Ok, I found it. It's a Yamaha (I knew it would be a "respected" manufacturer
to my embarrassment) MLC100.
I found the device and the MLA box on Yamaha's site. Unfortunately, there's
no technical specs, only device specs (a search for the manual came up
empty, but I've emailed their tech support too).
Here are the input output jacks on the teacher's console
Teacher Instrument, Aux Input 1, Aux Input 2, Tape Playback, Tape Record,
Aux Output 1, Aux Output 2, Unit Expand In, Unit Expand Out, Student
Instrument (x 16)
This student instrument x16 is what the little boxes (the MLA100s) are
connected to, via this thing I've been calling a MIDI cable (obviously,
we've only got 11 connected).
On the accessories page the mention NB100B 10M (I assume meters, roughly 30
feet which is what I thought they were, so these might be the actual cables
that came with the system, so they should be built to work correctly, one
would assume).
Unfortunately there's no charts or other info on the cables here. I'm off to
see if I can dig up some info on them.
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