(I'm not sure if this is Ling over on Facebook, but I just posted a reply to a similar question over there. In case it's just a coincidence, I'm reposting here as well):
There is no MIDI thru on the CZ-5000. That "MT" port does not mean MIDI Thru. It means "Music Terminal". It is not MIDI at all. It is an 8-pin DIN connector that Casio used to connect a 230S, a 5000, an SZ-1 sequencer or an RZ-1 drum machine to a cassette recorder/player. It was a way of storing tones and sequencer data to (or reloading from) standard cassette. The 3000 and the CZ-1 have MIDI Thru, but not the 5000.
So, you do not need to fix that MIDI port. Because it's not a MIDI port :)
MT is clumsy by today's standards, but can be very useful to store and retrieve sequences, which you can't (easily) do with MIDI or even the cartridges. The original MT cables are nearly impossible to find these days, but I managed to make one out of existing off-the-shelf parts this week. I use the cable to dump data to a small digital voice recorder as WAV files, so I'll never lose a sequence again.
But, yes, you can use the 5000 with a DAW. The Sequencer will send/receive some data (see the manual, page 49; and 50-53 for detailed examples.
Also, be sure to check out these pages. Lots of great information here that is either not documented in the manuals, or not written and explained as well. Hope all this helps!
--Robert
TinyLoops Invaluable Tips for the CZ-5000 -
CASIO CZ-5000 - Tips
Jim Atwoods CZ-5000 explorations -
Casio CZ-5000 Synthesizer in Japan