SL-990xp No MIDI Output

My SL-990XP stopped working earlier this year. I reseated the chips, and it worked…until this month. Now it has stopped working once again, and a reseat didn’t fix it.
Symptoms: No MIDI output, not even a flicker on the MIDI interface saying it’s getting any data. When I turn it on, the power LED lights up, then slowly fades out.
Any idea which component on the board needs to be replaced? (And does anyone have a component list/schematic?)