VERTICAL FLY-BACK 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. +25v
+5v
R504 39k Q507 Q505 R528 100k
R547 3k9
R505 2k2 Q502 R506 120 D507 12v
D508
R526 4k7
VProtect
R514 390
R508 680
C512 220nF
R517 15k
Field Scan Coils Q503
Drive
R507 8k2
Field Output
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.
V Prot Signal
Sharp Electronics (UK) Ltd Technical Support September 99 Page 10
