There is a wealth of knowledge here. https://jestineyong.com/audio-restoration-repair-bose-wave-music-system-awrcc1-awrcc2-awrcc7/ Main premise is capacitors age and go out of tolerance. I suspect maybe a few in power supply stages would cause erratic behaviour. (don't need to replace them all) China blesses us with low cost ESR meters if high end gear is out of reach. This video was useful https://www.youtube.com/watch?v=snuJdw60.... (maybe play at 2x speed and skip as necessary)

I have a refurbished 7490 and after pressing the power button some lights came on and the fan would spin but the display remained dark, moments later dark and quiet. I tried many things including external monitors, disconnecting everything ssd, ram, wireless, keyboard, etc. and couldn't get a POST display, or when main display disconnected on an external display. Disassembled and probed voltage test points - all seemed normal. Removed and inspected main board (all except disconnecting CMOS battery). somewhere some how (possibly holding control and escape on boot?) it booted enough for me to install a critical stability BIOS patch. System was used daily and was stable for a whole month. It is somewhat murky but likely crashed while in use which seems like a different issue. (Not my laptop, I just bought it and am responsible for its service) It appeared to be up to its no boot antics. After it sat for 3 nagging weeks I tried again and it booted. (when it wouldn't boot there were IC's that would warm up so...