3·4·8.
HV
Hold·Down Circuit
Feedback
voltage, normally
9.00
V, from
the
HV
block
is
input
to
the
positive
input
of
comparator
lCI
(1/2)
on
the
DD board.
It
is
compared
with a
reference voltage (VHV) selected hy
R305,
R345,
and
R314
of
9.60
±O.IO
V.
Normally,
the
output
of
lCI
(1/2)
is
low. When
the
voltage at the positive
input
exceeds VHV,
the
output
voltage at pin
CD
goes high and switches
on
Q309
thru
hold·circuit
IC
I
(1/2)
on
the
DE board. The collector
of
Q309
is
con·
nected
to
the
collector
of
Q302
to
disable
the
HV
drive pulse
when
Q309
switches
on,
holding down the
high voltage. The high voltage value when
hold
down
initiates
can
be calculated as follows:
HP
(PRT)
=
9.60
(±0.10)
x
3000
(+1/-3%)
=
2B.80
~li~
kV
3·4·9. Beam Protector
If
the
CRT
anode
current
increases
beyond
a
prede'
termined
value,
the
beam
protector
circuits inhibits
high voltage
operation.
This
monitor
incorpotates
two
individual beam
protectors
to
provide
double
over
current
protection.
Anode
current
is
supplied by
the
secondary
of
the
FBT
and
flows
thru
series resistors
R307
- R31
O.
Voltage
on
pin ®
of
connector
D·6 decreases as
the
high voltage
current
increases. Since
the
bleed cur-
rent
flowing
to
high-voltage feedback resistors
and
focus resistors
is
constant,
any
change in
anode
cur-
rent
is
proportional
to
the
voltage drop across sense
resistors
R307
- R31
O.
When
anode
current
exceeds
a
threshold
value,
the
voltage at the positive
input
of
IC2
(2/2)
on
the
DD
board
drops below
that
of
the
negative
input
(VHV), and
the
output
ch8nges from
a
"high"
to a
"low"
state. IC2
(2/2)
is configured as
a
comparator
with hysteresis
and
the
output
is held
"low".
As
a result, high-voltage pre driver
Q302
is
inhibited
and
prevents excessive
current
flow
to
the
CRT.
ICI
(2/2)
on
the
DD board comprises the
second
beam
protector
and
functions
in a similar
manner
to
lC2
(2/2).
The
current
threshold
of
beam
protector
#2
is set
at
a higher value
than
the
threshold
of
beam
protector
#1.
3·4-10. Scan Failure Protection
Scan
detection
circuits are utilized
to
inhibit
HV
generation
and
protect
the
CRT
phosphor
upon
loss
of
horizontal
or
vertical deflection.
The
collector
voltage for
Q302
(HV predriver)
is
obtained
. by rectifying
the
retrace pulse from
the
HOT.
Upon
loss
of
horizontal
deflection,
no
voltage
is supplied
to
the
collector
of
Q302
and
HV
is
in-
hibited.
3·5
Vertical deflection flyback pulses are
rectified
by
D 106 and charge
capacitor
C327
to
turn
Q30B
"on".
However, when vertical defleciton fails,
Q30B
is
switched
"off"
and
Q307
is
switched
"on".
The
collector
of
Q302
is
held low by
Q307,
inhibiting
HV
operation.
3-5. G BOARD (Power Supply)
3-5-1.
AC
Line Filter
The
AC
line filter consists
of
an
input
filter,
and
transformers
Tl
and
T2,
and
capacitors
CI,
C28,
C30,
and
C31
which
are
mounted
on
the
G
board.
3·5·2. AC Voltage Selector
AC
input
voltage
is
selected by switch
S902
on
the
connector
panel. When set for AC
100
to
120
V,
the
voltage rectifier circuit is configured as a voltage
doubler. When
set
for AC220
to
240
V,
the
voltage
rectifier circuit is
configured
as a full-wave rectifier.
3·5·3. Degauss
The degauss coil is
mounted
on
the metal
CRT
shield
to
automatically
demagnetize
the
CRT. The me-
chanical relay
RY52
and
one
of
the
two
posistors
THPI
and
THP2 provide an
attenuating
AC
current
to
the
degauss coil
L90
l.
Transistor Q56
controls
RY52
and
a
time
constant
circuit, R65 and CB8,
allows a degaussing
duration
time
of
approximately
five seconds
at
each
turn
on
cycle.
Until
RY52
is
seiected
on,
AC
current
thm
the
posistor
causes its
resistance
to
increase and
the
current
is gradually
attenuated.
3·5·4. OVP Circuit
The over voltage
protection
(OVP) circuit is designed
into
the
AC
primary
line circuit. The circuit
operates,
for example,
when
applying an
AC
input
voltage
of
220
V
to
the
monitor
with
the
AC
voltage selector
switch set
to
100
- 120 V. When
the
switch
is
set to
this
position,
a series circuit consisting
of
neon
lamp
NL I
and
resistor
R4
is
connected
across
the
AC
,
primary
lines
(from
Live
to
Neutral).
The
neon
lamp
fires
at
a voltage
of
230
±34 V
and
results in a blown
fuse
F901.
The
maximum
AC
voltage which triggers
this
protection
circuit is set
at
164 ±24 V,
with
the
voltage selection
switch
set
to
100 - 120 V AC.
3·5·5.
DC
Power Supply (Regulator)
DC
supply
voltages
(+145
V
<GDM-1602:
120
V>,
+60
V, +15
and
-15
V)
are
generated
by
a Y
-Z
type
switching
mode
regUlator.
Two
regulators are uti-
lized,
one
for
the
+B
voltage
and
one
for
+B2,
+15,
and
-15
voltages. This
description
is
for
the
+B
regulator.
lC51
compares
a
feedback
voltage
from
the
+B line
to
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