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A sample of the High Voltage
is taken from Pin 15 of T900 and
applied
to the Programmable
Shunt
Regulator,
IC901-1.
This
sample
can be adjusted
by R943 to attain
the desired
High
Voltage.
IC901
sends
the product
of this
sample
voltage
through
Q903 to IC900-8
to regulate
the output voltage.
The
output
of IC900
can be completely
stopped
if Q904 turns On
and inhibits the feedback
path between
IC901 and Q903.
This
input via Q904 is the shutdown
input for a scan type of failure.
There
is also
a sense
input
for the 15V Source
called
Low
Voltage
Detection.
Either
one of these
failures
will blank the
Picture Tubes and turn On Q904, placing the set in shutdown.
The Shutdown
Control
IC, tC902,
watches
for too much High
Voltage
and/or too much
Beam Current.
If IC902 is turned On
from either of these conditions,
it must be reset by turning
the
set Off and On again.
Beam Current is monitored
from the bot-
tom of the diode stack and applied to Q907/Q908.
The output
from
Q907/Q908
is applied
to the Picture
Control
circuitry
to
modify the contrast
level and to IC900-8
to help control the reg-
ulation of High Voltage.
When IC902 is turned On, Q901 is also
turned On, placing the set in shutdown.
The High Voltage
shutdown
comes
from the Shutdown/Phase
Sense
winding
on the HV Output
Transformer.
If the level of
this pulse becomes
too high, IC902 will turn On and shut the set
down.
8
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