Circuit Description— Type 1A1
Channel 1 Signal Pickoff Amplifier (Q 1 6 4 /
Q174)
Q164 and Q174 with its associated circuitry is a push-
pull amplifier for the Channel 1 signal arriving from the
emitter of Q163 and Q173. Voltage gain for the stage
is about 2 for Q164 and about 6 for Q174. In the collector
circuit of Q164 the signal at the junction of R164 and R165
is applied to the CH 1 SIGNAL OUT connector. The
polarity of the signal at the connector is the same as that
of the signal applied to the Channel 1 INPUT connector.
Output DC level of the signal at the CH 1 SIGNAL OUT
connector is about +0.45 volt.
Channel 1 Trigger Output Amplifier (Q 1 8 4 /
Q194)
The Channel 1 takeoff signal which is used as a trigger
output source, is obtained from the junction of divider
resistors R174 and R175 in the collector circuit of Q174.
This trigger takeoff signal is applied to the bases of Q184
and Q194.
Transistors Q184 and Q194 with associated circuitry
form a complementary amplifier having a signal-voltage
gain of about 3.3. The outputs from these two transistors
are combined to produce a single-ended signal. This sig
nal, which is used as a trigger source, is applied to the
CH 1 TRIGGER OUT connector and to pin 5 of the inter
connecting plug to the oscilloscope.
The trigger at pin 5 is available for use as an internal
trigger source. However, to make use of this trigger, the
associated oscilloscope must be capable of selecting it with
a Triggering Source switch. If the Channel 1 trigger cannot
be selected internally, external triggering must be used
instead. The Channel 1 trigger has the same polarity as
the signal applied to the Channel 1 INPUT connector.
Output DC level is approximately zero volts.
SWITCHING CIRCUIT
Selection of the input channel whose output is to be
applied to the Output Amplifier is accomplished by the
Switching Circuit. The Switching Circuit consists of the
following stages in order: Switching Multivibrator Q305/
Q315, Alternate Trigger (Blocking Oscillator) Q330, and the
Blanking Multivibrator Q343/Q353.
Switching Multivibrator (Q 3 0 5 /Q 3 1 5 )
The Switching Multivibrator stage (Q305/Q315) is basical
ly a bistable circuit that switches Channels 1 and 2 in the
Type 1A1. When Q305 conducts, Channel 1 signal or trace
is displayed. When Q315 conducts, Channel 2 signal or
trace is displayed. The setting of the MODE switch deter
mines whether the Switching Multivibrator rests in one of
its stable states (CH 1 or CH 2), is astable (CHOP), is
bistable (ALT— base triggered by the alternate trigger
pulse), or is dual-conducting (ADD).
(1 ) CH 1, CH 2
Assume that the MODE switch is set to CH 1. In this
position, base-biasing network R302, R303 and R304 in the
base circuit of Q305 is grounded at the switch end of
R302. A similar network in the base circuit of Q315 is
connected to + 3 9 volts at the switch end of R312. The
MODE switch disconnects emitter resistors R301 and R311
from the +39-volt supply. As a result, both emitters are now
returned to + 3 9 volts through D301, D311 and R300. Under
these conditions, the Q315 base-biasing network cuts off
Q315 and the base biasing network for Q305 turns on
Q305. Diode D303 is conducting while D313 is reverse
biased. These diodes control the base impedance of their
respective transistors so that proper currents are provided
for the operation of transistors in each of their states.
With Q305 conducting, its collector rests at +7.7 volts;
the collector voltage of Q315 during cutoff is +2.6 volts.
As described earlier, the + 7 .7 volts reverse biases the
Channel 1 shunt switching diodes D422 and D423. Chan
nel 1 series diodes D421 and D424 are forward biased
and they connect Channel 1 Input Amplifier to the Output
Amplifier. Simultaneously, the +2.6 volts at the collector
of Q315 causes Channel 2 shunt diodes D452 and D453
to become forward biased and series diodes D451 and
D454 to become reverse biased. Thus, Channel 2 Input
Amplifier is disconnected from the Output Amplifier.
If the MODE switch is set to the CH 2 position, just the
opposite occurs. The MODE switch connections cause Q305
to cut off and Q315 to conduct. Diode D303 becomes
reverse biased and D313 forward biases. Then, the +2.6
volts at the collector of Q305 causes the Channel 1 diode
switches to disconnect Channel 1 from the Output Ampli
fier. At the same time, the +7.7 volts at the collector
of Q315 causes Channel 2 diode switches to connect Chan
nel 2 to the Output Amplifier.
(2 ) Alternate Mode of Operation
When the MODE switch is set to the ALT position, the
Switching Multivibrator becomes a bistable circuit. Initially,
the circuit is resting in one of its stable states. When trig
gered by a positive-going pulse applied to the junction of
diodes D308 and D318, the circuit switches to its other
stable state, and remains there until triggered again.
With the MODE switch set to ALT, the switch ends of
R302 and R312 in both base-biasing networks are con
nected to + 3 9 volts. The emitters are tied to a common
point through D301 and D311 at R300. Using this method
of biasing, the Switching Multivibrator is converted into a
bistable circuit. With initial application of DC power, one
of the transistors begins to conduct first while the other
is cut off. Regenerative action causes the conducting tran
sistor to saturate, holding the other transistor in cutoff. Thus,
the circuit initially res*s in one of its stable states.
At the end of each sweep a positive-going trigger is
generated by the time-base circuit in the oscilloscope. This
alternate trace sync pulse is applied via the oscilloscope
Sync Amplifier tube cathode circuit to pin 8 of the inter
connecting plug. The sync pulse goes through pin 8 of the
plug and then through a single-pin connector on the Out
put Amplifier board connector to the Alternate Trigger
Blocking Oscillator stage (Q330). A positive-going trigger
of suitable waveshape and amplitude is generated in the
output winding of T330. The trigger is then applied to the
junction of diodes D308 and D318. The trigger forward
biases both diodes and goes through the diodes to the
4-6
