enough
solder to make a firm joint. After soldering, clean
the
area
around the solder connection with
an
approved
flux-removing solvent
(such
as
isopropyl alcohol)
and
allow
it to air dry.
Circuit boards
in
this instrument
may
have
as
many
as
four conductive layers. Conductive paths between the top
and
bottom board layers
may
connect to
one
or more
inner layers. If
any
inner-layer conductive path
becomes
broken
due
to poor soldering practices,
the
board
becomes
unusable
and
must
be
replaced.
Damage
of this
nature
can
void
the instrument warranty.
~
Only an experienced maintenance person proficient
in
the
use
of
vacuum-type desoldering equipment
should attempt repair
of
any circuit board
in
this
instrument.
Desoldering parts from multilayer circuit boards
is
espe-
cially critical. Many integrated circuits are static sensitive
and
may
be
damaged by solder extractors that generate
static charges. Perform work involving static-sensitive dev-
ices
only at a static-free work station while wearing a
grounded antistatic wrist strap.
Use
only
an
antistatic
vacuum-type solder extractor approved by a Tektronix
Service Center.
~
Attempts to unsolder,
remove,
and resolder leads
from
the
component side
of
a circuit board may
cause damage to
the
reverse side
of
the
circuit
board.
The
following techniques should
be
used to
replace
a
component
on
a circuit board:
1 .
Touch
the
vacuum
desoldering tool to the
lead
at the
solder connection. Never
place
the
iron
directly
on
the
board; doing
so
may
damage the board.
NOTE
Some components are difficult to remove from
the
circuit board
due
to a bend placed
in
the
component
leads during machine insertion.
To
make removal
of
machine-inserted components
easier,
straighten the
component leads on the reverse side
of
the circuit
board.
Maintenance-2445A/2455A Service
2.
When
removing a multipin component, especially
an
IC,
do not heat adjacent pins consecutively. Apply heat to
the
pins
at alternate sides
and
ends of the
IC
as
solder
is
removed.
Allow a moment for the circuit board to cool
before proceeding to the next
pin.
Excessive heat can cause the etched circuit conduc-
tors to separate from the circuit board. Never allow
the solder extractor tip to remain
at
one place
on
the
board for more than three seconds. Solder
wick,
spring-actuated
or
squeeze-bulb solder suckers, and
heat blocks (for desoldering multipin components)
must not be
used.
Damage caused by poor soldering
techniques can void the instrument
wa"anty.
3.
Bend
the
leads
of the replacement component to fit
the
holes
in
the circuit board. If the component
is
replaced
while
the
board
is
installed
in
the instrument, cut
the
leads
so
they protrude only a
small
amount through the reverse
side of the circuit board. Excess
lead
length
may
cause
shorting to other conductive parts.
4.
Insert the
leads
into the holes of the board
so
that
the replacement component
is
positioned the
same
as
the
original component. Most components should
be
firmly
seated against the circuit board.
5.
Touch the soldering iron to the connection
and
apply
enough solder to make a firm solder joint.
Do
not move
the component while the solder hardens.
6.
Cut off
any
excess lead protruding through
the
circuit
board
(if
not clipped to the correct length
in
step
3).
7.
Clean
the
area
around the solder connection with
an
approved flux-removing solvent.
Be
careful not to
remove
any
of
the
printed information from the circuit board.
8.
When
soldering to the ceramic CRT-termination net-
work, a slightly larger soldering iron
can
be
used.
It is
recommended that a solder containing about
3%
silver
be
used
when
soldering to the ceramic material to
avoid
des-
troying the bond.
The
bond
can
be
broken by repeated
use
of ordinary tin-lead solder or by the application of too
much
heat; however, occasional
use
of ordinary solder will
not break the
bond,
provided excessive heat
is
not applied
when making the connection.
6-23
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