I recently picked up a second Color Classic to try and fix my first one (which has a weird blue blotch in the top left of the CRT, which is not going away after being plugged in and "on" for a long time to degauss).
The machine boots immediately when the power switch on the back is set to "I", which the other one doesn't do (analogue board problem?). It also makes a periodic squeaking sound which doesn't sound like electrical arc'ing (as I've heard previously on a Mac Plus) but it does sound electrical in nature.
The speaker plays the boot chimes and beeps, so the computer is functioning properly. The monitor remains black the entire time, and doesn't feel "staticky" when I touch the glass, so I'm guessing it's not getting something it needs to power on.
The Color Classic progresses properly to the OS, and you can change the volume level through the buttons on the front. It will also respond to the three-finger salute as well as trying to zap the PRAM on startup.
If I push the power button on the keyboard after I imagine the boot process to have reached Finder, it doesn't beep like it should when bringing up the "Shut down?" dialogue box, so the OS is either stuck on some other dialogue box, the version of the OS installed doesn't have the "Shut down" dialogue box, or however the power button interrupt works isn't getting caught by the hardware. Since it works with the three-finger salute, I'm assuming the key works and the OS is just getting stuck.
The motherboard is good, as verified by swapping into my first Color Classic; both ADB ports work and the machine has the normal pseudo-softpower behavior.
I posted to the 68kmla already (see here) but I figured a Colour Classic specific forum might be of more help. I'm going to crack the case soon to take a look and see if there are any obviously blown caps or torched lines.
Any suggestions about what to check next would be greatly appreciated.