Star Tech Journal

July 1997
STAR* TECH JOURNAL
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SEGA POWER DRIFT S1T-DOWN
VIBRATION MOTOR PROBLEMS
Kerry Messana
Garnes Technician
Waterford, New York
PROBLEM
Over the past six months or so I
have had three Sega Power Drift
sit-downs come through my
shop. They suffered from various
problems, the typical ones,
power supply, monitor etc ...
What I did find was all three had
a common problem .. The vibra-
tion motor was being shutdown
due to trouble.
CAUSE
After a little troubleshooting with
the 'scope I found out why the
shaker was being shutdown. The
system uses a motor that has an
optic on its rear end that senses
motor speed. This is a simple but
odd optic interrupter with side
mounted ears. When reading the
signal back with a 'scope I found
that on all three games it did not
give me a valid low signal back
to the 74LS240 IC that buffers it.
The signal would pulse between
+5 volts and +2 volts. 2 volts is
not low enough to trigger a low
to a 74LS IC.
SournoN
At this point a fix was in order. I
contacted my distributor who in
tum got in touch with Sega. To
my dismay, they do not carry just
the optic, they only stock the
complete motor and by the time
the distributor handles it we were
going to be set back $165. Not
too good, especially when you
multiply it by three! The solution,
modify the circuit that reads the
optic because I couldn't find the
optic.
The stock circuit has a 74LS240
as an input buffer with a 4. 7K
ohm resistor pack used for pull-
up. The resistor is pulling up the
collector of the optic and in many
other similar circuits I have found
that the resistance is usually
much higher for the pull-up re-
sistor.
My idea was to modify the cir-
cuit with a higher value pull-up
that will still be strong enough to
pull the signal high but didn't
require the optic to work so hard
to get it low. This modification
will also have a neat side effect,
it will allow the motor control
board to be alot more tolerant of
a dirty optic and we know how
many people go in and clean the
optics on the back of their shaker
motors. Sometimes it's hard
enough to get them to wash be-
hind their ears! I found a value
of 22K ohm worked quite well
in this instance. Here is how I did
the modifications.
... continued on page 73