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Section VIII
SERVICE
GROUP
F
Model 3455A
d.
The same
operation takes place
for
1, .1.
2.
.2,
3,
.3.
after
the
.0
has
moved
to
the
far right
of
the
display.
If
any of
the
LEE>s
do
not
light,
replace
them
or troubleshoot
their
drive
circuits. If
some
numbers
of
the display are
bad, troubleshoot
the display
and
associated drive
circuits.
8-F-B.
Front Panol Servieino (Schomotic
10).
a.
If
the display
blanks out
any
zeroes,
try
replacing
AIU62.
b.
The
proper operation
of
the front panel
buttons can be checked by monitoring
A1U5^
pin
14.
The
level
at
pin 14
should
go
low
(TTL)
low,
any
time
a front panel
button
is
pressed.
If this
is
not
observed,
try
replacing
U57.
c.
If
the front
panel buttons
do
not operate, check
for
a high
level
(TTL
high)
at
A1
J8
pins 2
and
3.
A
high
level at
any
of
these pins
will
disable
some
of
the front panel buttons.
Check
for the correct
operation
of
AlUSl,
U50,
or
US3.
d.
If
the front panel has a
sticky switch,
try
the following:
With
a
low temperature
soldering
iron,
heat the solder
connection of
the
LED
within the switch.
While
the solder
is
warm,
push
the
button
in
and
out
several times.
This should
straighten
out
the
LED
and
relieve
any
pressure
on
the switch.
e.
If
the
procedure
in
the
precendent
paragraph does
not
relieve
sticky switches,
change
LEDs
A2CR5
to
CRl
1,
CR19
to
CR22, and
CR24
to
CR35
from
-hp- part
numlKt
099(W)547
to -hp- part
number
1990-0665.
These changes
should be
made
for
instruments with
serial
number
I622A0I336
and
below.
A
procedure
for
changing
or
replacing
LED's
are
in
paragraph
8-F-7.
f.
A
modification
to
reduce key
bounce
is
as follows:
Change
A1U57
from
a
9318
to
98L18
(part
number:
1820-0987),
A2R17
and R18 from
a 2.2
K ohm
resistor
pak
to
a 10
K ohm
resistor
pak
(part
number;
1810-0206). This
change should be
made
for
instruments with
serial
number
1622A00906
and
below.
NOTE
Switch
bounce
can be observed
by
pressing the
ENTER Z
button
and
then pressing
the
MA
TH
OFF
button only
once.
If
two 2
'j
are
displayed,
the
2455A
has key bounce.
g.
For
all
other
front
panel malfunctions use Troubleshooting
Diagram
8-H-26.
B-F-7.
Front Panel LEO'a Switch,
and Kay Cap Replacement
Procedure,
a.
Removal
Procedure.
1.
Remove
front
frame which
is
held
by
8
screws.
2.
Disconnect
two
connectors
between
front
panel
and
left
side
of
instrument.
3.
Remove
front
panel
and
ON/OFF
switch.
4.
Remove
II
screws holding Display/Swiich
board
to front panel
and remove
Display/-
Switch
(D/S)
board.
5.
Pull
key cap
off switch body.
6.
With
knife
or punch,
cut
off or
punch
through
the red switch
body mounting
studs (clean
excess
plastic
off
to
prepare
holes for
new
switch).
7.
Hold
display
board
upside
down
with key
facing
down
and
heat
LED
terminals
to
let
bad
LED
fall
out.
8.
Suck
out solder holes
to
prepare
for
new LED.
8-74
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