Board Issue 3: This allows OKI ICs to be used in place of TI 4532.
Again, all ICs must be of the same designation. Appropriate links (2)
must be fitted in the grid located on the board between the edge-
connector and the 'mic' jack socket. Again the extra memory is
obtained
by
plugging
in 4
logic
ICs and
eight
memory
ICs.
1.7 Hitachi vs NEC ROM
During manufacture, wiring links are selected and inserted in the
board according to whether a Hitachi or NEC ROM is used. These links
are marked 'H' and 'N' as appropriate and are located as follows.
Board Issue 1 - between the ROM and the heat sink
Board Issue 2 - beneath the raised portion of the heatsink
Board Issue 3 - adjacent to the loudspeaker
Clearly, if the ROM is changed for one of a different make, then these
links must be changed as well.
2. FAULT DIAGNOSIS
2.1 Techniques
In a
closed
loop system such
as a
computer, because
of the
inter-
dependence of numerous component parts, fault diagnosis is not
necessarily straight-forward. In addition, because of the high speed
cyclic operation, interpretation of any waveforms on control, data and
address lines as being valid depends to a large extent on practical
experience of the system. There are however, certain checks with
valid
waveforms
and
levels
that
can be
carried
out
before
substituting
any
integrated
circuits.
Experience
has
shown
that
the
best
method
of
initially
checking
waveforms
and
levels
can be to
compare
with
the
same point in a known serviceable board. The following pages provide
a basic fault-finding procedure and furnish a list of possible faults
along with suggested ways of curing them.
With
a
densely populated board such
as the ZX
SPECTRUM,
a
careful
physical examination of the board can sometimes indicate an obvious
fault.
Burst-out
discrete
components
or an
overheated
track
show
up
immediately, as do the attentions of an enthusiastic amateur. Bearing
in
mind
the
latter,
short
^circuits
,
caused
,by
hairline
solder
splatter can be or
several
ohms
resistance
and can
cause
some
very
misleading fault symptoms.
Provided
first
principles
are
adhered
to and a
common-sense
approach
is adopted, it will be found after a short space of time that fixing a
faulty Spectrum is very much a routine operation.
4.6
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