[sdiy] Matrix 6R MIDI input prob

[edanderson]

I've been hacking around with this dead-to-MIDI Matrix 6R for a bit now.  The problem seems to be in the MIDI input circuit- as seen in the lower right corner of this schem:

http://people.atl.mediaone.net/gail2001/m6proc3.jpg

Since the MIDI thru feeds off of the input "pre-amp" (not sure what it is- opto isolator??) and thru doesn't work, I'm thinkin that the problem lies there.  The schem *seems* to indicate one of two choices here; I've got a PC900V.

Can anyone recommend what sort of tests I could do to pinpoint the source of the trouble?

Thanks muchly.

Ed Anderson