Tektronix T912 - Page 57

Service
Information—
T912
Triggering.
SYMPTOM
1.
—Trace
free
runs,
does
not
trigger
in
AUTO
or
NORM
(see
Trigger
schematic).
a. Set
TRIGGERING
MODE
to AUTO.
Turn
LEVEL
control
cw and
ccw to both
limits.
Does the
trace flicker?
If
not,
triggering
signal
is
not reaching
sweep
circuit.
b. Check
voltage
at
junction
of R2147,
R2152,
and
R2143.
Does
voltage
vary
from
-2
to
+3
V while
turning
LEVEL
control
throughout
its
range? If
yes,
check
U2156B
output.
Does
U2156
output
level
change
while
turning
LEVEL
control as
above?
c.
Connect
a signal
to CH 1
. Check
for
trigger
pulse
at
U2156B
output.
If
no
signal,
suspect
U2156A
or B,
or
related
circuits.
SYMPTOM
2.—
Does
not trigger
in
AUTO.
a.
Check
for
trigger
pulse
at
pin 6 of
U2212B. If
none,
check
for
a HI at
U2212B,
pin
4,
while
varying
the
LEVEL
Control.
If
pin
4
does
not
go HI,
check
for
defective
U2212B,
U2224B,
or CR2227.
Sweep.
SYMPTOM
1. —
No sweep
on
crt (see
Sweep &
Horitontal
Amplifier
schematic).
a.
Push
BEAM
FINDER
button. If
trace
or
dot is right
of
center,
check
at R2243
(end
of resistor
toward
board
center)
for a
12
V
(approximately)
ramp.
b. If ramp
is not
present,
check
for
approximately
0.7 V
at
Q2274
base.
c. If
voltage
at
Q2274
base
is high
(approximately
8.0
volts)
check
Q2274,
Q2242,
Q2244, or
Q2246.
NOTE
When
troubleshooting
the sweep
or
horizontal
cir-
cuits,
the
Vertical
AMPLIFIER
may be
removed
from
the
instrument.
Sweep
may
lock
up
while troubleshooting.
If in
doubt,
switch
instrument
power
off and
back on. If
there are
no
problems,
trace
should
free run.
Horizontal.
SYMPTOM
1.—No trace
on
screen (see
Sweep
&
Horizontal
Amplifier
schematic).
a. Check
output
(Q2334-Q2344
collectors)
for
ap-
proximately
40 V
ramp.
If okay,
check
for
possibly
defective
crt leads.
SYMPTOM
2.—Trace
on
screen,
but
is
short.
a.
Check
horizontal
output
(Q2334-Q2344
collectors)
for
approximately
40
V ramp.
b. If no ramp
at
output,
check for 12
V ramp
at junction
of
R2243-R2244-R2245.
c. If
ramp
is
present,
check
0231
4,
Q2326,
Q2332,
Q2334, or 02344.
STORAGE.
SYMPTOM
1, —Will
not
go into
store
mode.
a. Does
crt flood
or
illuminate
when in
store?
If not,
check
for
about
1 9.3 V
on
collector
of
0944. If
none,
check
0944
and 0942.
SYMPTOM
2.
—No
trace
on
crt.
a.
Repeat
sweep
symptom
1,
parts
a and b.
SYMPTOM
2.
—Will
not erase
in
store
mode.
b.
Check
for
a HI
at
U2234C,
pin
8. If
not
HI
check
U2234C.
a. Check
for
erase
pulse
at collector
of 0944
of about
200
V
peak-to-peak.
If none,
check
0944,
0942, and the
ERASE
switch
S910A.
5-10
REV.
A APR
1977