Toshiba TW40F80 manual H Blanking, Outline, Theory of Operation, Circuit Operation

4-3. H Blanking

4-3-1. Outline

The H blanking circuit applies a blanking precisely for the horizontal flyback period so that undesirable pictures fold- ing does not appear at screen ends.

This unit allows the users to adjust an horizontal amplitude adjustment, so, picture quality at screen ends will be im- proved. This is one of the purposes of the blanking circuit.

Q487

ON period

4-3-2. Theory of Operation

The H blanking circuit determines the flyback period pre- cisely from the AFC pulse in the FBT and applies the period to emitter of the video output stage transistor on the CRT-D PC board.

4-3-3. Circuit Operation

As can be seen from Fig. 9-12, the flyback period of the AFC pulse in the FBT starts at a negative side from 0V. To detects this, the DC component is cut with C493. This is, C493 is always charged through D487 with a negative side (about –17V) of the AFC pulse. As a result, a voltage at point A in the waveform rises from the ground level. This wave- form is sliced in a circuit (R486, D486) to detect the flyback period. Thus obtained voltage is applied to Q901, Q911, and Q921 through D904, D914, D927 and cuts off them thereby blanking the resters.

			D486
0V			Slice level
Approx.
-17V	AFC Pulse	Waveform at
		point

Fig. 9-12

Point A

T461 (FBT)

AFC C493 10

L410	R486
	D486
R409
D487

+35V

Q487

R417

R438

Q921

D927	BLUE
	CRT/D
10
10	Q911
D914	GREEN
	CRT/D
10

10

Q901

	R906	D904	RED
			CRT/D
6	P904	CRT-D DCB

7 P903 Deflection/Power PCB

Q489

Q488

V blanking

Fig. 9-13

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