Genius 2793 User Manual

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.

HORIZONTAL RASTER WIDTH CONTROL CIRCUIT DESCRIPTION.

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.

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

043

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:

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

2. Correct pincushion distortion in the vertical axis.

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

The purpose of the horizontal width control is to:

EHT return on the FBT

Vertical yoke return.

V. current feedback resistor

H. Size Pot.

Beam current - - - - - - -

Vertical parabolic + Iv

Vertical current (Iv) - -

Horizontal size - - - - - -

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

082

and 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

050

and

053

prevent 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.

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

095

and level shifted by resistor

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

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

475

and

476

rectify the flyback waveform

present on the GND referenced node of the
horizontal tuned circuit. This current is conducted
through inductor

457

and integrated by capacitor

456

and 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

439

are 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 I R losses in the
yoke during trace time. Capacitor

432

and resistor

428

keeps the linearity coil from ringing after retrace.

The raster may be shifted by making solder
connections: left

or right

. The amount of the

shift is set by solder connections

, &

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

and

connections are made.

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.

490

029

051

031

040

038

082

050

053

041

043

042

030

052

009

010

097

095

058

055

056

460

464

463

461

060

049

458

462

457

456

441

442

440

438

431

432

428

447

423

424

425

185

195

189

057

022

023

026

021

437

439

477

478

475

476

477

478

477

478