Theory of Operation—2205 Service
CR349 to prevent external trigger signals or the line
trigger signal from reaching the Trigger Generator.
Signals from the Internal Trigger Amplifier are
passed to the Trigger Generator through forward-
biased
CR450.
CHANNEL 1. For triggering from Channel 1, the
SOURCE and VERTICAL MODE switches are set to
CH 1. Input pin 9 of U300C will be LO, which place a
LO at the output of U300C. This LO is passed
through U304B to an input of U300B. The output of
U300B goes Hi enabling Channel 1 signal through
U335. The Channel 2 signal path is disabled by the
output of U300A being LO.
CHANNEL 2. For triggering from Channel 2, the
SOURCE and VERTICAL MODE switches are set to
CH 2. Input pin 8 of U300C will be LO, which places a
LO at the output of U300C. This LO Is passes
through U304A to one of the inputs of U300A. The
output of U300A goes HI enabling Channel 2 signal
through U335. The Channel 1 signal path is disabled
by the output of U300B being LO.
VERT MODE. When the SOURCE switch is set to
VERT MODE, trigger source selection is determined
by the two VERTICAL MODE switches. The outputs of
U30QA and U300B will be HI, and triggering selection
will then be determined by the inputs of U304A and
U304B that are controlled by U540A in the Channel
Switch circuit.
When Channel 1 is selected (VERTICAL MODE
switch set to CH 1). the input to U304B will be HI.
The LO from the output of U3048 is applied to U300B
and causes the CH 1 TRIG line to go HI and enable
the Channel 1 trigger signal. The input to U304A will
be LO placing a HI at the input of U300A. The LO
from the output of U300A causes the Channel 2
trigger signal to be disabled.
When Channel 2 is selected (VERTICAL MODE
switch set to CH 2), the outputs of U340B, U3Q0B,
U3Q4A, and U300A will be the reverse of the states
described for Channel 1 selection. The Channel 2
signal will be selected and the Channel 1 trigger
signal disabled. M
When selecting ALT VERTICAL MODE, the inputs
of U304A and U304B will toggle with each sweep.
The outputs of the two gates will also toggle and the
Trigger signal source will alternate with the displayed
channel.
i In the ADD VERTICAL MODE position, both inputs
to U304A and U304B will be LO and both gate out
puts of U300A and U300B will be HI. Both Channel 1
and Channel 2 signal paths will be enabled and their
output current will be summed at the inputs of the
Internal Trigger Amplifier to produce the internal
trigger signal.
In CHOP VERTICAL MODE position, the
CHUR ENABLE line places a LO on both inputs of
U300D. The output of U300D is held HI and applied
to the inputs U304A and U304B and the signal to the
Internal Trigger Amplifier will be the same as for the
ADD mode.
Internal Trigger Amplifier
The Internal Trigger Amplifier converts the
differential trigger signals from the Vertical Pre
amplifiers into a single-ended signal that drives the
X-Axis Amplifier and the Trigger Generator.
Signal current is applied to the emitters of U380C
and U380D. The collector current of U380C is con
verted to a voltage across feedback resistor R357.
The opposite-phase collector current of U380A
causes a voltage drop across R359 that adds to the
voltage at the collector of U380D. This voltage
appears at the base of U415C which buffers and
level shifts the signal back to 0 V. The emitter signal
of U415C drives the X-Axis Amplifier, and the base
of Q410. The emitter signal of Q410 in turn drives
the Trigger Generator whenever CR450 is forward
biased.
External Trigger Amplifier
The External Trigger Amplifier buffers signals
applied to the EXT INPUT OR Z connector and line-
frequency trigger signals. The output of the amplifier
is applied to the Trigger Generator.
EXT. When the SOURCE switches are set to EXT,
aind either EXT=Z, EXT, or EXT/10 is selected, the
trigger source is the signal applied to the EXT INPUT
OR Z connector. In EXT/10 position the input signal
is attenuated by a factor of 10 through the compen
sated divider composed of R377, R378, C377, and
C378.
The external signal is applied to the gate of Q370.
FET Q371 supplies source current for Q370 such that
there will be no voltage drop across the gate-source
junction of Q370. FETS Q370 and Q371 are a
matched pair. The source-follower Q370 drives
emitter-follower transistor Q412 which lowers the
Amplifier output impedance. Protection-diode
CR381 damps the signal at the gate of Q370 to
about -9 V. The Amplifier output will drive the Trig
ger Generator through forward-biased CR451 when
ever the SOURCE switch is set to EXT. When the
SOURCE switch is not set to EXT, the base-emitter
junction of 0370 will be reverse biased via 0411.
CR452, CR348 or CR349, and the Amplifier will be
disabled.
3-7
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