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                                          FEB 20,1987
        DEAR SIRS;
           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 
        pin connectors).
                  (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 Robertson
John's Jukes Ltd.

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