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MAINTENANCE
7.9 FAN FILTER CLEANING/REPLACEMENT
The fan filter, which is located on the rear panel of the in-
strument,
should be checked periodically
for dirt build-
up and cleaned or replaced, as necessary. Use the pro-
cedure below to clean or replace the filter.
WARNING
Disconnect
the instrument
from the power
line
and all other
equipment
before
removing
the
top cover.
1. Remove the two screws that secure the top cover to
the rear panel and slide the cover off completely
to the
rear.
2. Using a pair of needle-noise pliers, carefully remove the
filter, as shown in Figure 7-16.
3. Soak the filter in a solution
of warm water and mild
detergent until clean. Rinse thoroughly
in clean water
and allow the filter to dry completely before installation.
4. If a replacement filter is required, one may be obtained
from Keithley Instruments,
Inc. Order part number FL7.
NOTE
In order to minimize
dirt build-up
within
the in-
strument, it is recommended
that you operate the
instrument
only with the proper filter installed.
5. Using a screwdriver or pliers, install the filter by guiding
it in between the rear panel and the fan guard. Make
sure the filter is properly
seated at all four corners.
7.10 i! OUTPUT
HIGH LEVEL MODIFICATION
As shipped,
the Z output high level is programmed
for
5V. However, the output can be changed to a nominal l5V
value by modification
of the I/O circuit board as follows:
I
1. Remove the I/O board from the instrument
as covered in
paragraph 7.7.
2. Install the correct pull-up resistor at location R409 (+5V)
or R410 (+UV).
Figure 7-17 shows the location of these
two resistors on the Ii0
board, while Table 7-17 lists
nominal, maximum, and minimum values. The nominal
9100 value for +5V is recommended
for compatibility
with TTL circuits.
3. Remove the resistor for the unused voltage value. For
example, if you are changing
from +15V to +5V, you
would install R409 and remove R410.
4. Install the I/O board within
the instrument.
Be sure to
secure the board to the rear with the two screws removed
earlier, and check to see that all cables are connectd
properly.
7.11 CIRCUIT
BOARD JUMPER
LOCATIONS
A number of movable jumpers, located on the A/D and
mother boards, are used to select such operating aspects
as interrupt
levels, address decoding, RAM/ROM
types,
and delay timing. Figure 7-18 shows the locations of these
jumpers, as well as the correct position
for each, just in
case you inadvertently
change one or more settings. Keep
in mind that future software or hardware revisions could
change the locations or settings of these jumpers;
thus,
it would be a good idea to mark the locations and settings
for your particular
Model 194 before working
on the cir-
cuit boards.
NOTE
The instrument
may be partially or completely in-
operative if one of the jumpers is moved to an im-
proper position.
Table 7-17. 2 Output Resistor Values
Nominal
High
output
+5v
+15v
*Resistance value based on TTL compatibility.
7-38
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