Theory of Operation-2225 Service
crt. The amplifier formed by Q780, Q785, and Q789
drives the left crt horizontal deflection plate in the
same manner as described above, with
Zener diode
VR792 shifting the collector signal level of Q780 to
the correct level to drive the emitter
Q785.
Horizontal Beam Find
The BEAM FlND switch is buffered by emitter follower
Q776. Diodes CR780 and CR790 are normally
reverse biased by R776 when BEAM FlND is off.
When BEAM FlND is active, Q776 is turned on, and
its emitter is driven negative to about -8 V. The volt-
age on the cathode of
VR776 drops to about 5 V,
causing
CR780 and CR790 to be forward biased.
Current through
CR780 and CR790 cause the output
common-mode voltage of the two shunt-feedback
amplifiers to be shifted negative to reduce the
available voltage swing at the crt plates. This stops
the trace from being deflected off-screen
horizontally.
FRONT PANEL
termination for the input signals and isolates the
signal sources from the rest of the Z-Axis Amplifier.
Common-base transistor Q829 passes a constant
current through
R832. This current is divided
between Q825 and
Q829, with the portion through
Q829 driving the shunt-feedback output amplifier
formed by
Q835, Q840, and Q845. The bias level of
Q825 therefore controls the emitter current available
to
Q829. Feedback-resistor R841 sets the
transresistance gain for changing the input current
to a proportional output voltage. Emitter-follower
Q835 is dc coupled to
Q840; and, for low-speed
signals, Q845 acts as a current source. Fast tran-
sitions couple through
C845, providing added
current gain through Q845 for fast voltage swings at
the output of the amplifier.
External Z-Axis input voltages establish proportional
input currents through
R823, and amplifier sensitivity
is determined by the transresistance gain of the
shunt-feedback amplifier. Diode
CR823 protects the
Z-Axis Amplifier if excessive signal levels are
applied to the EXT INPUT
OR
Z connector.
The INTENSITY potentiometer controls the base
The Front Panel circuitry is shown in diagram 6.
voltage of Q804 to set the amount of emitter current
Many of the switches and potentiometers are also
that flows through that transistor and, therefore, the
shown on the other schematic diagrams adjacent to
level of the Z-Axis signal.
the circuitry controlled. Diagram 6 provides a
diagram of the complete Front Panel to aid in
servicing that circuit board. The active circuitry on
the Front Panel includes the External Trigger buffer
Amplifier,
Q370B and Q370A, and the Horizontal
Position Control current source,
Q725. Operation of
the FET External Trigger Buffer Amplifier is similar to
the Channel 1 and Channel 2 Source Followers
described previously.
All mode switching for the Vertical, Horizontal, and
Trigger circuitry is done by the Front Panel switches.
Z-AXIS AMPLIFIER
The Z-Axis Amplifier, shown on diagram 7, controls
the crt intensity level via several input-signal
sources. The effect of these input signals is either to
increase or decrease trace intensity or to
completely blank portions of the display. The Z-Axis
signal current as determined by the Z-Axis switching
logic and the input current from the
EXT INPUT
OR
Z
connector (if in use), are summed at the emitter of
common-base amplifier
Q825. The summed current
thereby sets the collector current of the stage. The
common-base amplifier provides a low-impedance
When the sweep is displayed, the emitter of Q817 is
LO, causing
CR817 to be reverse biased. Diodes
CR816, CR821, and CR820 are also reverse biased.
This allows the current through
R818 to flow through
CR818 and turn on the Z-Axis.
When X-Y is displayed,
CR817 and CR816 are for-
ward biased, reverse biasing
CR821 and CR818.
Diode CR819 is reverse biased, allowing the intensity
to be set by the current through R820 and
CR820.
When ALT MAG is selected, diodes CR816, CR817,
CR819, and CR822 are all reverse biased, allowing
the intensity to be controlled by the current flowing
through R818 and
R821. This action therefore
increases the intensity of the MAG trace.
When CHOP Vertical MODE is selected, the CHOP
BLANK signal is sent to the collector of Q825 through
CR824 during the display-switching time. Diode
CR825 is reverse biased, and the forward bias of
Q829 rises to the blanking level. When blanked, the
output of the Z-Axis Amplifier drops to reduce the
crt beam current below viewing intensity.
At high beam currents, the crt cathode voltage
tends to drop off slightly. To compensate for this,
Scans
by
ARTEK
MEDL4
=>
To Next Page
Loading...