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voltage limits). This pulse is output at IC7003/pin 3 and coupled by C7011
to pin 6. The pulse boosts the +15Vdc Vcc at pin 6 to the amplitude of the
pulse. This method of developing the higher Vcc avoids having a large B+
supply voltage source that is not used most of the time. A loss of boost
voltage seems to cause no vertical deflection in most sets.
Vertical Feedback Signal
The return end of the yoke at CN4002/pin 5 is grounded through low value
resistors and a thermistor TH7001 for temperature compensation. The
small signal that is developed across these resistors is AC coupled and
fed back to IC71305/pin 11 for three purposes:
1. Linearity correction
2. Frequency correction
3. Picture tube protection
Linearity correction
Linearity correction takes place within IC1305 when the manufactured ver-
tical signal is compared to the signal returned at pin 11. In addition, linear-
ity and size corrections are performed from the service mode adjustments.
The adjustments are input to IC1305 as serial data from the main micro
(but the information is actually stored in an external memory).
Station Sync
The frequency and phase of the VD signal output IC1305 must be the
same as the station's vertical signal. Two inputs are used to accomplish
this. One input is the station sync that comes into IC1305/pin 28. The
second input is the sample of the vertical oscillator signal that enters
IC1305/pin 11. The two inputs are compared in IC1305 and the frequency
and phase of the vertical oscillator is corrected to match.
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Picture tube protection
If there is a loss of vertical deflection, the picture would collapse. The
scanning lines would be concentrated at the center of the screen (if there
was no DC voltage to offset or move the beam). This would instantly
damage the picture tube. To avoid this the vertical feedback signal is also
used for protection.
A loss of vertical signal for two fields (1/30 second) into IC1305/pin 11
causes the IC to stop the horizontal drive output from pin 29. Therefore if
there is a loss of horizontal drive signal from IC1305/pin 29, even though
there is +5Vdc applied to the IC1305 at pins 34 and 43, check the vertical
feedback signal path.
While this is occurring, serial data is output IC1305 to tell the main micro
to shut off the TV set and blink the standby light. The light will blink four
times, pause, and then repeat over and over until the set is unplugged or
the front panel master power button is pressed.
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