76FW-53H (serv.man13). DA100 and DW100 training course notes - Sharp TV Service Manual (repair manual). Page 13

Colour Television – DA100/DW100 Chassis

Sharp Electronics (UK) Ltd

Technical Support

September 99

Page 10

 V Prot Signal

At the point where vertical fly-back is initiated Q503 turns off and Q502 turns on allowing +25v to flow
from Q502 emitter to the field scan coils. Since there is –13v on the other side of the scan coils
derived from the vertical drive signal, we effectively have 38v across the field coils, which will force the
electron beam to the top of the screen.

The vertical drive signal that is outputted from IC801 pin 31 has a negative going pulse in addition to
the normal ramp waveform. This negative pulse is used to initiate vertical fly-back. This drive signal is
applied to the emitter of Q505 via R508. The potential divider R526 and R514 will fix Q505 base
voltage so that it will only turn on during the period of the negative going pulse. Once Q505 turns on
its collector voltage will fall turning on Q507 taking its emitter to ground, which results in:

Q503 turns off due to the removal of gate bias
Q502 turns on allowing +25v at the junction of Q502 collector and the scan coils due to the action
D507 conducting secondly enabling D507 to conduct.

From the Junction Q502 collector and
the scan coils the vertical fly-back pulse
is applied to IC801 pin 11 (Vprot Signal)
as an indicator of vertical output stage
operation. If IC801 cannot detect the
negative edge of this pulse the IC801 will
presume that the vertical stage is not
operating correctly and blank the RGB
drive.

+25v

+5v

VProtect

Field Scan Coils

R526
4k7

R514
390

R508
680

Q505

R528
100k

R504
39k

Q507

R547
3k9

C512
220nF

R505
2k2

R506
120

R517
15k

R507
8k2

D507
12v

Q502

D508

Q503

Drive

Field Output