Circuit Description— Type 1A1
bases of both transistors in the Switching Multivibrator stage.
The trigger affects only the conducting transistor in the
stage.
Assume for this discussion that Q305 is conducting and
Q315 is cut off. Since Q305 is conducting, it is affected
by the trigger. The trigger causes Q305 collector current
to decrease, which decreases collector voltage. The decreas
ing voltage is applied via C306 to the base of Q315, caus
ing it to conduct. Q315 collector current increases, caus
ing more positive voltage to be applied via C316 to the
base of Q305. This regenerative feedback continues until
Q315 is driven into saturation and Q305 is cut off.
Since the Switching Multivibrator controls the diode
switches, Channel 1 is turned off as Q305 cuts off and
Channel 2 is turned on as Q315 is driven into saturation,
thus completing one-half cycle of the Switching Multivi
brator action. For the other half cycle, the next trigger
applied through diodes D308 and D318 will cause Q315
to decrease its collector current and the regenerative feed
back action finally causes Q305 to saturate and Q315
to cut off. As a result, Channel 2 is turned off and Chan
nel 1 is turned on.
By rapidly coupling the changing voltages to the bases
of the transistors in the Switching Multivibrator, capaci
tors C306 and C316 speed up the regenerative feedback
action and ensure rapid switching of the transistors.
In the alternate mode of operation, diodes D301 and
D311 are forward biased, effectively shorting out C301
and C311. Thus, no pulses from the Switching Multivibrator
are coupled through these capacitors to drive the Blank
ing Multivibrator stage. Since the channels switch states
during the retrace interval and the trace is already blanked
out by the oscilloscope circuitry, no blanking pulses from
the Type 1A1 are needed.
In an oscilloscope which has an alternate sweep feature,
the two time bases in the oscilloscope can be displayed
alternately on the CRT by setting the oscilloscope Hori
zontal Display switch to the Alternate (A and B) position.
In this position of the switch the A and B sweep generators
are alternately generating sweeps. During the time that
the B Sweep Generator is generating its sweep, this same
generator also produces a negative-going (+45 volts to
ground) slave pulse. The pulse is applied to pin 7 of the
interconnecting plug between the oscilloscope and the Type
1A1. From pin 7 of the interconnecting plug the pulse is
applied through pin X of the Output Amplifier card to the
junction of R313 and D313 via C313.
The negative-going slave pulse, applied to the junction
of R313 and D313, ensures that Q315 is triggered into
conduction so that Channel 2 turns on while the B Sweep
Generator is generating its sweep. While Channel 2 is on,
Channel 1 is off because Q305 is cut off. At the end of
the B sweep, the slave signal terminates and the alternate
trace sync pulse from the B Sweep Generator triggers the
Alternate Trigger Blocking Oscillator. The trigger from the
Alternate Trigger Blocking Oscillator drives Q315 toward
cutoff, turning Channel 2 off and Channel 1 on. During
the time that Channel 1 is on, A Sweep Generator is gen
erating its sweep so the signal in Channel 1 can be dis
played. Channel 2, meanwhile, is turned off as long as
Q315 is cut off.
(3 ) Chopped Mode of Operation
When the MODE switch is set to the CHOP position, the
Switching Multivibrator becomes an astable circuit. It free
runs at approximately a 1 MHz rate, driving the diode
switches at the same rate. The diode switches alternately
turn the channels off and on. Thus, each channel is on for
about 0.5
fi
s while the other is off the same amount of
time.
With the MODE switch set to the CHOP position, the
base-biasing networks are connected to + 39 volts, the
same as for alternate mode of operation. The junction of
emitter resistors R301 and R311 is now connected via the
MODE switch to + 39 volts. In addition, the switch end of
R300 is grounded to reverse bias D301 and D311. The
reverse-biased diodes remove the low-impedance path from
across coupling capacitors C301 and C311. These capaci
tors AC couple the emitters together to make the Switching
Multivibrator free run at a rate determined by the resistance
and capacitance values used in the emitter circuits.
Each time the Switching Multivibrator switches states, a
fast negative-going pulse followed by a slow-rise positive
going ramp is produced at the junction of C301 and C311.
The ramp rises in amplitude until the Switching Multivibra
tor switches states, then the cycle is repeated. This ramp-
type pulse at the junction of the capacitors is the algebraic
addition of the timing voltage signals developed at the
emitters of the transistors. The ramp pulse is applied to
the base of Q343 in the Blanking Multivibrator stage.
(4) Added Mode O f Operation
Setting the MODE switch to the ADD position grounds the
switch end of R302 and R312 of the base-biasing networks.
The MODE switch disconnects the switch end of emitter resis
tors R301 and R311 from + 39 volts. Instead, the emitter of
the transistors are now tied through forward-biased diodes
D301 and D311 to R300. The switch end of R300 is con
nected to +39 volts via the MODE switch to apply forward-
bias voltage to the diodes. The bases of both transistors
are lowered toward ground since the switch ends of R302
and R312 are tied to ground. As a result, both transistors
go into conduction simultaneously. With both transistors
conducting, both channels are turned on via their respective
diode switches.
In order to make the diode switches operate properly in
the ADD position, the collectors of Q305 and Q315 must be
raised to about the same level as the collector level of the
single conducting transistor in the other modes of operation.
To raise the collector level, the switch end of R321 is dis
connected from ground so it can be connected in series with
R322. Resistor R322 raises the voltage level at the collectors
to the normal +7.7-volt level.
Alternate Trigger Blocking Oscillator (Q330)
The main function of the Alternate Trigger Blocking
Oscillator stage (Q330) is to provide a fast positive-going
trigger of definite shape and energy content for triggering
the Switching Multivibrator in alternate mode of operation.
Reshaping the trigger assures that the Type 1A1 will function
properly with any oscilloscope capable of accepting the
unit.
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