b. Channel B.
(1) Setting the MODE switch to B reverses the conditions described in a above. Now a constant negative voltage is
applied to the base of Q9600, cutting the transistor off. With Q9600 cut off, a positive voltage forward-biases diode CR9201,
which disables the channel A amplifiers. Diode CR9301 is now reverse-biased by the negative-going output of Q9601
(conducting), and the channel B amplifiers operate normally.
(2) When the channel A amplifiers are disabled, diodes CR9202 and CR9203 in the collector circuits of Q9204 and
Q9205 become reverse-biased by a positive voltage that is applied to their cathodes via resistors R9258 and R9259 (zones
A9, B9). Reverse-biasing these diodes effectively opens the collector circuit of both transistors (Q9204 and Q9205). This
isolates the active channel B amplifiers from the inactive channel A amplifiers, preventing capacitive loading, and ensuring
that the maximum available bandwidth is maintained.
(3) If the MODE switch is set to A, the channel A amplifiers are active, and the conditions described in (2) above are
reversed. Now diodes CR9302 and CR9303 are reverse-biased, opening the collector circuits of the channel B amplifiers.
c. Alternate Switching.
(1) Setting the MODE switch to ALT, alternately activates and deactivates the channel A and channel B amplifiers.
With the switch at ALT, there is no constant negative voltage applied to the base either Q9600 or Q9601 of the electronic
switch. The operating state of the electronic switch is now controlled by a negative pulse.
(2) Blanking multivibrator Q9701, Q9702 (zone D9) generates the negative pulse which is then applied to the
electronic switch via diode CR9601 (zone B8). Let's take a closer look at the blanking multivibrator. With the MODE switch at
ALT, the blanking multivibrator functions as a monostable multivibrator. Transistor Q9702 is normally cut off, and diodes
CR9704 and CR9708 (zone C8) are normally reverse-biased. Diode CR9704 is reverse-biased by a negative voltage applied
to its anode from a voltage divider in the delaying sweep plug-in. Diode CR9708 is reverse-biased by a negative voltage
applied to its anode via the MODE switch and R9705. The blanking multivibrator remains in this state until a positive trigger
pulse from the delaying sweep plug-in (representing the end of the sweep) overcomes the bias on diode CR9704. The
positive trigger is applied to the base of Q9702, driving it into the ON state. Diode CR9708 is sufficiently reverse-biased to
prevent the positive trigger from being applied to the collector of Q9701. After the trigger pulse has been applied, the
multivibrator reverts to its initial operating state. Diodes CR9701 and CR9707 (zone D9) prevent the collectors Q9701 and
Q9702 from going too positive.
(3) Since the positive trigger has a very small time duration and the time constant of the blanking multivibrator, acting
as a monostable multivibrator, is very small, a negative pulse of very short duration is generated. This negative trigger is
applied to the electronic switch via diode CR9601 (zone B8) and steering diodes CR9602 and CR9603, alternately changing
the operating state of the circuit. The operating state of the electronic switch, therefore, determines whether the channel A or
channel B amplifiers are activated.
(4) During the transition interval when one pair of amplifiers is activated and the other pair of amplifiers is activated
and the other pair is deactivated, a display should not appear on the CRT. To prevent a display, the same negative pulse
output of the blanking multivibrator is applied to amplifier Q9703 (zone D10). This negative pulse is inverted by Q9703 and
the resulting positive-going signal is applied to the cathode of the CRT. This positive signal reduces the cathode current and
results in the blanking of the CRT. Clamp circuit Q9704 and diode CR9702 prevent the collector of Q9703 from going too
d. Chopped. Setting the MODE switch to CHOP, removes the negative voltage that reverse-biases diode CR9708 (zone
C8). The negative voltage is now applied to diode CR9704, which is reverse-biased so that the trigger pulse is prevented
Figure 3-2. Interstage voltage attenuator equivalent circuit.