SFD-1001 Won't Format / Only Reads Bottom of Disks Jan 25, 2023 8:45:27 GMT -7
Post by Cube Inc. on Jan 25, 2023 8:45:27 GMT -7
I purchased a SFD-1001 from an European seller, and although I know it was 100% working when he shipped it, when it arrived I noticed a bit of an odd problem. At first everything looked good, I was able to read disks I'd created with my old SFD-1001 and 8250 drives, but when I started copying files off of them, some files read perfectly while others would not read at all, resulting in a "FILE NOT FOUND ERROR" message when I attempted to load them. After some further testing I decided to try formatting a blank disk, and although the drive would attempt to start the format, it sat there after banging the heads for about 2 minutes before erroring out with a "21, READ ERROR, 78, 0" error.
I checked the usual stuff: I cleaned the heads, I tried another couple of disks, I even tried turning it off and then on again (in an English accent). Same result every time. Then I started trying oddball stuff: I disconnected the read/write heads from the main PCB and tried a format. The format started with the same head bump, but then immediately errored out with a "75 FORMAT SPEED ERROR, 1, 0" error (Track 1, sector 0.) If memory serves me correctly, If I leave the heads connected but physically "lift" the top head off the disk, the format starts the same way but errors out with a "75 FORMAT SPEED ERROR, 78, 0" (Track 78, sector 0). Track 78 is the first track on the top side of the disk, so it seems that the head is reading [something] from the disk.
After examining the schematic for the SFD, I put a scope on the center tap pin of the bottom head and can see regular pulses of about 9VDC on that pin, even when the head is idle. However, when I scope the center tap pin of the top head, there are no pulses at any time. This makes me wonder if the LS7445 BCD decoder that controls the center tap pins for each head to determine which head is selected for reading or writing is either stuck sending power to only the bottom head, or it is being told to do so from further up the digital chain.
I'm going to continue investigating further, but I thought I'd put the question out there since I know there are at least a few members here who still have SFD-1001 drives: Have you ever encountered this situation with your own SFD's and if so, were you able to fix the problem?