Theory
of
Operation-2225 Service
the Channel Switching circuit is applied to the Delay-
line Driver stage. This stage converts a current input
into a voltage output and provides an impedance
match for the Delay Line. The Delay Line produces
approximately
90
ns of delay in the vertical signal.
This delay allows time for the Horizontal circuitry to
start the sweep before the vertical signal is applied
to the crt, so that the operator can see the signal
that triggered the sweep.
Final amplification of the vertical signal is done by
the Vertical Output Amplifier. This stage produces
the signal levels that vertically deflect the crt elec-
tron beam. The upper frequency response of the
Amplifier can be reduced by enabling the
XI0
Gain
circuitry. For locating the position of off-screen dis-
plays, the dynamic range of the Amplifier can be
limited with the Beam Find circuitry. This circuitry
also intensifies the trace and limits horizontal
deflection.
Triggering
The Trigger circuitry uses either the Internal Trigger
signal obtained from the input
signal(s), an External
Trigger signal, or a Line Trigger signal derived from
the ac-power-source to develop trigger signals for
the Sweep Generator. The P-P Auto Trigger circuit
sets the range of the Trigger Level to conform
approximately to the peak-to-peak amplitude of the
selected trigger signal when either Auto or TV Field
Trigger mode is selected. This allows triggering on
most signals without needing to adjust the TRIGGER
LEVEL control. In Norm mode, the TRIGGER LEVEL
control must be adjusted to the signal level before a
sweep will be triggered.
The triggering circuitry contains the TV Field Sync
circuit. This circuit provides stable triggering on
television vertical-sync pulses when in the TV Field
triggering mode. TV Line triggering is possible using
P-P AUTO trigger mode.
Sweep
The Sweep Logic circuit controls the sweep
generation and Z-Axis unblanking for the Sweep dis-
play. When the TRIGGER Mode switches are set to
either P-P AUTO or TV FIELD and no trigger signal is
present, the Auto Baseline circuit causes the Sweep
Logic circuit to produce a sweep for reference
purposes. In the NORM setting, the Auto Baseline
circuit is disabled and sweeps are not generated
until a trigger event occurs. This is useful for trig-
gering on low-repetition rate signals. The SGL SWP
(single sweep) trigger mode allows only one sweep
to be generated after being reset. Following the
single sweep, the Trigger circuit is disabled until the
SGL SWP RESET button is pressed again.
The Sweep Logic circuit controls the operation of the
Miller Sweep Generator circuit. The Sweep circuit
produces a linear sweep with a ramp time that is
controlled by the
SECIDIV switch setting. The sweep
signal is applied to the Horizontal Preamplifier for
initial amplification and then to the Horizontal Output
Amplifier to drive the crt horizontal deflection plates.
Horizontal
The Horizontal Preamplifier gain is increased by a
factor of
5, 10,
or
50
when the Horizontal MAG
control is used. Horizontal positioning of the display
is accomplished in the Horizontal Preamplifier
circuit.
In the
X-Y
mode of operation, the Channel
1
signal
from the internal Trigger circuitry passes through the
X-Y
Amplifier to the Horizontal Preamplifier. In this
operating mode, the Channel
1
Internal Trigger
signal supplies the horizontal deflection to the crt,
and the Miller Sweep circuit is disabled to inhibit
sweep generation.
The Z-Axis drive from the Sweep Logic circuit is
applied to the Z-Axis Amplifier. The output signal
from the Z-Axis Amplifier circuit sets the crt
intensity. When using Chop Vertical mode, a
blanking signal from the Chop Oscillator circuit
blanks the crt display while switching between the
vertical channels.
The DC Restorer circuit applies the output voltage of
the Z-Axis Amplifier between the cathode and grid
of the crt. High dc potentials on these elements
prohibit direct coupling to the crt.
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