First post - thank you for the awesome information I've been able to track down so far. I haven't tried everything yet, but I think I've got enough to start a thread. Apologies if this is covered elsewhere and I missed it.
I've had my grandmother's OG Macintosh (I think it's legit, Model M0001) sitting in a corner of basement for 10 years, and I finally decided to try it out. Symptoms: flip the power switch, sometimes it gives the classic "bong" and sometimes it gives a muffled, slow-mo version of the sound. The screen doesn't show anything, or sometimes shows garbled checkerboard,