Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 21
East/West correction
Principle of circuit operation
If the width of the picture were not varied during the line scanning process, it would result in a picture that
would be wide at the top and bottom, but narrow as it progresses towards the centre. This effect is
especially noticeable on large CRT’s due to the distance the beam has to travel. The circuit used to correct
for this distortion is called the east/west correction circuit and is described below.
At the end of line flyback time, the charge on C608 and C610 decays slowly to provide the first half of the
scan. When the centre of the scan is reached Q602 will switch on and the scanning coils are supplied with
current via the charging action of C613 ( the S correction capacitor ). Therefore, it can be seen that by
changing the charge on C610, the width of the picture can be changed.
Circuit operation
Pin 45 of IC201 provides a parabola waveform at field rate that contains all the correction signals supplied
from the NVM for picture geometry. This is added to the negative feedback signal from the output stage
and is fed into IC502
IC502 ( pins 1, 2 and 3 ) comprises a pulse width modulator whose non-inverting input, pin 3, determines
the width of the pulse. The signal present on the inverting input of the amplifier is a ramp waveform,
generated from a line pulse off the line output transformer.
At any instant in time, the voltage on the inverting input of IC502 will directly affect the timing of the signal
on the output, i.e. when the ramp waveform crosses the threshold set by the parabola, the output of the
amplifier will switch to a high level ( see the diagram below ). The maximum amplitude of the signal
connected to pin 2 ( ramp ) is three volts and the maximum level of the signal on pin 3 ( parabola ) is one
and a half volts. Therefore when the parabola is at its maximum level, the duty cycle of the output will be
50/50 and when the control voltage is lower the mark will increase to point set by its base line voltage.
Pulse width modulator
This ramp waveform is generated by the action of C516 and R519/520 under control of Q505. Initially the
base of Q505 will be low and therefore C516 will be allowed to charge via R519 and R520. When Q505
is turned on as its base rises above 0.65V, the charging will stop and the capacitor will discharge via R520
and the collector/emitter junction of the transistor. Because the charge time is approximately eleven times
that of the discharge time ( R519 + R520 as opposed to just R520 ), the ramp will be gentle compared with
the discharge.
Figure 22 : IC502 Ideal Waveforms
Drive Circuit
This resultant pulse width modulated waveform is now fed into the base of Q506. The collector of this
transistor is taken to the 150V supply via R526 and R527 which allows the voltage at this point to rise to
approximately sixty volts. The actual signal generated is a pulse, whose width is dependant upon the on
time of the pulse width modulated waveform, but as signal inversion takes place, the on time of the
modulating signal will result in the off time of the pulse.
C610 provides the earth return for the line scanning coils, there will also be a pulse waveform at this the