YesApostrophe

Thanks Bibilit, I finally found another post that was having a similar issue. I think it might've been you that talked about the U3 component. I think you are right, I just don't know how to test it. This computer is actually my dad's, so I told him what I had done so far, recommended U3 and the CR20 and CR21 for replacement, and suggested he find someone who is less likely than me to destroy it with poor soldering skills! I will let you all know if he finds someone and if those replacements work. Thank you!

Update: I removed and tested a bunch of capacitors that the Classic Mac Repair Notes indicates are prone to failure, all were good (C24-C26, C29-C31). I did find out that CR29 was bad, so I replaced it with an equivalent diode (hopefully). I plan to wire it back up and test this afternoon. Main issue I've identified so far: Measuring the 12V and 5V bus while the computer is running reads inconsistent results. Sometimes it's 12.5V and >5V, sometimes it's more like 9V and 3V. It's stable while the computer is on, but each time I turn it on it goes to one or the other voltage

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,