Commodore SX-64 Portable Computer Restoration

2/19/22

The computer powers on, the diskette LED goes on for a few seconds, and the screen shows a color background. without a border or text. Pressing the diskette reset button causes the diskette LED to go on for a few seconds. There is no diskette seek activity. The diskette controller has its own 6502 CPU, and will turn on the spindle motor for a few seconds, but won't seek until the main CPU dells it to do something.

We looked at the J19 Serial Bus Cable (IEEE-488) signals for the floppy diskette controller. The clock and data signals are always low. We disconnected the cable between the FDC board and the I/O board, and the serial data signals went to 5V. So it looks like CIA #2 on the I/O board is holding the diskette controller CLK and DATA signals low.

We looked at the input and output clock signals for the CPU chip. All look OK and are 1 MHz.

We looked at the data signals on the CPU. We can see tri-state activity on all 8x