Vertical Positioning - Tektronix 545A Instruction Manual

Maintenance
—
Type
545A
To determine which
circuit
is
at
fault,
adjust the Time-
Base
controls
for a
free-running
sweep
at
1
millisec/cm
(STABILITY
control
full
right). Set
the
INTENSITY
control
to
midscale. Using a screwdriver with an
insulated handle,
short the deflection plates together at the
neck
pins
on the
crt.
These are
the pins
marked BLUE
(UPPER)
and
BROWN
(LOWER). Be extremely
careful
not
to short either pin to the
metal
shield
around
the
crt,
or to the pin
marked
ORANGE
(GEOM).
If
the
dc unbalance
is
being produced
in
the
vertical deflection
circuit
the trace
will
appear
at
or near
the center of the
crt
when
the vertical deflection plates are
shorted
together.
If
the trace does
not
appear, however,
the trouble does not
lie in
the
vertical circuit
and
the
dc
balance
of the horizontal
circuit
can be checked
in
a
like
manner.
If it is
determined that the
vertical deflection
circuit
is
unbalanced,
the next step
is
to
check
the
Delay
Line. For
this
check,
and
for others
to
follow,
it is
convenient
to
have
a short lead (about 6
to
10
inches) with
a rubber-covered
alligator
clip
on each end
(see
Fig.
5-12).
Connect
this
lead across the input
to the
Delay
Line
(at
the output from
the Distributed Amplifier).
If
the trace appears on
the
crt
the
Delay
Line
is
not at
fault.
If
the trace does not appear,
check
for
an open
line.
This
can be done by
turning
off
the instrument
and
checking the continuity of both
sides of
the
line
with
an ohmmeter.
If
the Delay
Line
is
not
at
fault,
connect
the shorting
strap across the input of the Distributed Amplifier.
If
the
trace
does
not
appear
when
these
grids
are connected
together, the trouble
lies in
the Distributed Amplifier.
Check
for
an open
grid or plate
line.
Check
RT
206
and
R1216
at the grid line termination,
and
check R1071 and R1073
and
the
associated
circuitry
at the
plate
line
termination.
Refer
to
the simplified
diagram
of a
distributed amplifier
shown
in
Fig.
4-1 for the location of these
components:
then
refer to
the schematic
diagram
for your instrument
for
the
exact
value.
Fig.
4-1
shows
the
normal
location of these
components
in
a
vertical amplifier.
If
the trouble
is
being caused
by
tubes,
it
may
help
to
connect
the shorting strap
between
the grids of that pair of
distributed amplifier
tubes closest to the input
of the
delay
line.
Then
move
the shorting
strap
back
stage-by-stage
toward
the input of the distributed amplifier
until
a point
is
reached
at
which
the trace no longer appears
as the
oppos-
ite
grids
are shorted
together.
The
trouble
will
then
lie in
that part of the
circuit
between
the shorting strap
and
the
Delay
Line.
It
is
unlikely that
just
one tube
in
the Distributed Amplifier could be
responsi-
ble for the trouble
because
the current from each tube
is
only a small
portion of the
total
current flowing
in
the
plate
circuit.
If
the trace appears
when
the input grids
to
the
Distrib-
uted Amplifier are connected
together, the trouble
is
occur-
ring
ahead
of the Distributed Amplifier
in
one
of the input
stages.
If
the tube replacement does
not correct the
trouble,
then look for
open peaking
coils, defective
resistors,
and
shorted or leaky
capacitors.
After replacing tubes
in
the distributed amplifier or other
sections of the
Type 545A
vertical amplifier,
it is
necessary
to
check
the various stages
for
balance between paired
tubes on opposite
sides of the amplifier.
To check
the
bal-
ance
of the
6DK6
stages,
insert
a Type TU-2
Test
Load
Unit
in
your oscilloscope. Short together the
vertical
deflection
plates of the
CRT
to
determine the
Electrical
Center
and
adjust
VERTICAL
POSITIONING
so that the trace
is
at the
electrical
center.
Jumper
the grid
lines of
the
6DK6
stages
together. Then attach a meter lead
or suitable connector
between
the
+225
volt
output of the TU-2
and
the
cathodes
of
each
pair of
6DK6
tubes
in
order.
Unbalance
of the
Fig.
5-12. Shorting
strap useful
in
locating an unbalanced stage
in
vertical or horizontal
amplifier.
Insulating tape
covers the
100-ohm
resis
lor.
5-8