ISSUE 6, 1984 : Computer 3


MODEL : PLUS 4



                    Troubleshooting Aides

NOTE: Visual inspection is critical in this unit!
The upright position of many of the components used on the
board can create problems. It is possible for them to be 
shorted to the shield or to each other. Make sure they are
evently spaced and do not contact the shield.
       Areas of the PCB particulary vulnerable to this
problem are:
          Bottom right corner - caps
          Bottom center - J1-J6
          Bottom left - Q3 shorted to FB57
          Top left - ferrite beads
          Center - Twisted caps (just outside of RF can)

1) NO VIDEO - Absolutely no video on screen
       A) Check for 5 volts
          O.K.      If not: 1) Check fuse
                            2) Check for twisted or bent caps
                               ( 5 V. short to ground )
                            3) Check L1
       B) Check for oscillation at pin 14 of U1
          O.K.      If not: 1) Check for good connection at
                               pin 14 of U1
                            2) Check for good connection at
                               R1 thru R7
       C) Check for LUM signal at pin 23 of of U1, at pin 4 of
          the modulator, at pin 8 of the modulator, at FB3
          and at pin1 of CN7
          O.K.      If not: 1) Check for LUM signal shorted
                               to ground
                            2) Check for open traces
                            3) Check modulator
       D) Check for reset
          O.K.


       E) Check for control signals

          Signal  I.C   Pin             Signal  I.C    Pin
            AEC    U1    35               R/W    U1     7
            AEC    U2     4                 O    U1    12
            CAS    U1    11               MUX    U1     9
            CS1    U1     6               IRQ    U1     8
            CS1    U20   15               RDY    U2     2
            CS0    U1     5               BA     U1    34
            CS0    U20    1               RAS    U1    10