Theory of Operation—2213 Service
When S611 is set to NORM, +8.6 V is applied through
the switch to R525 and R517. Transistor Q519 is biased
into saturation by the positive voltage, and both CR503
and CR504 become forward biased. This action reverse
biases peak detector transistors Q503 and Q504 to prevent
the trigger signal from affecting the TRIGGER LEVEL
control range.
With CR503 and CR504 forward biased, the voltage
divider network (R525, R527, R526, and R528) sets the
input voltage to U507A pin 3 and U507B pin 5. A fixed
positive output voltage from U507A pin 1 is applied to one
end of R455, and a fixed negative output voltage from
U507B pin 7 is applied to the other end of R455.
Trigger Level Comparator
The Trigger Level Comparator circuit determines both
the trigger level and slope at which a triggering signal is
produced. Transistors U460E and U460B form a com
parator circuit. It compares the trigger signal level applied
to the base of U460E with the reference dc level set by the
TRIGGER LEVEL potentiometer (R455) and applied
to the base of U460B. Slope switching is accomplished by
controlling the biasing of transistor pairs U460A-U460D
and U460C-U460F.
When AUTO or TV FIELD triggering is selected, the
Auto Trigger circuit maintains a dc level range at the base
of U460B that is dependent upon the amplitude of the
trigger input signal. In this instance, the Comparator
(U460E and U460B) determines the point on the input
trigger waveform at which the Schmitt Trigger circuit will
produce an output.
When NORM triggering is selected, the TRIGGER
LEVEL potentiometer (R455) is set manually to a dc level
that will produce a trigger signal at the output of the
Comparator. If the trigger signal amplitude at the base of
U460E is below the reference level, the Schmitt Trigger
circuit will never switch. If the trigger signal is above the
reference level, the Schmitt Trigger circuit output will
switch HI and remain HI until either the trigger signal is
decreased or the reference dc level is increased.
The TRIGGER SLOPE switch (S464) controls the
bias on U460C and U460F. When set to the positive slope
position, the ground is removed from the bottom end of
R464, and the forward bias is then determined by the
voltage divider formed by R462 and R463. Both U460C
and U460F are biased into conduction and carry the signal
current from the Comparator transistors. Moving the
SLOPE switch to the negative slope position grounds the
bottom of R464 and reduces the bias level of U460C and
U460F. The fixed bias level on the bases of U460A and
U460D is now higher than the bias on U460C and U460F
so that U460A and U460D carry the signal current from
the Comparator transitors. The collectors of U460A and
U460D are cross connected to the collectors of U460F and
U460C, so the resulting trigger signal output is inverted.
Inverting Amplifier and TV Trigger Circuit
Current from one transistor of the conducting pair of
transistors chosen by SLOPE switch S464 is applied to
U480C pin 10. Current from the other side of the Com
parator is applied to pin 14 at the output side of U480C
through R468. Pin 11 of U480C is at a LO logic level
except when TV FIELD triggering is enabled. This LO
does not affect circuit operation in either AUTO or NORM
triggering.
NOR-gate U480C is an emitter-coupled logic (ECL)
device that is operated in the linear region. In the linear
region, U480C acts as a high-speed inverting amplifier.
Common-mode signals such as noise or thermal drift in
the Comparator output signal are cancelled by U480C
and associated circuitry. These types of offsets equally
affect the outputs from both sides of the Comparator.
Changing current to pin 10 of U480C causes a correspond
ing voltage change at U480C pin 14. The voltage change
at one end of R468 is equal in amount and opposite in
direction to the voltage change at the other end since the
same common-mode signal from the other half of the
Comparator is applied to the other end of R468.
When the TRIGGER MODE switch is set to TV FIELD,
+8.6 V is applied to the TV Trig Enable signal line. Tran
sistors Q474 and Q476 are biased on via R474, and U480C
pin 11 is set HI, causing the output of U480C at pin 14 to
be LO.
Current flowing through R466 from either U460C or
U460D causes a voltage drop that establishes the bias
voltage on the base of 0474. Current flowing through R473
and R472 produces a voltage drop across R473 that estab
lishes the bias voltage on the base of Q476. The circuit
components are selected such that when the Comparator
output voltages from both halves are equal, the base
voltages to both Q474 and Q476 will be the same. With
equal base voltages, each transistor will conduct an equal
amount of current.
When the Comparator output becomes unbalanced, due
to an input trigger signal, unequal biasing of Q474 and
Q47G occurs. In response to a changing bias condition, the
collector currents vary proportionally.
3-14
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