The tape drive works.
I was down to the point of starting to poke around on the circuit board with a scope when the stars finally aligned. In the process I learned a lot about the mechanism worked, especially during its initial song and dance number.
I had to replace the capstan rubber. I followed the instructions at
http://www.hpmuseum.net/display_item.php?hw=257
(the o-ring method) and it worked perfectly. In personal correspondence with the author of that information, I was told that the diameter of the o-ring is not particularly critical, which would also imply that the durometer of the rubber is not critical.
It turned out that some of the noise that I had thought was indicating damage to the drive mechanism was a normal operational sound. The thing sounds as if it's tearing itself apart. The source of the noise is a stepper motor that is used to adjust the height of the tape head. During initialization, if the drive is having difficulty reading the tape, the stepper will move the tape head to a mechanical stop. Apparently there is no limit switch to stop the motor, so the stepper keeps trying to advance the tape head past the stop, making a horrible clatter. It didn't help that right after making that sound is when the drive would display the dreaded "Fault". So I believed that the sound and the Fault light were related and they were, in a way, but not as directly related as it appeared.
Some of my frustration was partly my own doing. The tapes used by the 9144A tape drive are very special. They are formatted at the factory with "keys" written periodically along the length of the tape. The drive can't recognize a tape without the keys. I had been very leary of using my HP labeled tapes in initial testing, fearing that one of my system tapes or spectrum libraries would be torn up by an errant tape mechanism. I had no reason to believe that one QIC tape was any different from another QIC. So I did a lot of my initial testing with a non-HP branded tape that appeared to have been used as a scratch tape.
Eventually I did learn that the tape is special, but by that time, I had gotten into the habit of reaching for that scratch tape as constant when testing other variables. Well, the scratch tape was apparently from some other system and simply had been tossed in the box when the GC/MS was packed up for sale. It is not readable by the 9144. I searched through the tapes that had an HP label again and found one labeled "Utilities - Not supported". OK. If I have to lose a tape during testing, I could probably lose that one.
It didn't work immediately, but the behavior of the drive changed a little. I had gotten accustomed to how much the head moved and what noises it made (and does it make noises!) and in what order. So the fact that the head moved in a slightly different set of steps and that I was seeing clear evidence of data being read on the scope was pretty exciting. It still made the nasty noises and still faulted, but it was doing _something_.
It took a lot of head cleaning, power cycling, and a couple of cartridge disassemblies, but it continued to get better. At one point I went to eject the cartridge and found that the mechanical eject had been locked. The drive had accepted the tape! The drive faulted during the unload procedure, but the fault appeared to have occured because the tape lost tension. The tape was looped back on itself severely enough that I disassembled the cartridge again and retensioned the tape. The next load was accepted and the unload worked properly.
I reassembled the unit and - with a little trepidation - reinserted the utilities tape.
It worked.
Sometimes things work out
