Dynamic Focus Block - Sony DTV-01 Manual

Dynamic Focus Block

Static Focus
An electron beam within the picture tube consists of many electrons that
are slowed down by the focus electrode and then accelerated by the fol-
lowing (G5) electrode into a fine point at the screen. The focus point is
positioned by adjusting the voltage at the focus electrode relative to the
accelerating voltage. The G5 voltage is usually fixed at the HV potential
from the flyback secondary.
Electrodes
Focus Accelerating
Elect
Beam
G5
Picture Tube Focus
The focus points form an arc as the electron beam is moved from side to
side (swept) by the magnetic field created by the external horizontal de-
flection yoke. The picture tube glass screens were made into a similar arc
(actually sphere) to maintain focus.
Picture Tube top view
Yoke
Electron
Beam
Focus Arc
Modern picture tube screens are flat to reduce annoying room glare. This
means the focus point must be moved up at the left and right sides to
meet the flat picture tube screen.
focus
right side
Flat picture tube screen
left side
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Dynamic Focus
DC Voltage for Static Focus Voltage (fv) is applied to the focus electrode of
the picture tube from the FBT T7002. This establishes focus on a curve or
arc just behind the flat CRT screen.
Dynamic focus voltage comes from transformer DFT T7001. The dy-
namic focus voltage is added to the static focus voltage from FBT T7002
to create the final focus voltage (fv) that is applied to the picture tube's
focus electrode. This voltage corrects the screen's left and right side fo-
cus.
Circuit Block Operation
The four blocks along the top and the three blocks along the bottom of the
dynamic focus block diagram make up the dynamic focus stage that feeds
the dynamic focus transformer T7001. The top and bottom blocks have
different purposes:
D y n a m i c F o c u s C ir c u it
C i r c u it r y
F o u r C r e a t e a n d r e g u la t e t h e D C v o lt a g e
U p p e r a p p lie d t o t h e D F T T 7 0 0 1 . A h ig h e r
B lo c k s v o lt a g e is u s e d w h e n a w id e p ic t u r e is
s e le c t e d .
T h r e e M a r k s t h e le ft a n d r ig h t s id e o f t h e
L o w e r s c r e e n f o r d y n a m ic f o c u s in g .
B lo c k s
Top Circuit Blocks
The pulse mfg. A, buffer, driver and LPF blocks along the top create and
regulate the supply voltage for Driver Q7012. The higher the supply volt-
age applied, the greater the dynamic focus voltage amplitude is generated
for focus correction. A larger dynamic focus voltage is required when the
deflection is at full width.
A ramp signal is needed to begin making this supply voltage. Capacitor
C1615 is charged to make this ramp. C1615 is reset using flyback pulses
via Q1614 so the ramp is at the horizontal scan frequency. This ramp is
applied to pulse manufacturing blocks A & B.
P u r p o s e
I C 1 5 0 2 , Q 1 5 0 1 ,
1 5 1 0 , Q 7 0 0 9
I C 1 5 0 2 , Q 1 5 0 5 ,
Q 1 5 0 7 , Q 1 5 0 8 ,
Q 7 0 1 2
M a j o r P a r t s