The 1977 North Star Horizon I found in a previous video still wasn’t booting, so I went after the basics first and then worked outward from there. I tested the power supply at the floppy power connections on the board and got 5.01V on the 5-volt rail and 11.86V on the 12-volt rail. With that looking good, I went ahead and pushed every socketed IC back down into place, since a lot of them had worked loose over time.
Before trying the computer itself, I checked the Hazeltine 1500 terminal that came with it. It powered up, gave me a cursor, and passed a simple loopback test on the serial port with pins 2 and 3 jumpered, so the keyboard and display were both alive. The real problem was mechanical: the smoked front screen and gasket had slipped badly, so the whole front looked crooked even though the terminal was otherwise working.
I opened the terminal, pulled the CRT assembly, and found that the gasket had deteriorated and bunched up around the front of the tube. I cleaned the old sticky residue off the glass and plastic with Ronsonol lighter fluid followed by 91% isopropyl alcohol, then reassembled everything. While I had it apart, I also found the flyback transformer mount had broken loose, so I drilled an extra hole in the aluminum plate and secured it with heavy zip ties. Once it was back together, the terminal came up cleanly and displayed text just fine.
With the terminal sorted out, I installed the cards in the Horizon and checked for shorts on the S-100 bus before applying power. I verified the rails on an empty slot and then powered the machine with the cards installed. One memory card LED came on, the floppy drive light came on briefly and then went out, and nothing let out any smoke, so I put the top back on and connected the terminal.
The first boot attempt didn’t show anything useful on the screen, even though the floppy activity looked promising. I corrected one mistake with the disk orientation, but the bigger issue turned out to be the serial cable. I had started with an adapter setup, then swapped in a true DB-25 RS-232 cable, and that was what finally brought the machine to life.
At that point the Horizon booted and displayed “North Star DOS 5.1.” I listed the disk directory with the `LI` command and confirmed there were files on the disk, including BASIC. Booting into BASIC from drive 1 wasn’t reliable, but using drive 2 worked and I got a BASIC prompt and ran a small program that printed “HELLO TO COLONEL SANDERS AND HIS KENTUCKY FRIED CHICKEN.”
I also rebooted into CP/M, which came up as “56K CP/M” and responded to familiar commands like `DIR`. The machine was clearly running, but the floppy subsystem still had issues. Formatting and loading weren’t consistent, and later the drives developed a temperature-sensitive startup problem, so the Horizon did boot, but the drives still needed more cleaning, lubrication, and likely alignment work.
— Aaron
Links
Timestamps
00:00 – Intro
01:02 – Testing The Power Supply
05:02 – PCBWay Ad
05:27 – Fixing The Terminal
23:01 – Looking For Shorts
28:27 – First Boot
36:28 – DOS History Quickie
39:38 – Wrap Up