McIntosh MPI 4, Руководство пользователя, Страница: 22 / 24

FIG. 34 LOW PASS FILTER RESPONSE
trace mode the right channel is offset from the left channel
by an amount determined by the TRACE SEPARATION
control. In the single trace mode the switching amplifier
passes the desired channel (left or right) to the vertical
amplifier.
Vertical and horizontal amplifiers are identical wide band
DC coupled amplifiers. Each consists of two cascaded
differential pairs. The input pair is a Darlington configura-
tion for high input impedance. The second pair comprises
high-voltage transistors that drive the deflection plates
of the cathode ray tube.
AUTOMATIC INTENSITY CONTROL
An automatic intensity control circuit is connected to
the output of the horizontal amplifier and senses the AC
component of the horizontal output. The circuit works with
the INTENSITY control; when modulation is not present
intensity is reduced to avoid burning the phosphor on the
face of the cathode ray tube. At all but full intensity set-
tings the spot is extinguished. At full intensity the spot
does not extinguish so it may be located and adjusted for
position on the screen.
TRIGGER AMPLIFIER
The trigger amplifier operates in single and dual trace
modes. Depending on the TRIGGER SOURCE switch set-
ting, it uses left or right preamplifier outputs, or line
frequency signals, to provide a steady presentation of small
vertical deflection signals on the cathode ray tube. The
trigger amplifier is a two-transistor high gain amplifier
whose output voltage is compatible with the digital IC
in the sweep generator.
SWEEP GENERATOR
When the sweep is triggered a linear ramp is generated
by an R-C network consisting of a high-voltage source
charging a capacitor through a high-value resistor. The
resistor value is set by the SWEEP control, while the capaci-
tor value is selected by SWEEP FREQUENCY push-buttons.
The high-value resistor and high voltage effectively serve
as a current source.
The linear ramp is fed into an emitter follower which
provides the time base output and also drives a differential
pair. The amplitude of the time base output is controlled
by the SWEEP EXPANSION. The differential pair senses
when the ramp has reached a certain voltage level. In
single and dual trace modes this level is a fixed voltage
and it determines when the sweep terminates. In the level
mode this voltage corresponds to alternate outputs from
the two peak detectors and determines the height of the
two columns. When this voltage is reached, the differential
pair generates a trigger hold-off pulse long enough to
discharge the capacitor in the R-C network. During the
hold-off period the sweep cannot be restarted and the
cathode ray tube is blanked.
The hold-off pulse feeds one-half of a dual type D flip-
flop and sets the flip-flop. When the hold-off pulse termi-
nates, the flip-flop allows the capacitor to charge again
after it receives a signal from the trigger amplifier. The
other half of the flip-flop switches each time the ramp
terminates the sweep. This half generates a square wave
signal that drives the Left-Right switching amplifier. During
the LEVEL mode the square wave alternates the column
position to the left and right for a two column display.
POWER SUPPLIES
There are five power supplies in the MPI 4: -5 volts;
+ and -- 15 volts; 200 volts; and -1000 volts. Each is
regulated to assure that the calibrated display does not
change due to line voltage variations.
The - 5 and - 15 volt supplies are derived from a full-
wave rectifier and are regulated by zener diodes. The 200
volt supply is derived from a half-wave rectifier and is
regulated by an active voltage regulator. The -1000
volts supply, which provides cathode ray tube anode
acceleration voltage, is obtained from a zener-regulated
voltage doubler.
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