If
these
tests
prove satisfactory check
IC1 pin 14, IC2 pin 11 and IC5
pin 28 for +5V. Also check for ground at IC1 pin 40, IC2 pin 29 and
IC5 pin 14. If all is still satisfactory at this point and IC1 is the
plug-in type, replace IC1. If replacement of IC1 does not cure the
fault, check the address and data lines of IC1, IC2 and IC5 for active
data.
It is
possible
that
one of the
Z80A
or
RAM
control
lines
has
become
faulty therefore,
comparing
with
a
known
serviceable
board
if
possible, check the waveforms at the following points at origin and
destination:
(a)
IC2:
Pin 16 - INT Pin 17 - NM1
Pin 20 - IDREQ Pin 24 - WAIT
Pin 19 - MREQ Pin 25 : fflCTEQ
Pin 21 - M Pin /b KtbM
Pin 22 - WR
(b) IC22: Pin 4 - "RAT Pin 3 - WITE
Pin 15 - CA3~
If no
fault
has
been
found
and the
computer
still
has not
initialised,
the fault could lie in the RAM. If the computer is a 48k machine the
32k of expansion RAM can be isolated. An easy way of doing this,
assuming that the ICs are not fitted into sockets, is to remove the
+5V
from
IC25.
This
is
probably
best
done
by
very carefully cutting
the
track
to
IC25
pin 16. If
this
operation
clears
the
fault
condition, the expansion RAM will have to be further isolated. In
either instance the point has been reached where it is necessary to
start replacing ICs.
Where
the
fault
persists
after isolating
the 32k
expansion
RAM it
would
be
necessary
to
start
changing
individual
ICs in the
order
of
IC13
to IC6
(RAM),
IC1
(ULA),
IC2
(Z80A).
After each change
of 1C the
unit must be powered up to check for correct initialisation.
A method of fault-finding that can be used on both the 16k and 48k
versions
is to
make
up a
'test
1C'
device.
This
may be
done
by
using
an 1C
test
clip,
to
which
is
attached
a
serviceable
1C (of the
relevant
type e.g. 4116
or
4532),
to
bridge across each suspect
1C in
turn. This method is not guaranteed to work but can often save a lot
of time unnecessarily changing suspect ICs.
Memory Check
Where
a
Spectrum
has
initialised
correctly
but a RAM
memory
fault
is
suspected, it is possible to find the faulty address and relate it to
a faulty 1C by carrying out the following procedure:
Key in the instruction:
PRINT PEEK 23732 + PEEK 23733 * 256
The value printed should be:
4.8
To Next Page
Loading...
