Theory of Operation—5B42
going trigger signal at pin 10 of U320 causes apositive
output from pin 14 of U335C which starts the sweep.
U335B and Cform alatch, which generates the sweep gate.
When the sweep ends, the junction of R356 and R357 goes
high, setting latch U330C and Dso that ahigh is on pin 14
of U330C, This high resets latch U335B and C. In this state,
U335C can not initiate asweep gate. When the holdoff
time passes, the junction of R356 and R357 goes low, A
low at pin 6of U33Q8, or the next negative-going trigger,
causes letch U330C and Dto reset, enabling latch U335B
and C. The next 'positive Output from U320 initiates the
sweep gate by lowering pin 13 of U335C. When negative
slope Triggering is desired, ahigh is placed on pin 9of
U32C, which allows U320 to operate as an inverting
amplifier with again of five.
Main Trigger Generator (SN B060000 and up)
Integrated circuit U300 converts the trigger comparator
output signal to agate that is used for sweep control. With
pin 1connected to ground l+SLOPE), apositive-going
signal on the input (pin 13} causes the output (pin 3) to
rise to about 4.1 V. Pin 14 is negative-going under these
conditions. The output, gate occurs when aTrigger signal
passes through the hysteresis limits at the input of U300
(pin 13 rs within 20 mV of pin 14). Placing pin 1at +5 V
(—SLOPE) creates an output gate at pin 3, when pin 13 is
negative-going and pin 14 positive-going. After completion
of the sweep, and during holdoff Time, pins 6and 10 are
high [about +4.2 V), This action inhibits the trigger gener-
ator until these pins drop to about +3.2 V, which occurs
after completion of holdoff time.
Delayed Trigger Generator (SN B059999 and below)
This generator operates, in the triggered mode, in the
same manner as the main trigger generator. In the delayed
sweep and intensified sweep modes, the delayed sweep
starts when the junction of R438 and R44Q goes low. The
highs placed on pins 14 and 15 of U410, through diodes
CR4T0 and CR411, cause acontinuous low at pin 6of
U420E3. This causes pin 12 of U425C to go low at the
beginning of the delayed pickoff gate. Pin 9of U425B is
held high, causing acontinuous low on pin 13 of U426C.
Two lows at the inputs of U425C cause ahigh at pin 14
starting the delayed sweep runup.
Delayed Trigger Generator (SN B060000 and up)
Inthe Trig After Delay mode, this generator operates in
the same manner as the Main Trigger Generator, U300. The
delayed sweep starts when two conditions are met; the
delay gate at pin Gand 10 of U40Q must be low (approxi-
mately +3.2 V), and atrigger signal must pass through the
hysteresis limits at the input of U400 (i.e., pin 13 is within
20 mV of pin 14).
Sweep Control
U450, with additional external circuitry, controls the
main sweep generator. In the automatic triggering mode,
pin 19 of U450 is connected to ground. If pin 1of U450
receives no trigger pulses from the main trigger generator
for aperiod of time determined by R495 and C495,
circuitry in U450 outputs anegative-going square wave.
This negative-going square, wave from pin 3of U450, drives
the base of Q430 negative. The collector goes positive and
stays positive for the duration of the sweep. Apositive
pulse at the emitter of Q480, from the trigger switches, also
causes the collector of 0480 to go positive, starting the
sweep. This positive pulse is also applied to the unblanking
circuitry, in the plug-in and the mainframe, to unblank the
CRT during the sweep.
The sweep ramp is fed to the base of Q470, which forms
acomparator with Q460, Q470 is conducting and Q460 is
off. As the ramp rises, current flow in Q470 is reduced until
CR460 is forward biased, and Q460 comes sharply into
conduction. This positive pulse at pin 16 of U45Q causes
the auto gate (from pin 3) to go positive, terminating the
sweep through Q4S0. If the unit is operating in the
triggered mode, apositive gate appears at pin 17, lowering
the sweep gate via the main trigger generator and G480,
terminating the sweep}
As the sweep ramp decays toward ground, Q460 and
Q470 switch, so that pin 16 of U450 again goes low. No
negative pulse can appear at pin 3 to start the next sweep.
Pin 17 remains high, locking out the main trigger generator
until holdoff time is completed. This time is determined by
C50C, R50G, 0501 and CB02. These components are
switched according to sweep rates, as shown in the switch
details. Holdoff is necessary to allow all sweep -generating
circuits to discharge before the start of the next sweep.
When the external horizontal amplifier mode is used, pin
18 of U45Q is connected through CR455 to +5 V. This
disables the sweep until the switch is opened.
In the single sweep mode, the automatic trigger capa-
bility is removed by ungrounding pin 19 of U450. Pin 12 is
connected to +5 V, The next trigger operates the sweep in
the normal manner. The gate at pin 17, however, remains
high, locking out further triggers until pin 11 of U450 is
grounded and pin 15 is connected to +5 Vthrough the
spring-loaded RESET button. VR49Q and CR490 provide
-5 Vfor U450.
Main Sweep Generator
The sweep ramp is generated in U550. The ramp is fed
to the horizontal amplifiers and the sweep control from pin
8. Timing capacitors Ctand timing resistors Rtdetermine
the rate of rise. The DELAY TIME MULT potentiometer
2-2 REV. B, JUNE, 1977
