Star Tech Journal

STAR*TECB Journal
April 1988
14
Gottlieb System. 80/SOA Transistors/Coils 9
John Robertson, President
John's Jukes LID.
Vancouver, British Columbia, Canada
This tip is designed to stop Gottlieb system
80 and 80A pinball machines from blowing
driver transistors and coils.
PROBLEM
The problem stems from an error in design of
the driver board (A3) with respect to the
solenoid grounds and the logic ground. If
you examine the schematic for the driver
board (A3) you will see that pulldown resis-
tors R55, R58 and R6 l are connected to the
logic ground, but that the driver transistors
Q58, Q62 and Q64 emiters are connected to
the solenoid ground.
I. Replace the pin on plug A2-J I pin 2 with
Molex part #08-50-0189. These pins have
added side wipers to the face wipe action
(and in fact work very well in Bally and Stem
power pin connectors).
2. Add ajumper on the jumper plug between
Al-J4 (I) and A3-Jl (1) using Molex part
#08-50-0106 and a short piece of wire.
:
c::::::3
.
-
Al
t ONTIIOI..
~
c=d
!i,
•
.,
'..
GROUND REFERENCE DIFFERENCE
Now if you trace the logic ground back from
the driver board, there is a single ground line
connecting to the control board (Al) through
the jumper plug between Al-J4 (I) and A3-
Jl (I). Continuing back through the control
board (Al), the logic ground is connected to
the power supply with two ground lines. And
going back still further, there is only one
ground line at power supply (A2). JI pin 2. In
fact, this pin connector is the primary cause
of the problem. There can develop, between
the pin and the connector, a resistance of
several ohms. Then what happens, the +5
supply will then create a situation where the
logic ground can be raised, relative to sole-
noid ground, as much as I to 2 volts. Of
course what happens then is that the driver
transistors are now getting more than 0. 7
volts at their bases and biasing "on" hard.
This is known as: Fry city.
One last suggestion, if you can't get the
Molex pins, splice a jumper from the green
line (plug A2-J 1) to the jumper plug between
the power supply A2-J2 (green wires) and
control board Al -JI.
SOLUTION
There are two steps required to cure this very
hot situation:
I also recommend that these modifications
be done immediately. It will save you time
and certainly money in the long run.
RECOMMENDATIONS
I would recommend that the factory make
known this problem, as I feel this very prob-
lem has prejudiced a lot of operators against
these otherwise well-designed machines.
I also would like to see another ground pin
used on the power supply connector A2-J I,
perhaps using the through line at A2-J I (3)
as a second ground for pinballs.
f