Circuit Description— Type 1A1
1 signal passes through these diodes to the Output Am
plifier second stage.
Meanwhile, +2.6 volts is applied from the Switching
Multivibrator's cutoff transistor Q315 to the cathode junc
tions of the Channel 2 shunt diodes D452 and D453 (see
Fig. 4-2). This is the lowest voltage seen by cathodes of
the Channel 2 diode switches. As a result, Channel 2 shunt
diodes conduct and the voltage drop across the shunt diodes
sets their anode level at +3.6 volts. The +3.6 volts reverse
biases Channel 2 series diodes and blocks the signal from
going to the Output Amplifier second stage. With the
shunt diodes conducting, the Channel 2 signal is shunted into
the common-mode point located at the cathode junction of
the shunt diodes.
In the CH 2 position of the MODE switch, the opposite
condition exists— Channel 1 is off and Channel 2 is on
because the Switching Multivibrator changes state. The
Switching Multivibrator's cutoff transistor Q305 applies
+2.6 volts to the cathode junction of Channel 1 shunt
diodes. Under these conditions, Channel 1 shunt diodes
conduct and reverse bias the series diodes. The reverse-
biased series diodes disconnect the Channel 1 signal from
the Output Amplifier second stage. Simultaneously, Chan
nel 2 diodes conduct and reverse bias the shunt diodes.
With Channel 2 series diodes conducting, the Channel 2
signal passes through the series diodes to the Output
Amplifier second stage.
During dual-trace operation when the MODE switch is
set to either ALT or CHOP position, the diode switches con
nect and disconnect their respective channels to the Out
put Amplifier second stage alternately at the same rate
as the Switching Multivibrator rate. The cycle of opera
tion for the diode switches is as previously described.
When the MODE switch is set to ADD, the Switching
Multivibrator goes into a state in which both of its tran
sistors are conducting simultaneously. The conducting tran
sistors apply +7.7 volts to the cathode junctions of the
shunt diodes in both channels. As a result, the shunt diodes
reverse bias and series diodes conduct. The signals in both
channels pass through their respective series diodes and
algebraically add in the input circuit of the Output Ampli
fier second stage.
In the ADD mode of operation, R429 is shorted by the
MODE switch, thus decreasing the effective resistance to
ground in the base and diode switch circuits. With two
channels on, the decreased resistance sets the cathode volt
ages of the series diodes in both channels and the bases of
Q464 and Q474 to their proper levels. The voltage levels
will then be the same as those of the turned-on channel
in the other modes of operation. Proper voltage levels in
this portion of the circuitry allow the Switching Multivi
brator to operate at its correct design levels.
Output Amplifier Second Stage (Q 4 6 4 /V 4 6 4 /
Q 4 7 4 /V 4 7 4 )
This stage is a push-pull, hybrid, cascode configuration.
The hybrid circuit is used to raise the + 4 .6-volt input level
at the bases of Q464 and Q474 to the +67.5-volt output
level required for driving the oscilloscope vertical ampli
fier for linear operation.
Signals applied to the bases of transistors Q464 and
Q474 cause current variations in the base-collector circuit
of the transistors. Since the transistors are connected in
series with the cathode circuit of V464 and V474 and the
grids of the tubes are at AC ground, any current variations
in the cathode circuit of the tubes produce corresponding
in-phase current signals at the plate of the tubes. The sig
nals at the plates are then applied through pins 1 and 3
of the interconnecting plug to the oscilloscope vertical
amplifier.
Voltage signals applied to the bases of Q464 and Q474
are inverted 180° at the collectors. No phase inversion
occurs as the signals go through V464 and V474. With the
PULL FOR INVERT switch set to normal, a positive-going
signal applied to the input connector of a channel will
be positive going at pin 1 of the interconnecting plug to
the oscilloscope and negative going at pin 3 of the same
plug (see Block Diagram, Section 10 for signal polarity
comparisons).
Variable peaking inductors L460 and L470 in the base
circuit of Q464 and Q474 provide interstage high-frequency
compensation adjustments for the high frequencies. Variable
capacitors C466 and C476 are emitter compensation adjust
ments for the high frequencies.
Resistors R464 and R474 aid in matching the output
impedance of the Type 1A1 to the input impedance of the
oscilloscope vertical amplifier.
Channel
1
Signal Pickoff Emitter Follower
(Q 1 6 3 /Q 1 7 3 )
The Channel 1 signal is taken off in a push-pull fashion
from the emitters of the second Emitter Follower stage
(Q153A/Q153B). This is the signal which is applied to the
Channel 1 Signal Pickoff Emitter Follower stage (Q163/
Q173) and then to the following stages for use as Chan
nel 1 Signal and Trigger outputs from the Type 1A1.
The reasons for taking the signal off at the emitters of
Q153A and Q153B are as follows:
1. The emitters are low-impedance points where the
signal can be extracted with least effect on the bandwidth
or transient response of the Type 1A1.
2. The takeoff points are isolated from the diode switches.
3. A gain of 10 is obtained through the Input Amplifier
stage when the VOLTS/CM switch is set to the .005 posi
tion.
4. The push-pull takeoff signal is not affected by use
of the PULL FOR INVERT switch, POSITION control, VARI
ABLE VOLTS/CM control, GAIN control or MODE switch.
5. By using push-pull takeoff, common-mode signals such
as noise, hum and DC drift are cancelled in the common
collector circuit of Q163 and Q173, and in the common
emitter circuit of Q164 and Q174.
®T
4-5
