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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