HP 9895A 8'' Floppy Drive - Problem

[Michael Fehlhammer]

Paul, you are perfectly right. The drive is a cdc drive, BR8A8-A. there are service manuals available that come close, but do not match exactly.
We swapped the drive heads and proved them to be ok. As Paul has assumed the problem lies on the floppy controller board. After some fiddling around the first drive (which worked fine initially) now also fails. It still reads correctly, but now it destroys data on the disk (probably the directory) when writing anything (proved by a second 9895A in working condition). It also cannot format a disk.
So now we have drive 0 which can read but not write, and drive 1 which can't read.
(writing can't be tested, since verify fails and a write command will never finish).

The floppy controller boards are revision 3, which unfortunately is not described in the available manuals (CDC 9404, 9406).
Moreover, the two boards differ slightly, and obviously there have been repair attempts for board 1. For example, sealed potentiometers have been adjusted subsequently, and ICs have been replaced.
We probably will have to find out which components control read and write access and replace one after another. Maybe we will need some help from the community to identify special ICs. There seems to be no easy way, but hints and tips are welcome.
By the way, we experimented with the radial alignment of the print heads. At least we were able to improve read access in that sense that drive 0 finally managed to read disks that were recorded by another drive.