Tektronix 2215A, Инструкция по эксплуатации

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Theory of Operation—2215A Service
The Sweep Logic circuitry controls the holdoff time,
starts the sweep upon reception of a trigger signal, and ter­
minates the sweep at the proper sweep level. When using
P-P Auto or TV Field triggering, the Sweep Logic circuitry
will cause the Sweep Generator to free run, producing a
baseline trace if a trigger signal is not received within a
predetermined time period.
A Miller Sw eep Generator
The A Miller Sweep Generator produces a linear voltage
ramp that drives the Horizontal Amplifier. It produces this
ramp by maintaining a constant current through timing ca­
pacitors to obtain a linearly increasing voltage.
Field-effect transistors Q704A and Q704B are matched
devices with Q704B sourcing current for Q704A. Since the
gate and source of Q704B are connected together, the
source current of Q704A will be of a magnitude such that
there is no voltage drop across its gate-source junction.
When the sweep is not running, Q701 is biased on to
hold the timing capacitors in a discharged state. The low
impedance of Q701 in the feedback path holds the A Miller
Sweep output near ground potential. The voltage across
Q701, in addition to the base-emitter voltage of Q706, pre­
vents saturation of the output device.
A sweep ramp is initiated when Q576 is biased off. This
will bias off Q701 and the timing capacitors can charge at a
rate determined by timing resistors R701 and R702 and the
position of A SEC/DIV switch S701. One end of timing resis­
tor R701 is connected to the wiper of R721 and the other
end is connected to the input of the Miller integrator. Due to
feedback from the circuit output through the timing capac­
itors, the integrator input voltage remains fixed and estab­
lishes a constant voltage across the timing resistors. This
constant voltage, which produces a constant current
through the timing capacitors, results in a linearly increasing
voltage at the output of the A Miller Sweep circuit.
When the output reaches approximately 12 V, the Sweep
Logic circuitry will initiate the holdoff period in which Q701 is
turned on and the A Sweep Generator is reset. This holdoff
period is necessary so that the timing capacitors can be fully
discharged before another sweep starts. Capacitors C702
and C703 are always in the charging circuit and are used for
high sweep speeds. Capacitor C701A in series with C701B
are used for medium sweep speeds, and C701B is used for
slow sweep speeds.
The Sec/Div Variable circuitry utilizes an operational am­
plifier to maintain a constant reference voltage at one end of
R721 independent of the circuit load. The voltage applied to
the timing resistors varies with the rotational position of
R721, the SEC/DIV Variable control. A fixed dc voltage is
applied to the noninverting input of the operational amplifier
and feedback resistors R717 and R718 establish double
that voltage at the anode of VR720.
A Sw eep Logic
The A Sweep Logic circuitry controls sweep generation,
as a function of incoming trigger signals and the A Trigger
mode selected.
Incoming trigger signals from the output of U480B will
clock U502, a one-shot multivibrator, and cause the Q out­
put to go from LO to HI. If another trigger signal is not
received by U502 within a time period determined by the
time constant of R503 and C501, the Q output will return
LO. Whenever trigger signals are being received, the Q out­
put of U502 will bias on Q509 and illuminate TRIG'D LED
DS518. The output state of U502 is used in the Auto Base­
line circuit as described in the “P-P AUTO and TV FIELD”
section.
NORM. When NORM Trigger mode is selected, input pin
12 of U532D is held HI by S401B, causing the gate output
to also be HI. The output of U532C will then be LO and
U506A will not be held reset. Input pin 4 of U532A is held HI
by S401C, causing the output to be LO which places a LO
on the D input of U506A. Trigger signals received at the
clock input of U506A will then clock this LO to the Q output.
During the previous holdoff period, U506A had been set
by U532B so that the Q output went LO. This biased on
Q576 and the A Miller Sweep was prevented from running.
Whenever U506A is clocked following holdoff by a trigger
signal, the LO on the D input will be transferred to the Q
output and the Q output will go HI. This will bias off Q576
and the A Miller Sweep will generate the sweep ramp as
described in the ‘ A Miller Sweep Generator” section. When
the ramp voltage is about 12 V, Q525 will be biased on. The
output of U532B will change from LO to HI, setting U506A
and biasing on Q576. With Q576 conducting, holdoff one-
shot U504B will be triggered and the A Miller Sweep Gen­
erator will be reset to turn off Q525.
With U504B triggered, the Q output changes from HI to
LO and will stay LO for a time duration determined by the
Var Holdoff circuitry and the A SEC/DIV switch position.
VAR HOLDOFF potentiometer R9521 determines the
amount of charging current available to charge C518, 0519
or 0520 at pin 15 to the threshold voltage level on pin 14.
During the time the Q output is LO, the set input of U506A is
held HI so that no trigger pulses can initiate a new sweep.
When pin 15 of U504B reaches the threshold voltage on pin
14, the Q output goes HI to end the holdoff period and re-
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