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SECTION
MAINTENANCE
4.1
ACCESS INSTRUCTIONS
Removal
of the dust cover affords quick access to the
majority of
components
within the
unit.
However,
it
may
be
necessary to
remove
the
Power
Supply Board
or other
parts to gain access
to those
components
not
exposed by
removing
the dust
cover.
4.1.1
Removing
the
Dust Cover
To
remove
the dust cover
(case)
from
the
Model
144, the
following
procedure should be
followed:
1.
Unplug
the
power
cord
and unscrew
the
two
captive
screws
on
the
rear panel.
2
.
Remove
the
rear
casting
and with
the instrument
setting
face
down,
lift
the cover
off slowly.
3.
Reverse
this
procedure
when
replacing the
dust
cover.
Tighten the
two
captive
screws only
finger-
tight.
4.1.2
Removing
the
Power Supply Board
1.
Remove two
screws holding the
Power Supply
Board
to
the
two
side panels.
2.
Loosen
the
two
screws (from the outside of the
rear
plate)
which
holds the heat
sink for
the four
power
supply
transistors.
3.
Flip
the
Power
Supply Board out
of the
chassis,
toward
the
rear,
and
mount
the board
on
the outside of the
rear plate
using
the
same two
screws
and
the
same
two
holes used to
mount
the heat
sink.
NOTICE
This instrument can
be operated with the
power
supply board
mounted on
the outside of the
rear
plate.
However,
it
should never be operated
without
having the heat sink of the
power
supply board
connected
to the
rear plate
of the
unit, as
the
rear plate
is
also
used
as
a
portion of
the heat
sink for
the
power
transistors.
NOTICE
4.1.3
Removing
the
Sweep
Board
1.
Remove
the front panel
knobs
labeled
TRIG LEVEL,
MODE, VARIABLE
and
SWEEP
TIME.
2.
Loosen one
of
the
set
screw
in
the shaft-coupler of the
SWEEP WIDTH
control.
3.
Loosen
the screws
which
hold the
Sweep
Board
to the
two
side panels.
4.
Label
and
disconnect the four
wires
connected
to
the
TRIG
IN and
SWP/GCV
OUT
connectors.
5.
Remove
the
Sweep
Board by
sliding
the
Board toward
the
rear
panel.
4.1.4
Removing
the Attenuator Bracket
1.
Remove
the
OUTPUT ATTEN
knob.
2.
Label
and
unsolder the wires
connected from
the
atten-
uator to the
BNC
connector and
main
circuit
board.
3.
Loosen
the
two
screws
which
mount
the bracket
to the
Main
Circuit
Board.
4.1.5
Replacing
Q54
and
Q55
1.
To
remove
the
transistor,
unscrew
the nuts holding the
transistor
to the base of the heat
sink
and
unsolder the
transistor leads
on
the
main
circuit
board.
Then
remove
the
transistor.
2.
Before
replacing the
transistor,
put
some
thermal
com-
pound
or
silicon
grease
along
both
sides
of the
bottom
edge of
the
transistor for
good
thermal
contact,
4.1.6
Front
Panel
Removal
1.
Remove
the
rear
panel
and dust
cover
as
described
above,
and unplug
all
the knobs.
2.
Unsolder
all
BNC
connectors,
label
and unsolder
FRE-
QUENCY
DIAL
potentiometer
leads
and unscrew
the
two
nuts
which
hold the
power
transistors into
the
heat
sinks.
4-1
12/70
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