Look
on
the
etch
side
of
the
Board
under
the
RAMs. You might find an installation error. A
socket
lead
is
easily
bent.
Maybe
the
lead was
folded
under
the
socket
body
and
not
soldered
in a hole.
There
may
be
just
enough pressure
on
the
lead
and
the
pad
to
allow spastic
operation.
Look
for
a
smooth
coating
of
solder instead
of
the
pointed
cone
you
would
expect
if a wire
or
lead was
protruding
through
the
Board.
Don't
limit
your
visual inspection
to
socketed
parts
only.
It
is
possible
to
find a
bent
lead
on
a stand-
ard package,
although
it
is
not
as
common
as a
bent
socket
lead.
Another
type
of
defect
that
can cause intermit-
tent
failure
is
the
open
feedthrough.
See if
you
can find
any
solder-filled holes where
the
solder
did
not
come
all
the
way
through
the
Board
(the
plating
may
be cracked inside
the
hole and
the
solder
did
not
flow
past
the
crack).
Solder
a
small piece
of
lead wire
through
the
suspect hole
during
repair. Defective
feedthroughs
usually
occur
in
groups. If
you
fix
one
hole, look care-
fully
at
surrounding
holes
for
other
defects.
Hints, Ideas, Suggestions
And"""
WhatNow?
One
of
the
advantages
of
digital
computers
is
the
repetition
of
basic circuits. Once you
know
how
a basic logic cell works, it will
operate
in
the
same way all
through
the
machine,
no
mat-
ter
how
many
times it
is
repeated.
The
gate may
not
generate
the
same
type
of
signal
that
another
one
did
two
inches away,
but
the
out-
put
does respond
in
the
same way
under
the
same
input
conditions.
One
of
the
disadvantages
of
troubleshooting
digital
computers
is
the
machine language soft-
ware.
There
is
some kind
of
program inside
there,
pushing
the
buttons
and pulling
the
levers.
Nobody
has
bothered
to
tell you
exactly
how
it
performs
a task. You
know
it does
exist
though,
like migraine
potential,
ready
to
give you a
headache
and
a half.
In
the
Circuit Description
Section
we briefly
told
you
how
the
keyboard
scan
software
operates,
but
the
discussion never
does get
down
to
the
nuts
and bolts. So
what
do
you
do
if you
think
the
problem
exists in soft-
ware
and
the
only
way you can confirm
your
suspicions
is
to
know
exactly
what
software
is
doing? Before you can
examine
exactly
what
software
is
doing
in
the
Keyboard, you'll have
to
get yourself an expensive logic analyzer, a ream
of
computer
print
outs,
and
spend a
lotta
time
figuring
out
how
the
program operates. Unless
you're
willing
to
spend
the
time
and
money,
software
listings are
not
going
to
help
in
trouble-
shooting.
This section
of
the
book
will give you hints
and
ideas
to
try
when
you're
faced with software-
type
problems
that
screw up
the
hardware.
The
hints will play
the
advantages against
the
disad-
vantages
of
digital
troubleshooting
and help you
confirm
or
deny
suspicions
about
where
the
problem
really resides.
77
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