Theory
of Operation-2225
Service
CHANNEL
1.
When the Trigger SOURCE is set to
CH 1, Channel 1 is the trigger source whether
displayed or not. The Channel 1 signal is also the
trigger source under other settings of the Trigger
SOURCE and Vertical MODE switches that call for the
Channel 1 signal to be displayed. Those conditions
are:
Trigger SOURCE set to VERT MODE and the Vertical
MODE is set to CH 1, or
Trigger SOURCE set to VERT MODE and the Vertical
MODE is set to BOTH and ALT.
CHANNEL
2.
When the Trigger SOURCE is set to
CH 2, then Channel 2 provides the trigger signal
whether Channel 2 is displayed or not. As with
Channel 1, other Trigger SOURCE and Vertical
MODE settings will call up the Channel 2 as the
trigger signal when Channel 2 is displayed. Those
conditions are:
Trigger
SOLIRCE set to VERT MODE and the Vertical
MODE is set to CH 2, or
Trigger SOURCE set to VERT MODE and the Vertical
MODE is set to BOTH and ALT.
VERT
MODE.
When the SOURCE switch is set to
VERT MODE the trigger source selection is
determined by the Vertical MODE switch. Vertical
MODEs of CH 1, CH 2, and BOTH in ALT are
described above. Vertical
MODEs of BOTH in ADD or
CHOP result in the trigger source being the
arithmetic sum of the Channel 1 and Channel 2 input
signals.
EXT.
When the SOURCE switches are set to EXT,
and either
EXT=Z or EXT, the trigger source is the
signal applied to the EXT INPUT
OR
Z
connector.
With EXT and
EXTI10 selected, the trigger signal is
as above but attenuated by a factor of 10. With EXT
and
LINE selected, the line-frequency signal,
generated in the power supply, is passed to the
External Trigger Input Amplifier (shown on diagram
6). In each case, the buffer consisting of
Q370A and
Q370B, drives differential amplifier U340. This ampli-
fier has the same form as the CH 1 and CH 2
preamplifiers. External offset adjustment is provided
by
R360. The LO logic signal generated by U308B,
EXTEN, switches on the external trigger path.
Trigger Amplifier
The Trigger Amplifier converts the differential signals
from the vertical and external preamplifiers into a
single-ended analog trigger signal that drives the
X-Axis amplifier (for X-Y Mode displays) and the
Trigger Generator.
Transistors Q363 and Q365 act as a cascade stage
to add the signals passed by the preamplifiers to the
offset current provided by the coupling control
amplifiers on diagram 3. The resulting differential
output drives the differential pair Q366 and
Q367.
The collector load of transistor Q367 is R388. That
load is driven via
cascode transistor Q368 and
"diode-connected" transistor
U380D. Transistor
Q366 drives current mirror
U370D and U370B. Diode
CR370 ensures that the collector-base voltage of
U370D is not too low, and CR369 compensates for
U370C, to equalize the collector potentials of U370B
and U370D.
The collector current of U370C is the output of the
current mirror and is equal to the collector current of
Q366. R388 passes a current equal to the difference
in the collectors of Q366 and Q367 (the trigger
signal). Transistor
U380C acts as an impedance
buffer, whose voltage drop is compensated by
U380D. The output from the emitter of U380C is the
analog trigger signal. In X-Y mode,
U380B
is
biased
off, allowing the trigger signal to be passed to the
X-Axis Amplifier.
U380E is switched off when HF
REJECT is selected. This allows C372 to be switched
in by
U380A, thereby shunting signals of frequencies
about 30 kHz and above.
Peak Rectifiers
The analog trigger signal is passed to the positive
and negative Peak Rectifier circuits, The Peak
Rectifiers generate voltages equal to the positive
and negative peaks of the analog trigger waveform
in P-P AUTO and TV FIELD modes. In NORM and SGL
SWP modes, the Peak Rectifier outputs assume a
voltage of about the full peak-to-peak limits of the
trigger signal.
The analog trigger signal is applied to the bases of
U415B and U435A. In P-P AUTO, C418 charges to
the positive peak of the analog trigger signal less the
U415B base-emitter drop. The base-emitter drop of
U415D compensates so that the output of U425B is
equal to the positive peak of the analog trigger
signal. In NORM Trigger mode, the base drive to
U415A rises to about +3 V, which drives the output
of
U425A to this level.
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