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Pincushion Correction
Electronic pincushion correction is necessary because it is difficult for the
yoke to generate perfectly uniform magnetic fields in all areas. Without
this circuit a yoke will display a reduced scan creating a caved in hour-
glass picture.
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By increasing the current through the yoke, a larger magnetic field is cre-
ated that will stretch the picture, filling the screen. To produce a straight
picture, the magnetic field must be increased gradually while the electron
beam is scanning from the top down to the center. From the center of the
picture down, the magnetic field intensity should then gradually decrease.
The correction signal required is at a 60 Hz. vertical rate. This signal
comes from the Digital Deflection Control IC1305/pin 20.
Circuitry
The Digital Deflection Control IC1305 makes the vertical drive signal and
the pincushion correction signal. The correction signal leaves IC1305/pin
20 as a parabola waveform labeled EW (east/west correction).
The low frequency 60Hz signal is added line by line using a horizontal AFC
pulse input IC1803/pin 8. The output signal at IC1803/pin 14 looks like the
E/W parabola chopped up at a horizontal rate.
105
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C h a n n e l 1
C h a n n e l 2
C h a n n e l 3
C h a n n e l 4
T im e b a s e
The 12Vp-p chopped parabola signal is amplified by MOSFET Q4027 and
applied to the low end of the yoke (-) via HLC coil L4006. When Q4027
conducts, it shunts the PMC coil L4005, providing a higher current path to
ground for the energy in the yoke. This increases magnetic deflection.
The amount of deflection is dependent upon Q4027's conduction. The
conduction is based upon the gate's parabola voltage level.
The following waveforms were taken at the same location as above, but at
a 10usec/div time base so you can see the horizontal pulses chopping the
vertical parabola.
P M 3 3 9 4 , F L U K E & P H I L I P S
c h 1 : p k p k = 2 . 0 6 V
ch1
T
c h 3 : p k p k = 1 2 . 0 V
ch2
2
ch3
ch4
1
3
C H 1 ! 1 . 0 0 V =
C H 2 ! 2 0 0 m V ~
C H 3 ! 5 . 0 0 V = B W L
4
C H 4 ! 1 0 0 V = C H P M T B 5 . 0 0 m s
c h 1 -
W a v e f o r m P in 1 - P a r a b o la S ig n a l P r o c e s s in g
N a m e
L o c a t io n
E / W p a r a b o la
I C 1 3 0 5 / p in 2 0
R e d u c e d p a r a b o la
I C 1 8 0 3 / p in 9
C h o p p e d p a r a b o la
I C 1 8 0 3 / p in 1 4
C N 1 3 0 4 / p in 1
P in O u t p u t
Q 4 0 2 7 / d r a in
5 m s e c / d iv
V o lt a g e / d iv
1 V p - p
0 . 3 V p - p
1 2 V p - p
1 4 2 V p - p
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