PLUS 4 Troubleshooting                              Page 2


       2) BAD VIDEO - Scrolling lines on screen - Random
          blocks on screen - Blurred display

            A) Check J1-J6 for shorts to ground or each other
               O.K.

            B) Check reset for correct operation
               O.K.

            C) Check U1 for proper operation
               O.K.   If not: 1) Check socket for good solder
                              2) Check for bad U1
            D) Check U2 for proper operation
               O.K.   If not: 1) Check socket
                              2) Check for bad U2
            E) Check RAM data lines for correct amplitude
               O.K.   If not: 1) Check for hot surface of RAM
                              2) Jump out RAM to verify
            F) Check multiplexers U9, U10 - signals at RP3
               and RP4 should be simlar in frequency and
               amplitude.
               O.K.   If not: 1) Suspect U9 or U10

            G) Check ROM for chip select signal at pin 22 of
               U23 and U24
               O.K.   If not: 1) Check for signal generation
                                 at U20
            H) Check that all ROM addresses are present and
               correct amplitude
               O.K.   If not: 1) Trace problem address A0-A15

            I) Check U19, U23, U24 by replacement with known
               good


       3) NO POWER
            A) Verify voltage +5 and +9 volts
                              1) Check for shorts to ground
                              2) Check switch
                              3) Check power supply


       4) BAD BASIC - Random characters on screen - Random
                      colors - Power-up message is missing
            A) Check Basic ROM U23
            B) Check B thru I above (Bad Video)