TM 9-6625-1753-14
CR8205, CR8209, CR8211, and CR8216. With both diodes conducting, charging-capacitor Cc (actually a capacitor which
3-13.
Normal Sweep (Fig. 4-5)
you select with TIME/DIV switch S850) is short-circuited. In addition, the grid of V1 (V8203) and the cathode of V2 (V8204)
a. Normal Sweep Gate.
are clamped to a small negative voltage (approximately -2 volts) by diode-clamp CR8212 via diode CR8213 and the
disconnect diodes.
(1) After passing through transformer T8201, the negative trigger goes to bistable multivibrator Q8206, Q8207 (the
normal-sweep gate multivibrator) via trigger diode CR8203. In its initial state, transistor Q8206 is conducting and Q8207 is cut
(3) Timing capacitor Ct charges to the plate voltage of V1 via resistor RT. Assume that this voltage becomes large
off. The negative trigger pulse applied to Q8206 then cuts it off. This causes Q8207 to conduct, and the operating state of
enough to fire the neon bulb connected across the capacitor. Since the neon bulb drops approximately 50 volts, this will be
the multivibrator changes. In this new operating state, positive output appears at the collector of Q8206 and a negative output
the final charge across the capacitor.
appears at the collector of Q8207. Trigger diode CR8203 and the lockout circuit, ( below), prevent the triggering of the
d
multivibrator until after the sweep is completed.
(4) When the sweep gate multivibrator is triggered, a negative signal is applied to the disconnect diodes, reverse-
biasing them. This removes the short circuit across capacitor Cc and the clamping voltage from the grid of V1 and the
(2) The negative output from Q8207 goes to the sawtooth ge nerator via diode-clamp CR8212, disconnect diodes
cathode of V2.
CR8205, CR8209, CR8211, CR8216, and diode CR8213. The positive output from Q8206 goes to delayed-sweep gate
multivibrator Q8106, Q8107 (zone D9), and unblanking amplifier Q8405 (zone D13). The length of time that the normal-
(5) Removing the clamping voltage causes the grid of V1 to go more negative and its plate voltage to increase.
sweep gate multivibrator remains in this particular state depends on the duration of the sawtooth generator output (b below).
Since the neon bulb drops a constant 50 volts, a higher potential is applied to the grid of V2.
When the sawtooth reaches its maximum output, it causes turn-off diode CR8204 to conduct, causing the multivibrator to
change state.
(6) Since V2 is a cathode follower, its cathode is at approximately the same potential as its grid. Capacitor Cc,
connected to the cathode, starts charging to the grid potential. Its charge path is through resistor Rg and V2. As Cc charges,
b. Sawtooth Generator.
it forces the grid of V1 to go more negative, resulting in an even larger plate voltage. The grid of V2 then receives this higher
voltage via the neon bulb. This process continues until the negative potential (sweep gate multivibrator output) is no longer
(1) The sawtooth generator is a Miller integrator (V8203) coupled to a cathode follower (V8204). Since this is not the
applied to the disconnect diodes. Notice that as the voltage, to which the charge on capacitor Cc increases, the slope of the
type of circuit you see with any regularity, let's take a closer look at it. Figure 3-4A is a simplified schematic of the sawtooth
charging curve becomes straighter (more linear). Figure 3-4B illustrates this. Since the output is taken from the cathode of
generator.
V2, to which Cc is connected, the time constant of Rg and Cc determines the angle of the slope or ramp. Notice that for
capacitor Cc to charge linearly, a constant current must be applied to it. Cathode follower V2 acts as a variable resistor and
(2) First, assume that the sweep gate multivibrator hasn't been triggered, and, therefore, it sends a positive signal to
provides this function. With a linear voltage applied to its grid, the resistance exhibited by V2 also varies linearly, producing a
disconnect diodes, forward-biasing them. Although the simplified schematic shows only two diodes, there are actually four,
constant current. When the charge on the disconnect diodes is reversed by a gate from the normal-sweep gate multivibrator,
capacitor Cc is shorted and discharges. The circuit is then ready to begin another cycle, as described in c (3) below.
(7) The circuit through which capacitor Cc charges is shown in figure 4-5 (zone A10). Setting TIME/DIV switch S850
to the desired position (any one of 23 positions), you select the proper values of Rg and Cc to produce the desired time
constant. This is illustrated by a single resistor-capacitor combination in the functional schematic.
(8) Assuming that the TIME/DIV switch is set to the desired position, then capacitor Cc starts charging when the
disconnect diodes are reverse-biased. Remember that the charging path for Cc was via resistor Rg and cathode follower V2
(V8204). Looking at the functional schematic, note that resistor Rg is also connected to emitter follower Q8204, which is the
source of negative potential from which capacitor Cc charges. SWEEP CAL, and SWEEP VERNIER resistors R824R and
R8242, respectively, are connected to the base of Q8204 via DISPLAY LOGIC switch S840, and control the operating level of
the emitter follower. This, in turn, determines the amplitude of the negative potential.
(9) Continuing with the signal flow, the positive-going output of the sawtooth generator goes to the horizontal
deflection amplifier (par. 3-15) via RC attenuator R8623, C8606 and the DISPLAY LOGIC switch. In addition to the horizontal
deflection amplifier, the sawtooth voltage also goes to turn-off emitter follower Q8208 (zone A8) and comparator Q8601,
Q8602 (zone B10).
c. Sweep Duration.
(1) The width of the negative-gate output, from the normal-sweep gate multivibrator, determines the duration of the
sawtooth-generator output. The positive ramp output of the sawtooth generator, in conjunction with turn-off emitter follower
Q8208 and SWP LENGTH resistor R8263 controls the duration of the negative gate.
(2) Turn-off emitter follower Q8208 receives the positive-going ramp from the sawtooth generator. After passing
through the emitter follower, the ramp voltage goes to turn-off diode CR8204. Adjusting SWP LENGTH resistor R8263 in the
emitter circuit of Q8208 varies the dc operating level of the ramp voltage. Since diode CR8204 is normally reverse-biased,
the normal-sweep gate multivibrator tends to remain in its activated state, producing a negative output. With the SWP
Figure 3-4. Miller integrator (run-up) circuit.
3-8
