Mark and Stu,
Thanks so much for your help! This is exactly the information I was looking for, and I am going to start working on diagnosing the issue right now.
In case this additional information is helpful, here's what I've done so far:
- Temporarily reconnected the display board directly to the player to confirm it still works: Confirmed, the display board still works, buttons light up and everything.
- Tried both the backward-facing connector and the downward-facing connector on the display extender, to rule out a connector problem. Both connector paths reproduce the issue.
- When connected via the display extender, noticed an additional symptom of the problem that I hadn't mentioned before: When the player boots, the standby LED blinks once very briefly at the very beginning of startup, instead of slowly pulsing during the entire boot process like it normally does. At the same time, the custom button LEDs also blink once very briefly and faintly, then go dark and stay dark.
Okay, now I'm going to go get the multitester and start tracing stuff based on your recommendations.