Theory of Operation-2225 Service
In P-P AUTO, C431 charges to the negative peak of
the analog trigger signal, and
Q435 will only switch
on if the base drive to U435 is less than that of
U435B. If Q435 switches on, then C431 will
discharge to a more negative voltage so the output
of
U425A will track the negative peak of the analog
trigger signal. In NORM mode,
U415E switches on,
and C431 charges to about -3 V via
CR431. Trigger
LEVEL control
R426 selects a trigger level voltage
between the peak rectifier outputs to give trigger
operation over a sufficient dynamic range.
Coupling Circuit
The Trigger Amplifier is optimized for bandwidth, not
dynamic range. A current is added to the summing
stage of
Q363 and Q365 (via R397 and R398) to shift
the desired switching point on the analog trigger
signal to the threshold of the Schmitt Trigger circuit
(fixed at zero volts). The selection of current drivers
to feed the Trigger Amplifier is achieved by emitter
switching of differential pairs
U445C and U4450,
U445A and U445B, and U435C and U435D. In
NORMAL DC coupling, a fixed current proportional to
the voltage on the LEVEL control is passed to the
summing stage by
U445C and U445D. This is
enabled by logic signal DC from
U308A being HI to
bias on
(2420.
In NORMAL AC coupling, the dc component of the
analog trigger signal is extracted by a low-pass filter
circuit
R470, C471, C472, and U415C. The dc
component is added to the LEVEL voltage, and the
result is fed into amplifier
U450A. The output of
U450A controls differential pair U435C and U435D
and completes the feedback loop that adjusts the
offset current so that the input of
U450A is held at
zero volts. This forces the DC component of the
analog trigger signal to be equal and opposite to the
LEVEL voltage, giving AC coupling with DC shift. LF
REJECT operates in exactly the same way, except
that the time constant of the low-pass filter is
changed by switching off
U415C, allowing C473 to
dominate the circuit. P-P AUTO operates by
establishing a feedback loop with
U450B to hold the
voltage on LEVEL at zero.
Note that
P-P
AUTO does
not distinguish between DC and AC coupling.
Trigger Level Comparator
The Trigger Level Comparator compares the level of
trigger signals selected by the Trigger SOURCE
switch to a zero voltage level. Positive- or
negative-
slope triggering is selected by the front-panel
Trigger SLOPE switch.
The analog trigger signal drives the base of
U460B.
The transistors of U460 form a differential amplifier.
With the input to
U460E grounded, it is effectively
a
"single-ended" to differential amplifier. The cross-
coupled collector outputs can reverse the direction
of the signal fed to the succeeding stage depending
on the selection by the SLOPE control.
Schmitt Trigger and TV Trigger Circuit
This circuitry generates a signal that drives the
Trigger Logic as a function of the Trigger Level
Comparator output signal and the Trigger MODE
switches.
The output signals from the Trigger Level
Comparator drive Q400 and
(2401. These transistors
are configured as a current mirror that converts the
differential output to a single-ended current to drive
amplifier
U480C. Slope Balance potentiometer R481
corrects for dc offsets between positive and
negative slopes. Shunt feedback amplifier
U480C
converts a current input to a voltage output to drive
the input of the Schmitt Trigger,
U480D, through
R485. Positive feedback for the Schmitt Trigger is
provided by Trigger Sensitivity potentiometer
R489,
and C489 reduces trigger jitter by increasing positive
feedback at higher frequencies. The setting of R489
determines the circuit hysteresis.
When TV FIELD is not selected, the
l'vF
signal
connected to R487 is HI (unasserted)
.
Transistors
Q488 and Q489 are biased off, and a LO is placed
on one input of
U480A by R492-R493. This LO input
will cause
U480A to invert the output from U480D.
With Q489 off, a LO will be placed on one input of
U480B by R495, and U480B will also act as an
inverter. The Trigger signal at the output of
U480B is
therefore the same as the input signal to
U480A.
When TV FIELD is selected, the line is LO
(asserted). The outputs of
U480D will determine the
conduction states of Q488 and
(2489, and the input
of
U480A connected to R492 will be HI. The output of
U480A will be LO, and U480B will invert the signal at
its other input. Signals at the collector of
Q489 are
filtered by
C495, R495 and C496 to reject TV Video
I
information and average the TV horizontal-sync
pulses. Setting the trigger-level threshold near the
center of the horizontal-sync-pulse swing
establishes the untriggered level. When the TV
vertical-sync block occurs, the output of the filter
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