Service manual
CPU BOARD SECTION
The CPU
board
is
composed
of
the
following
six blocks. When
it
gets in
trouble,
first
locate
which
block
is
concerned
with
the
trouble,
and
next
try
to
check
for
its
corresponding
circuits;
the
wiring
diagrams
of
every
block
will
be
shown separately.
CPU
in
trouble
Display
block
No
Cassette
recorder
block
No
Keyboard
block
No
Audio
block
Clock
block
No
Program
block
Yes
Yes
Yes
Yes
Yes
Yes
•
Checking
methods
of
each
circuit
Check
for
video
RAM
circuit.
Check
for
IC5
(8255)
and
its
peripheral
parts.
Check
for
IC5
(8255)
and
its
peripheral
parts.
Check
for
IC18
(8253)
and
its
peripheral
parts.
Check
for
IC18
(8253)
and
its
peripheral
parts.
Gets
in
trouble
instantly
with
power
on
.
1.
By
touching
IC
insulating parts by fingers:
•
If
they seem
too
hot
by heat generation;
Yes
Memory
circuit
and
its
peripheral
parts.
IC
is
defective,
IC
load
is
heavy
or
components are touching each other -
ROM
and
V-RAM
are
exempted from this checking, however.
• If a circuitry state
is
changed to another; Soldering is poor, socket contact
is
improper, or printed-
wiring
is
erroneous.
2.
By
using a synchroscope:
•
If
the relation between input and
output
of
TillC
is
iltogical, this means defective
IC
gate.
• Check if the voltage level of
TTllC
is
as specified: High level; over 2.4V,
Low
level; below O.5V.
-5-
MZ-80K