Read Sharp 28JW-74H (serv.man2) Service Manual online
50Hz Widescreen Colour Television – GA20 Chassis
Sharp Electronics (UK) Ltd
Technical Support
Issued June 02
Page 8
Scan Coils
S-Correction
Fly-Back Tuning
Capacitor
Efficiency
Diode
Horizontal
Output
Direction Of Scan
This causes current to flow through the scan coils from the S-correction capacitor, which results in a
linear build up of magnetic energy in the scan coils that will move the beam spot from the centre
position towards the right hand edge of the CRT.
Scan Coils
S-Correction
Fly-Back Tuning
Capacitor
Efficiency
Diode
Horizontal
Output
Direction Of Scan
At the point where horizontal drive pulse returns to zero, and the horizontal output transistor turns off.
The magnetic energy that has built up in the scan coils begins to collapse at a rate determined by the
flyback tuning capacitors. A back emf is produced by this collapsing magnetic field which charges the
flyback tuning capacitor. The spot will now move from the right hand edge of the CRT towards the left
at a higher speed. When the beam spot reaches the centre position the current through the scan coils
(magnetic field totally collapsed) is at a minimum and the voltage across the flyback tuning capacitor
is at a maximum. The charge that has been built up in the flyback tuning capacitor will now flow in the
reverse direction through the scan coils keeping the beam spot moving towards the left hand side of
the CRT.
When the voltage across the tuning capacitor becomes zero all the energy is now stored in the scan
coils as a magnetic field, flyback is completed.
Scan Coils
S-Correction
Fly-Back Tuning
Capacitor
Efficiency
Diode
Horizontal
Output
Direction Of Scan
As a linear scanning current is required, the oscillation set up in the scan coils and tuning capacitors
during flyback has to be stopped. If resistance were used to damp the oscillation (15625 times/sec)
the stored energy would be wasted. The oscillation is damped by a diode connected across the
collector/emitter (can be encapsulated within the transistor).
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