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XOR S-4 Repairs

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  by Bill Degnan - 08/12/2017 18:17
XOR S-4 CPU / IO card
Closeup of the CPU I/O card voltage regulators and related components. Click image to larger view.

It had been a while since I used this system. I took it out because it's capable of formatting a few 8 inch IBM 3740 format disks (SDSS) needed for testing Digital RX01 drives. DEC did not provide a way to format disks.

The system did not boot. Great. By probing components I discovered I had a blown cap near the 7812 and a bad 1488 driver on the CPU I/O card, possibly blown because of the bad voltage regulator. I made the necessary repairs to return the system to operational status.

XOR S-4 CPU / IO card
The 47.uF cap near the 7812 was fried, the heat sink gets pretty hot and that probably diminished it's lifespan. In addtion to the cap I also replaced the heat sink paste and put in a new 12V regulator. Note how the new cap is bent up to help keep it cool. click image for larger view.

XOR S-4 CPU / IO card serial drivers
The serial drivers. It's a clever design, using one cable for two serial ports on the other end with a special adapter to split out the signals on the other emnd. Click image for larger view.

IBM 3740 format uses 128 bytes 26 sectors per track, ~241K capacity. There were a lot of disk formats back then, I had to double check I was building the right thing as the old systems are flexible yet cryptic.

The XOR S-4 system I am using has Mitubishi M2896-63-02U drives, they're 8" DS/DD and the disk controller uses a 1795 FDC chip.

NOTE TO FUTURE SELF: The bottom of the two serial ports by default is to be used by the terminal at 9600 baud. Use a NULL Modem cable. When you turn on the system it should put you to the monitor prompt (*). To boot from the left disk drive (drive A) hit CTRL+C on the terminal keyboard.



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