3DQDVRQLF
24
1.3 Horizontal deflection - FBT
1.3.1 General
description:
CRT produces an image on the screen when a controlled electron beam is focused on the screen
phosphors. To achieve this, the electrons must first be created and accelerated towards the screen
using a high voltage electric field. The movement of the beam must then be controlled so as to sweep
the entire screen from the top left-hand corner to the bottom right-hand corner.
The FBT carries out three functions:
1. Supplying the horizontal deflection coils with the sawtooth waveform current necessary to effect the
line sweep
2. Supplying high voltage to the tube anode and voltages in the focus control grid and G2
3. Supplying subsidiary voltages by means of secondary windings
1.3.2 Horizontal deflection
Basic
operations
Tr
D
Ct
Ly
Cs
110/123V
HV
T402
FBT
L1
From
TDA8374
Pin 40
Fig. 17 : Basic horizontal deflection circuit
with:
Tr : 0401 Transistor
D : D409
Ct : C409//C410
Cs : C411
The horizontal deflection output circuit uses the transistor switching function, combined with a damper
diode. The horizontal driver pulses from the TDA8844 are amplified in the horizontal drive circuit to get
sufficient base-drive current for the high voltage switching transistor Q401.
The horizontal deflection output circuit makes sawtooth wave current flow through a deflection coil(L
y
).
The electric field which controls the electron beam is proportional to the current flowing in the
deflection yoke. This current, called i
y
, has a sawtooth waveform to allow the electron beam to sweep
the entire screen from the left-hand side to the right-han4 side as shown in fig. 19. During the retrace
time (between t
2
and t
4
), the CRT has received the blanking command and consequently, the retrace
of the electron beam does not appear on screen
There is a (damped) linearity corrector coil (L405) in series with the horizontal deflection coil. During
the scan, there is some loss in the resistance of the deflection coil In the first part of a line, the linearity
coil stores some energy in a permanent magnet until it is saturated This improves the linearity of the
horizontal scan speed.
Waveforms obtained
Figures 18 and 19 present all the waveform produced by the operations as mentioned in the next
paragraph.
t
1
t
2
t
4
t
3
t
2
t
4
t
7
Fig. 18 : The electron beam sweep the entire screen
