Tektronix T912 - Page 56

Service
Information—
T91
2
Does
beam
appear
on
screen?
If
not,
rotate
INTEN-
SITY
control
clockwise
while
holding
BEAM
FINDER
button
in.
until
beam
appears
or
control
is
fully
clockwise
If
beam
does
not
appear,
the
trouble
may
be
in
the
power
supplies
(see
Power
Supply
schematic).
b
Check
all
low
voltage
power
supplies,
starting
with
8
V
(
8 V
IS
reference
supply),
then
+8
V,
and
+120
Vat
appropriate
test
points.
(1)
If no
voltage
is
present,
check
F700.
(2)
If
-8 V
is
low
(or
zero),
check
Q772
Q774
Q776
or
U742.
(3)
If
-8
V
is
correct
but
+8
V
is
low
(or
zero),
check
Q752,
Q754,
Q756,
and
U742
(U742
is
used
for
both
-8
and
+8
V
supplies).
(2)
Check
for
positive-going
pulse
at
Q416
emitter.
This
pulse
amplitude
should
vary
from
20
to
80
mV
with
change
(fully
cw
to
fully
ccw)
in
INTENSITY
control
position.
(3)
Check
for
0-40
V,
peak-to-peak
unblanking
pulse
(varies
with
INTENSITY
control
position)
at
Q426
collector.
(4)
If no
unblanking
pulse,
check
Q426
Q424
Q416
or
Q434.
Vertical.
SYMPTOM
1.
—No
trace
on crt
with
BEAM
finder
button
pushed,
or
vertical
POSITION
control
does
not
center
display
(see
Vert
Input,
Vert
Switching,
and
Crt
&
Vertical
Amplifier
schematics).
a. If
trace
is
on
screen,
but
about
2
cm
above
graticule
center,
it
indicates
trouble
in
vertical
amplifier.
c.
Check
high-voltage
supplies
(see
Crt
&
Vertical
Amplifier
schematic).
(1)
Check
for
2.7
kV
at pin
1,
P465,orpin2,
crt
base
socket.
Use
DVM
for
all
voltage
checks
in
this
circuit
to
prevent
circuit
loading.
b.
Short
P4-9
to P4-11
(A8,
Vertical
board).
If
tracedoes
not
center,
trouble
is
in
output
circuits.
Check
Q1
12 0116
Q122,
0126,
0132,
Q142,
0136,
and
0146.
c
If
trace
centers
with
pin
9 and
11
short
circuited
trouble
is ahead
of P4.
(2)
If
no -2.7
kv,
check
for
50
kHz
sine
wave
(approximately
200
V,
peak-to-peak)
at
pin
5
of T460
(high-voltage
transformer
primary).
d.
Short
circuit
04322
collector
to
04324
collector
If
trace
centers,
trouble
is
ahead
of
delay-line
drivers
04336-04346.
(3)
If
neither
2
7 kV
or
50
kHz
sine
wave
is
present
check
0454,
0458,
and
0446.
CAUTION
Do
not
unload
the
secondary
of
the
high-voltage
transformer,
T460;
the
transformer
may
be
damag-
e.
Check
voltage
at
04302
and
04303
emitters.
Each
should
read
approximately
+5
V.
If
emitter
voltage
okay
trouble
is
either
in
04302
or
04303,
or in
switching
circuit
(short
circuited
CR4314,
CR4303,
or
open
CR4304).
SYMPTOM
2.—
With
50
mV
input
and
VOLTS/DI
V set
to
10
mV,
crt
display
position
is
low
and
does
not
position
above
graticule
center.
Z
Axis.
SYMPTOM—
No
intensity
or
no
control
over
intensity
(BEAM
FINDER
button
pushed).
a.
Does
beam
come
on
screen?
If
not,
and
low-
and
high-voltage
supplies
are
correct,
trouble
may
be
in
Z-
Axis
circuit
(see
Crt
&
Vertical
Amplifier
schematic).
(1
)
Check
for
approximately
60
V
swing
between
crt-
socket
pins
2 and
3. If
no
voltage
swing,
trouble
may
be
in
unblanking.
a.
Press
BEAM
FINDER
button.
Iftrace
does
not
appear
on
screen,
rotate
Vertical
POSITION
control.
b. If
trace
appears,
but
decreases
in
amplitude
at
graticule
center,
suspect
vertical
output
circuit.
c.
Short
circuit
01
36
collector
to
0146
collector.
Iftrace
centers,
short
circuit
0112
collector
to
0122
collector.
Trace
should
center.
If
not,
suspect
0112,
0116,
0132,
or
0136.
REV.
B
JUL
1977
5-9