Tektronix 2213, Инструкция по эксплуатации

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Theory of Operation—2213 Service
The collector current changes from Q474 are filtered
by a network composed o f C476, C477, R477, and R478.
The filte r network rejects TV video information and
averages the TV horizontal-sync pulses. Setting the trigger-
level threshold at near the center o f the horizontal-sync-
pulse swing establishes the untriggered level. When the TV
vertical-sync block occurs, the output o f the filte r rises to
a level th at w ill cause the Schm itt Trigger circuit to switch.
Precise TV field synchronization is obtained as a result o f
the filtering action.
The o utput signal from the filte r is applied to U480B
pin 6. The Schm itt Trigger circuit responds only to the TV
sync signal because pin 7 is held LO by the output o f
U480C.
Schmitt Trigger Circuit
With a LO on U480B pin 7, the output at pin 3 goes
LO as soon as the signal on U480B pin 6 reaches the
switching threshold. The LO is applied to U480A pin 4 and,
together w ith the fixed LO on pin 5, causes the o utput o f
U480B pin 6 via R480 to reinforce the switching action.
As a result, the output signal at U480A pin 2 switches
rapidly.
When the level from the filte r network falls to the LO
threshold level, the feedback supplied by R480 holds the
Schmitt Trigger switched HI fo r a short time. The amount
o f time involved prevents noise occurring exactly at the
threshold level from causing false triggering.
When either AUTO or NORM triggering is selected,
input pin 6 of U480B is held LO, and the Comparator
output signal on U480B pin 7 supplies the input to the
Schmitt Trigger circuit.
The output o f the Schm itt Trigger circuit is obtained
from U480D pins 9 and 15. The differential output signal
derived from U480D is applied to a two-transistor level-
shifting circuit composed o f Q492 and Q493. The level-
shifting circuit converts the ECL logic levels to T T L logic
levels required fo r the Sweep Generator. A signal obtained
from the collector o f Q493 is used to drive the Auto
Baseline circuit.
Auto Baseline Circuit
The A uto Baseline circuit (composed o f U640A, Q605,
and associated components) is enabled in both AUTO and
TV FIELD triggering modes. This circuit provides a signal
to the Sweep Generator circuit (Diagram 5) that initiates
a sweep if a triggering signal is not received by the Schmitt
Trigger circuit w ith in a period o f about 100 ms. A second
output from the circuit illuminates the TR IG 'D LED on
the instrument fro n t panel when the sweep is triggered.
When adequate triggering signals are being received, the
output o f Q493 is applied to pin 5 o f monostable m ulti­
vibrator U640A. The negative-going edge o f the signal
causes pin 6 o f U 6 4 0 A to switch HI. The HI forward biases
CR615, and Q605 is then biased into conduction. With
Q605 conducting, the Auto Baseline signal line is held LO
to prevent the Sweep Generator circuit from free running.
The amount o f time th a t pin 6 o f U 640A stays HI
w ith o u t receiving an input signal is determined by tim ing
components R614 and C614. If a trigger signal is not
received in about 100 ms, pin 6 o f U640A w ill go LO and
Q605 w ill be biased o ff. The A u to Baseline signal line then
goes HI through pull-up resistor R610, and the Sweep
Generator free runs to produce the baseline trace.
In NORM triggering mode, the A uto Disable signal
(+8.6 V) is applied to the base circu it o f Q605 via CR611
and R611. The signal holds Q605 forward biased and
prevents the Sweep Generator from free running.
The other function o f the A uto Baseline circuit is to
illuminate the TR IG 'D LED when the sweep is properly
triggered. As long as U640A pin 6 remains HI (triggering
signals occurring w ith the proper tim e), T R IG 'D LED
DS618 w ill be illuminated. The trigger mode in use does
not affect the operation o f the T R IG 'D LED.
SWEEP GENERATOR AND LOGIC
The Sweep Generator and Logic circuitry, shown on
Diagram 5, produces a sawtooth voltage that is amplified
by the Horizontal A m plifier to provide horizontal deflec­
tio n on the crt. This sawtooth voltage (sweep) is produced
on command from the Sweep Logic circuits. The Sweep
Generator circuits also produce gate waveforms that are
used by the A uto Intensity and Z-Axis circuits to establish
the correct tim ing o f the c rt unblanking and intensity
levels used fo r viewing the display. See Figure 3-5 fo r the
Sweep tim ing diagram.
The Sweep Logic circuitry controls the h o ld o ff time,
starts the sweep upon reception o f a trigger signal, and
terminates the sweep at the proper sweep level. When using
AUTO or T V FIELD triggering, the Sweep Logic circuitry
w ill cause the Sweep Generator to free run, producing a
baseline trace if a trigger signal is not received w ith in the
predetermined time period.
Miller Sweep Generator
The Miller Sweep circuit is composed o f Q630A, Q630B,
Q631, and associated tim ing components. The circuit
operates to hold the charging current to the tim ing
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