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Chapter 2—Functional Description
Model 330, 340SC and 370SC Service Manual 2-27
The purpose for having an oscillator for the vertical sweep circuit is to maintain a sweep
even in the event of loss of vertical sync signal to prevent damage to the CRT. The
vertical oscillator has a free-run frequency of approximately 35Hz when there is no input.
Horizontal Phase Locked Loop
The incoming signal, /HSYNCR initially is sent through a Frequency to Voltage
converter. The output from the F to V is used to provide a program voltage to the PLL, set
the horizontal phase adjustment range, and to set the frequency of the horizontal power
supply. The Horizontal PLL, like the vertical oscillator, takes the input pulsed signal,
/HSYNCR, from the RTG, locks to it, and produces a pulsed output of the same
frequency. The Horizontal PLL has the additional function of controlling the phase of the
output signal relative to the input signal. To do this, the horizontal PLL receives an input
from the SCB via the serial bus that indicates the desired phase relationship. The
incoming signal /HSYNCR is then compared with the flyback pulse (derived separately
from the /HVFLBCK signal listed in the I/O section) to measure the phase relationship.
To control horizontal phase, the operator presses the PHASE button on the remote
control, then adjusts phase with the left and right arrow keys. As with the vertical
oscillator, the Horizontal PLL provides for a minimum free-run frequency in the event of
loss of horizontal sync signal. That frequency is approximately 12.5kHz.
Horizontal Centering
Horizontal centering of each raster (R, G, and B) is accomplished by applying a direct
current bias to each horizontal deflection coil. The DC comes from a programmable
current source that is in series with the main deflection coil. The current source is capable
of providing either positive or negative polarity. Control input to the current sources
comes from the SCB via the serial interface. Pressing the POS (Position) button on the
remote control, then selecting the desired color can independently control the centering of
each individual color. The left and right arrow keys are then pressed to adjust horizontal
position. When controlling the position, Green is a master, i.e.: when Green is selected,
all three-(3) colors move. Red and Blue are independent.
Horizontal Power Supply
The horizontal power supply is a switching power supply that provides an output voltage
proportional to the horizontal frequency and width. The variable output of the power
supply is a negative voltage providing a sink for the horizontal deflection current. The
trace speed of the CRT spot will be determined by the voltage applied across the
deflection coil. The voltage at the power supply output directly determines this in turn. So
when the output from the power supply increases, a given width of raster can be obtained
in a shorter period of time, thus supporting a higher horizontal frequency. In addition to
providing for the maintenance of a constant raster size for varying frequency, a variable
power supply is also necessary for control of the raster width. The output voltage of the
horizontal power supply is controlled primarily by two (2) inputs.