Sage II - Checking the Refresh Signals; Checking the RAM Signals

SAGE II SERVICE MANUAL
[l.OJ
CPU BOARD
TROUBLESIIOOTING
CHECKING
TI{E REFRESH
SIGNALS
VIII.03
CIIECKING
THE
REFRESH
SIGtrAtS:
Check
for
the
64
Khz
refresh
signal at
pin
3
of
V32, If this
ls
not
correct, or the
timing
l-s
sllghtly of
f
then the
delay line
(tJ42)
is
bad
(known
to happen).
U34,
lJ32
or a
pc
board
fault
can
also
cause
problerns
for
the
refresh
signal.
At
thts
polntr
tro
matter
how
dead the board ls
you
should
have
refresh
slgnals to the
MMs,
To check
thts
send
nothing
but
refresh
signals to the
RAMs
by holding the
reset
swttch in.
Check
RAO RA7 for
missing signals
or
a
slgnal shorted
to another I
ine.
Slgnals
whlch
are
shorted
together wtll show
a mld-level
voltage
between
0
and
+5V at lnEervals
during the refresh.
They
may cause
lnconslstent
errors.
Good slgnals wtl1
change
quickly
between
ground
and +5V
wlthout
pauslng
ln
the
nlddle.
VIII.
04
CIIECKING
THE R.AI{
SIGNALS :
Proper
functloning
of
the
RAMs
is
not
necessary
for
the
system to
display the
slgn-on
message when using either
the
RAI'I test
ROMs
or
the
normal
Boot
R0Ms.
The
RAM clrcuitry
can only affect the
system
start-up
lf lt lnterferes
wlth
the data
bus.
Check
the MC6889
buffers
to
see lf
they
are
ehorted
to the
data
bus.
If the read
signal
ls
faulty,
the
buffers
w111
always
drlve the
bus on the
processor
slde.
To test
this
remove
buffers
U63, U62, U30 and
tJ23.
Reset and
see lf
the
sign-on
meSsage
appears.
The other
posslble
point
of
lnterference
with
the data bus
can be
caused
by the
data
lines
betng
shorted
to the
parity
decoder
chlps
U64
and
U3l.
I7