9-16
L402 of hor output circuit is a choke coil which corresponds
to deflection yoke in ordinary TV. Relay S401 switches over
capacitance of resonant apacitor. When scanning frequency
is low, relay closes to short C402. To keep high voltage
constant, input voltage of FBT is made reduced with
descending of frequency, but the voltage change becomes
discontinuous at around 28kHz because of switching over
resonant capacitor. (Fig. 20)
QH43 which switches over d.c. bias at pin 3 of error amp
IC407, prevents high voltage from rising abnormally, when
the scanning frequency changes from low condition to high
due to change of input signal. If this QH43 is omitted, high
voltage would jump up instantly when relay S401 opens
from the closed state. This is the reason why control operation
of chopper regulator cannot instantly follow when the relay
opens to reduce capacitance of resonant capacitor, and even
to increase peak value of flyback pulse. Therefore, the
circuit is designed so that the reference voltage of error amp
is for a moment reduced just before the relay opens and high
voltage is kept down.
Focus pack of FBT, unlike in TV, produces focus voltage Fv
and screen voltage Fs by dividing high voltage.
Accordingly, in this model, high voltage does not remain for
a long time after power switch is turned off.
+200V source which is used in Video Out circuit, is produced
from pin3, and +38V source which is supplied to X-Ray
Protection circuit, is produced form pins 5 and 6. These all
are rectified in retrace period.
As mentioned above, because of detecting high voltage
through focus pack, flowing of leakage current to the focus
electrode due to failure of picture tube causes high voltage
to change.
Vin
15
28
40
fH(kHz)
Fig. 20
The purpose of switching-over is, as mentioned in section of
Deflection Circuit, to obtain voltage necessary to get over-
scan by raising output voltage of chopper regulator for HV
REGU which supplys power source also to Deflection
Circuit, because amplitude in TV mode is large by over-
scan.
