described
when an adequate trigger pulse is present. However, when
an adequate trigger pulse is not present, a free-running
reference trace is produced in the AUTO mode. This occurs
as follows:
A uto M ultivibrator. U650A and U650B form a mono
stable m ultivibrator. Quiescently w ith no trigger pulses
being generated by the Trigger Generator circuit, U650B is
conducting and U650A is biased o ff by only about 150
m illivolts. With U650A not conducting, its collector moves
3-15
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