Hmm. Things are working again. Something must be flakey. Oh Joy!
I’ve been working with a working core + din4 this evening
running the MIDIO 128 application. I then attempted to
upgrade my core to MIOS v1.8. I think the upload failed
but now when I power on I don’t get any MIDI output
and just a row of black boxes on the LCD. Did I brickify
the PIC somehow? Hints? Debugging tips? This core
has been working fine for some months.
I think that’s a common symptom of a bunch of different wiring/assembly/PIC burning mistakes we might make in a hurry. 
I had a core a while back which did that. Mine would do a bad boot “most” of the time, and you could often get a proper one by quickly flicking the power off and on a few times.
I fixed it, but I’m sorry to say, I can’t remember exactly what it was. There may be an archived post in here. When I looked, there were other problem posts like that, which weren’t from the same thing. I also managed to kill a 2x40 LCD during the troubleshooting, while tinkering with the rectifier circuit.
Hope you get it worked out!
George