Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 27
Vertical Stage
Vertical time base generation
A R/C oscillator within IC201 provides a vertical drive signal for the vertical drive and output stages .
External components are used to control the stability and phasing of this signal ( note that the external
crystal is not used as a reference source - the vertical stage will operate with this component disconnected
from the circuit ) When a video signal is present, this oscillator is locked to the incoming vertical
synchronisation pulse.
Figure 28 : Vertical Stage Block Diagram
The oscillator has two outputs that exit IC201 from pins 46 ( V OUT P ) and 47 ( V OUT N ). Pin 47
provides the main drive signal, while pin 46 provides the correction signal, i.e. the vertical geometry signal.
The geometry settings are contained within the non volatile memory and are accessed via the I C bus and
2
the microprocessor.
As can be seen from the diagram on the next page, the vertical drive signals from IC201 pass into the
operation amplifier IC502 on pins 5 and 6. This amplifier sums the two inputs, the resultant output being
fed into pin 6 of IC501. Feed back from the vertical stage comes from the bottom end of the vertical
scanning coils via R516 and R539. This is to ensure that the vertical stage does not drive the coils
incorrectly due to inefficiencies within the vertical drive or output circuits, i.e. it provides negative feed
back. Also connected to this feed back point is the output from pin 47 of IC201, this pin is effectively the
frame correction signal.
The input to the second operational amplifier, IC501, is also affected by the vertical scanning coils drive
signal ( top end of the vertical scanning coils ). This enters via R511 ( to the inverting input on pin 6 ) and
R509 ( to the non-inverting input on pin 5 ). This provides positive feed back and therefore will cause the
stage to oscillate. This oscillation is free running, but remains at about 140kHz ( this is set mainly by C509
and R509 ). The output from pin 7 is a square wave whose frequency and mark/space ratio is dependant
upon a number of factors. One is the drive signal itself, two the frame geometry settings and three the
frequency of oscillation generated by the feedback to IC501. This signal then drives Q503 and Q504 in
the frame drive circuit described in the next section.
Both IC501 and IC502 are supplied on pin 8, +5V and 4, -15V. This ensures that the output from pin 7
of IC501 is swinging both positive and negative.
Vertical geometry is set using the software service mode and therefore all adjustments are stored in the
NVM, IC104. Please refer to the Service Set Up Mode section of these notes for information on how to
set the geometry of the vertical stage output.