Genius 3693, ISO XFR-75W, 2093, 1493, 2793, 1793 Horizontal Raster Width Control Circuit Description

043 .

HORIZONTAL RASTER WIDTH CONTROL CIRCUIT DESCRIPTION.

The purpose of the horizontal width control is to:

1.Provide a convenient means for adjusting the horizontal raster size.

2.Correct pincushion distortion in the vertical axis.

3.Correct horizontal raster distortion caused by periods of high beam current.

The horizontal width control circuit is comprised of two main parts; The control circuit and the diode modulator (DM). The control circuit combines four signals in the monitor to produce the width control circuit. These signals are:

The power output stage of the horizontal width

control circuit is a high efficiency switching mode
driver. The FBT pulse is integrated by capacitor			057
through resistor		and level shifted by resistor
	095		058

to produce a saw tooth waveform. See waveform block TP 39. By connecting one input of the comparator, in the LM392 049 , to this sawtooth signal and the other input to the control amplifier a switched signal with a duty cycle dependent on the control voltage is produced at the output. Resistors 055 and 056 form a voltage divider which limits the control voltage amplitude to be within the sawtooth waveform. Resistor 060 acts as a pullup for the comparator output. Resistor 461 couples the power MOSFET 460 to the comparator. Capacitor 463 and resistor 464

1.Horizontal size - - - - - - H. Size Pot.

2.Vertical current (Iv) - - V. current feedback resistor

3.Vertical parabolic + Iv Vertical yoke return.

4.Beam current - - - - - - - EHT return on the FBT

The diode modulator controls the horizontal yoke current which affects the horizontal size. This is accomplished by the diode forward current. In effect, the diode shorts out the horizontal width coil to the extent of the diode forward current during the previous horizontal trace time. The current used to control the diode forward current comes from the diode modulator and is controlled by the control circuit and the switching mode driver.

The horizontal size voltage from the remote control PCB 490 is applied directly to the current node (LM392 Pin 5) of the control amplifier by resistor

For pincushion correction, two separate signals are used. The inverted vertical current waveform

(TP 34) and the yoke return waveform (TP 33). The yoke return waveform includes a parabolic and linear component. The inverse of the linear component is added to the yoke return waveform to correct the pincushion. The vertical current waveform (Iv) is inverted by an Op Amp and resistors 029 and 051 . Resistor 031 level shifts the inverted Iv to + 6V.

The (vertical parabolic + Iv) is AC coupled by capacitor

082and resistor 038 and 040 . It is then amplified by an Op Amp connected as a voltage

follower. Resistor	038	protects the Op Amp		against
arc related voltage spikes. Load resistors				and
			050

053prevent cross over distortion of the Op Amps by using only the current source transistors.

The inverted Iv and (parabolic voltage +Iv) are added to the current node of the control amplifier by resistors 041 , 042 , 030 , & 052 which then makes up the pincushion correction signal.

are connected as a snubber circuit to reduces noise due to rapid drain transitions.

When the MOSFET is on (gate voltage high) current increases in inductor 458 and when the MOSFET is turned off the current is dumped in to the 24-27V line through diode 462 . The magnitude of this current, from the diode modulator, is determined by the duty cycle of the MOSFET which is a function of the control voltage.

Diodes		477	and		478	with current equalizing
resistors			and			rectify the flyback waveform
	475			476

present on the GND referenced node of the horizontal tuned circuit. This current is conducted through inductor 457 and integrated by capacitor

456and then is controlled by the driver circuit. Diodes 477 and 478 are the diode modulator diodes and the forward current which the drive circuit controls is the current which determines the turn on delay of the GND referenced node of the horizontal tuned circuit. An increase in the current of diodes 477 and 478 produces a greater delay in the GND referenced node, and reduces the amplitude of the flyback pulse at this node, which results in an increased horizontal size.

Capacitors 441 and 442 are the primary horizontal tuning capacitors and must be the specified value for a given chassis horizontal frequency and yoke combination for proper operation of the monitor. Capacitors 437 and

439are the diode modulator horizontal tuning capacitors. Diodes 440 and 438 clamp the GND referenced node voltage to GND. Horizontal linearity coil 431 stores energy from the flyback pulse and injects it into the horizontal yoke in the reverse direction of the yoke current to decrease deflection at

the start of trace to balance the decreased deflection at the end of the horizontal trace due to I2R losses in the yoke during trace time. Capacitor 432 and resistor

428keeps the linearity coil from ringing after retrace.

The beam current from the FBT is converted to a voltage by resistors 009 and is filtered by capacitor 010 . Resistor 097 then connects the signal to the current node of the control amplifier, which accomplishes the blooming correction function.

These circuits are designed around a virtual ground, the +6 volt line. This line is generated by buffering a voltage divider 022 and 023 with an OP Amp.

Resistor 021 and capacitor 026 form the output filter.

The raster may be shifted by making solder connections: left SL or right SRR . The amount of the shift is set by solder connections S1 , S2 , & S4 . Inductor 447 permits only the DC current to pass to the yoke return. Resistors 423 , 424 , & 425 define the size of the shift together with the V+ plus 5V and V+ minus 5V supplies. Resistor 189 supplies a load on the V+ ±5V lines to avoid over-voltage of the filter capacitors. Resistors 185 , 195 act as fuses to protect the PCB in the case where both SL and SR connections are made.

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