759 INDUSTRIAL DRIVE
This tip is to stop Gottlieb System 1, 80 & 80A pinball machines
from blowing driver transistors and coils.
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 pull down resistors R55, R58, and R61 are connected to
the LOGIC GROUND, but that the driver transistors Q58, Q62, and
Q64 emitters are connected to the SOLENOID GROUND. Now if you
trace the LOGIC GND. back from the DRIVER BOARD., there is a single
ground line connecting to the CONTROL BOARD (A1) through the
jumper plug between A1-J4 (1) and A3-J1 (1). Continuing back
through the CONTROL BOARD (A1) the LOGIC GND. 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) J1 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 is the +5 supply then will
create a situation where the LOGIC GROUND can be raised relative
to SOLENOID GROUND as much as 1 to 2 volts. Of course what
happens then is the driver transistors are now getting >.7 volts
at their bases and biasing on. Fry city...
There are two steps required to cure this situation:
(1) replace the pin on plug A2-J1 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 STERN power
(2) add a jumper on the jumper plug between A1-J4 (1)
and A3-J1 (1) using MOLEX part# 08-50-0106 and a short piece of
I would recommend that the factory make known this problem as
I feel this problem 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-J1, perhaps using the through line at A2-J1
(3) as a second ground for pinballs.
One last suggestion, if you can't get the MOLEX pins, splice a
jumper from the green line (plug A2-J1) to the jumper plug
between the POWER SUPPLY A2-J2 (green wires) and CONTROL BOARD
John's Jukes Ltd.