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WJ-8718A/MFP
MAINTENANCE
1.
Refer to Section
ill
of this manual for detaHed discussions of ICs,
included in the appropriate detailed circuit descriptions.
The sche-
matic diagrams in Section
VI
provide pin diagrams.
2.
Use the logic pulser to stimulate the inputs, while using the logic
probe or logic clip to observe the response at the outputs.
Make a
note of each node that fails to respond to the stimulus, in the manner
predicted by the truth table. The logic clip is especially useful for
counters.
(The term node is used to emphasize the fact that the
failure of an IC to respond properly may be due to short-circuited
tracks, or other failures external to the IC under test.)
NOTE
When an IC fails, it usually fails completely.
Thus, it is not normally necessary to observe
the timing of the output signals.
If an IC
performs correctly under static test, it can be
assumed to be good.
3.
When one or more failing nodes have been located, use the logic
probe to check the output of the IC that is driving the node.
The
logic probe will indicate a faulty level by showing a dim li.ght. If a
faulty level is indicated, the IC driving the node should be replaced.
4.
Check for a short circuit to ground or Vcc by placing both the logic
probe and logic pulser on the terminal. The logic pUlser can override
normal TTL output levels, but cannot override power supply po-
tentials. Therefore, the presence of a pulse indicates that the node is
not short-circuited, and the absence of a pulse indicates the node is
shorted to Vcc
(if
it is high) or to ground (if it is low).
If
a short
circuit is indicated, proceed to step 5. If no short circuit to .ground
or Vcc is indicated, proceed to step 6.
5.
A node short-circuited to ground or Vcc could be the result of either
a short-circuited track on the board or an internal short in an IC.
First, examine all branches of track conne-eted to the node with the
current tracer. Use the current tracer to isolate a defective IC: the
circuit that draws the most current is generally defective.
6.
If neither an open output (step 3), nor a node shorted to ground or Vcc
(step 4) is indicated, look for a short between two nodes. This can be
done more easily by turning OFF the unit's power, and using an
ohmmeter to test for continuity between nodes that should not be
common.
(Refer to the schematic diagrams.) If a short circuit
between nodes is indicated, proceed to step 7.
If a short circuit is
not evident, proceed to step 8.
7.
The most probable cause of a short circuit between two nodes is a
solder bridge on the board.
If the two short-circuited nodes are
connected
to
the
same
IC,
the
short
circuit
might
be
4-11
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