J.B. Wood wrote:

For those who use MIDI-capable musical instruments: I've got a nearly
25-year old MD 80 MIDI expander (thru) box that works fine but I've
noticed that connecting the MIDI out from some devices to an input on
the MD 80 doesn't always result in a signal at any of the MD 80's 4
output (thru) ports as there should be. Yet connecting a different MIDI
device the same way works. I'm a bit puzzled as to this incompatibility
given the relative simplicity of a MIDI connection. Your comment and
time is greatly appreciated. Sincerely,

That device looks suspiciously similar to the old Yamaha YME8 thru
box I bought in the late eighties and used for decades. A thru box is
usually a very simple circuit: one optoisolator at each input followed
by a buffer, say one or more 74ls04 or the like with their outputs
wired as a current loop. There is not much that could fail, other than
maybe the power supply filtering capacitors both on the device and the
wall wart. I would check those first.