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Table 5-6. Display Board Checks
E
i
;tep Item/Component
Required Conditions
Remarks
1
Turn on power; select the 2mh range.
Engage zero check
2
P1006, pin 5
+ 3.33v
Vlcdl
3
P1006, pin 6
+ 1.66V
Vlcd2
4
P1006, pin 7
+5v
+5%
Power to display.
5
P1006, pin 2
81.92kHz
Display Clock
6
P1006, pin 1
OV to + 5V Pulses
Data From pp
7
P1006, pin 12
OV to + 5V Pulses
Data From pp
8
P1006, pin 13
OV to + 5V Pulses
Data From pp
9
P1006, pin 14
+ 5v to ov Pulses
Data From pp
10
LCD
Check that LCD is positioned properly.
11
Connector (P1008) Check that connector is not reversed.
If reversed, display test will run, then
display will blank.
12
Strip Connectors*
Check that they are positioned properly.
*See Figure 6-2.
5.6.6 Signal Conditioning
The circuit can be checked by using the diagnostic program
(troubleshooting
modes). See paragraph 5.4.2.
5.7 INPUT AMPLIFIER
(U1051 BALANCING
Part changes in the input section of the Model 485 may make
it necessary to balance the offset of the input amplifier. If the
instrument
cannot
be zeroed, as explained
in paragraph
2.7.2, perform the following
procedure:
WARNING
Figure 5-2. Jumper
Locations
Disconnect
line cord and remove
all test
leads from the terminals
of the Model 485.
1. Remove the top cover as explained in paragraph 5.2.
2. Remove the shield. Four screws secure it to the mother
board.
Table 6-7. Offset Jumper Set Up
t
3. Referring to Figure 5-2, locate jumpers WlOl
and W102
on the mother
board. If the jumpers
have been cut or
removed,
replace them with
new ones.
4. Connect the line cord and turn the power on.
5. Select the 2nA range and zero check.
6. Center the front oenel ZERO pot and note the offset pre-
5.6 LINE FUSE REPLACEMENT
0
a104
sent on the display.
7. Follow Table 5-7 and cut the appropriate jumper.
8. Disconnect
the line cord and install the shield and top
cover.
The line fuse is located internally in the Model 485. For exact
fuse location, refer to Figure 6-l. To replace the fuse proceed
as follows:
6-7
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