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u-Sci A2 and Disk II questions


New Member
Oct 21, 2021
New to Apple II Plus and think I screwed it up big time. I bought an Apple II Plus system at an estate sale and it has a u-Sci (Microsci ?) drive as drive 1 and an Apple Disk II as drive 2. I couldn't get the u-Sci to boot any disk. I have verified from another person with a working Disk II drive the disks are good and bootable.

With the case off the u-Sci drive I see the carriage do the rattling thing (which I think is normal?), but that is all it does. It won't move back to the back of the drive. With the power off I greased the rails very lightly, and it moves freely. No matter where I position it while unit is off, the head won't move other than the jittery rattle on first power-up.

1)Any ideas how to troubleshoot this drive? I can't seem to find any information about it on the interweb.

As it was I thought I would try swapping the ribbon cables on the Disk II interface card. It was easier for me to do this with the card out of the drive. (By the way I had the cases off of both drives to clean the heads and never put them back on while doing the tests.) I took the Disk II card out (of slot 6) and swapped the cables (actually now that I think about it I just put the Disk II cable in the Disk 1 connector and didn't reconnect the u-Sci). My own fault, I put the Disk II card in slot 7 instead of 6 without noticing what I did. I turned the computer on and POP & the magic smoke came out of an IC on the Disk II analog card, which I saw & smelled all too well because I had the case off of it. Chip ULN2003 - I think Darlington Array.

So, I can get a new ULN2003 but
2) is there a way to see what other things I blew on there (if any)
3) I guess I should mark slot #7 as "DO NOT USE"...? lol

Last edited:
It has nothing to do with putting the disk II controller in the wrong slot. What has happened many times to people is the disk ribbon gets put on the disk II controller card offset by a pair of pins which blows the 74LS125 on the disk II drive analog board.

Larry G
I’ve used a disk controller in slot 7 many times. The only reason people almost always use slot 6 is because it became customary so a lot of software expects it there.

Most likely the ULN2003 is the only one blown. Unless you have a chip tester there’s really no way to check the other chips. Be careful plugging the cable into the Disk II card. Most people have plugged it in incorrectly at least once.
Thank you for this information, that makes perfect sense. I was getting told I had put the cable in backwards, which would have been extremely awkward. This is more likely since I wasn't paying close attention to getting it in exact alignment and had it over a pair of pins! I'll order a 74LS125. Cheers for this