I was using "TU" as the generic, as the 77 only came in Massbus TU77, the 78 came in Massbus TU78 and STI TA78 flavors, and the 79 only came in STI TA79.
I'm not sure if you could mix and match 78/79 drives as secondaries* with the other model as a primary* - all of my drives were primaries.
I did have a "TU79" which was a TA79 drive with a TM78 formatter in it (which is the reason I think mix and match might be possible). I did that conversion because at the time I only had SDI cards in my HSC50 - subsequenly I won a pair of universal (disk or tape, depending on microcode) controller cards for my HSCs at a DECUS Symposium. Prior to the universal cards, cards were either SDI or STI (presumably just a different set of PROMs).
* primary/secondary used instead of the period-correct terminology due to #BLM
The main improvement was a rubber foam gasket in the well where you mounted the tape, located between the Easy-Load II seal and the air port on the drive. The drives originally shipped with no gasket. They also shipped with the wrong airflow through the air port, but that was a less important part of the fix. DEC only made 100 FCO kits for a field population of ~6000 drives and it was an "Improvement" FCO, meaning the drive had to be on a Field Service contract and the customer had to complain in order to get the FCO. The most famous mis-categorized "Improvement" FCO was the power harness in the BA23, which tended to set the whole chassis (and sometimes a whole car assembly line) on fire.
The TU77 never officially got the load fix FCO, although the same kit works fine on a TU77.
The "whopper" TU78 FCO that took 3 tries for DEC to get right was the read-after-write soft error fix at 6250 BPI on TU78s. The boards shipped with the wrong capacitors. Rev A of the FCO replaced the capacitors, but positioned improperly. Rev B of the FCO fixed the problems with Rev A. Rev C added a shield that was forgotten in Rev A and Rev B. The capacitors and shield weren't the only problem - the Write and Preamp boards needed to be replaced, along with the Write to Preamp cable. The Read board needed modification, as did the 9 M8950 boards in the formatter and their ground wires. All in all, a total cluster-flip(chip).
Thanks. Lots of good information. Then my understanding of the Tx79 was correct, and DEC only had the TA79 actually. I thought the TA79 was neat in that it also had a sliding cover in front of the loading hub, so you could mount tapes without having to open the door.
Yes, about the HSC cards, I know. We have all variants. The HSC50s are long gone, but we still have a HSC90 around, in case we want to play with things that way. (No SCSI for it, though.)
I never had any slave tape drives for any formatter, so I have zero experience around that as well.
Interesting that you could have a TM78 with the Tx79 drive. Didn't know that. I think we got rid of our TA79 at some point, so probably not so useful to find this out now, but anyway...
I do think both our TU77 and TU78 drives have that gasket. I definitely remember seeing one. I could possibly be mistaken and only the TU78 have it. Something to check next time I'm near them.
The TU77 was on service until the early 90s, along with the DEC-2060 it was hooked up to. At some point we got a TU78, and I hooked that one up as well. Worked fine.
When we sortof turned off the DEC-20, I realized I wanted to run the TU78 on our 11/70 instead, but I never got that sorted. Documentation was conflicting, and in the end I realized that officially it was not supported. I did find the RSX device driver, but for some reason it didn't want to play. But it might have been that the TU78 developed a hardware error at the same time.
It's one of those things I'd like to get back to at some point. Since we still have both machine and drive, it would be cool to see if I could manage to get RSX to play with it.
But after we got a TU81 as well, the need sortof wasn't that urgent...
