Philips 29PT6441/85, Service Manual

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Vertical Deflection
The Frame stage consists fully of discrete components. This has the advantage for
better flash behavior than when an IC was used.
The Frame differential drive signal from the Hercules comes from a current source.
Resistors R3460 and R3461 convert them into a voltage, and feed them into the
differential amplifier TS7455 and TS7456. The output of TS7456 is input to the next
amplification stage of TS7452. Finally, TS7451 and TS7453 deliver the Vertical yoke
current to the coil and feedback through the sensing resistors R3471 and R3472.
D6458 and TS7454 are used to bias TS7451 and TS7453, to get rid of zero crossovers,
which can cause horizontal lines at the screen center.
The negative supply is from -12V and the positive scanning supply is from +12V through
D6459. The flyback supply is derived from D6455, D6456 and C2456. This circuit is a
voltage doubler, which stores energy in C2456 during the Line flyback period and
delivers the energy to C2465 during the Line scanning period. Throughout the Frame
period, the charging and discharging of C2456 works alternatively. However, at the first
half of the Frame scanning, TS7451 is “on” and consumes all the charge from C2456.
When entering 2nd half Frame period, TS7451 is “off”, so C2456 will gradually charge
up to the required flyback supply.
C2463, R3464 and D6457 are for boosting the base voltage of TS7451 during the
flyback period and the 1st half Frame period as well. C2463 is charged by D6457 during
the 2nd half scanning. R3467 and R3468 are for oscillation damping.
The V_guard protection is to protect the Frame stage if a fault condition happens. The
V_guard will sense the pulse with voltage > 3.8 V and period < 900 us. Any signal out of
this range will be considered as fault, and the chassis will be shut down.