Theory of Operation-2225 Service
Vertical Beam Find Internal Trigger
Beam Find is used to reduce the vertical trace
deflection to within the graticule area for locating
off-screen and over-scanned traces. BEAM FlND
switch S390 adjusts the Delay Line Driver amplifier
biasing to limit the voltage swing at the crt plates.
When S390 (diagram 6) is in the normal position
(not pressed), the BEAM FlND voltage level on R226
is about 0.4 V. When the BEAM FlND switch is
pressed, the voltage level on R226 goes to about
-8.6 V. This level forces the output of
biases Q202 and Q203 such that
dynamic range is limited.
U225 LO and
the amplifier
Alternate Sweep Separation
The circuit consisting of Q283, Q284, Q285, and
associated components provides a means of
vertically positioning the Alternate (Magnified)
sweep, with respect to the
XI mode trace during
Alternate Horizontal Mode displays. During the
Alternate (Magnified) sweep interval, the
SEP
signal
from the Alternate Display switching circuit is LO,
and Q283 is biased off. This allows TRACE SEP
potentiometer R280 to affect the bias on one side of
a differential current source composed of Q284 and
Q285. The potentiometer supplies a dc offset
current to the Vertical Output Amplifier that changes
the position of the Alternate trace on the screen.
During the
XI Mode sweep interval the
SEP
signal is
HI (unasserted), and Q283 is biased on. The base
voltages of Q284 and Q285 are then the same, and
equal current is supplied to both sides of the
amplifier so that no offset of the trace occurs.
TRIGGER
The Trigger Amplifier, shown on diagram
3,
provides
signals to the Trigger Generator from either the
Vertical Preamplifiers, the EXT INPUT connector, or
the power line. The SOURCE switch selects between
Channel 1, Channel 2, line, or external trigger
sources. The
COLlPLlNG switch selects AC, DC,
LF REJECT, or HF REJECT trigger-signal coupling.
Signals from the Vertical Preamplifiers drive the
CH 1 and CH 2 lnternal Trigger Amplifier with
channel selection determined by the Vertical and
Horizontal MODE switches. Trigger
pickoff from the
Preamplifiers is accomplished by
U315B and U315C
for Channel
1
and U325A and U325B for Channel 2.
The circuitry associated with Channel 2 is the same
as Channel 1 except that it does not have a
trigger-
offset adjustment.
Differential vertical signals from the Channel 1 Pre-
amplifier go to
U315B and U315C. These emitter-
follower transistors each drive one input transistor in
U335. The collectors of the U335 input transistors in
turn supply emitter current to two pairs of
current-
steering transistors. The compensation and biasing
network connected between the emitters of the input
transistors in U335 is fixed for Channel 2 but not for
Channel 1. Potentiometer R338 in the emitter circuit
i
adjusts the bias levels of the two input transistors to
match the dc offsets of the Channel 1 and Channel 2
Trigger Amplifiers.
I
One transistor in each side of the output differential
amplifier pairs of U335 has its base bias set to zero
volts. The bias voltage of the other transistor in each
pair is controlled by the CH 1 TRlG signal from the
Trigger Switch circuitry. When the CH 1 TRlG signal
is LO, the transistors in each output pair with the
collectors connected together are biased on, and
the other transistors in the output pairs are off. The
collector signal currents of the conducting tran-
sistors are equal in magnitude but of opposite
polarity, so signal cancellation occurs. When the
I
CH 1 TRIG signal is HI, the other transistors in each
pair are biased on, and a differential signal is
developed across output load resistors R339 and
R340 to drive the lnternal Trigger Amplifier.
lnternal Trigger Amplifier
lnternal trigger channels are chosen by the SOURCE
switch being set to CH
1,
VERT MODE, or CH 2. The
logic function required to generate CH 1 TRlG and
CH 2 TRlG is performed by
U300, U304, CR300,
CR301, and CR302. External Trigger is selected by
the SOURCE switches being set to EXT, and
EXT=Z
or EXT or EXTI10. Line Trigger is selected by the
SOURCE switches being set to EXT and LINE.
Scam
by
ARTEK
MEDIA
+
To Next Page
Loading...