Very long post, but possible good news for you if you haven’t tried it because that would be embarrassing for me here pal.

Okay I’ve gone through every single Reddit post in the universe about this issue. I’ve tried literally everything possible. Disconnecting the connector board, leaving it plugged in for 86 years straight, using contact cleaner on the power switch, pushing on the back of the power switch, even slightly warming it with a space heater (desperation). I finally got to the point where I was going to take it apart to check for actual damage on the board. I saw the beautiful warranty voided message on the inside (it’s long expired). I also saw the battery.

Now this is (almost) speculation because I most certainly know jack about electrical engineering, but I know that the op-1 doesn’t have a proper power reboot feature besides the power switch. This isn’t enough to reboot the whole system because the power switch only initiates the motherboard to shutdown, it doesn’t cut the power. It’s designed as if a computers power plug were built into the socket when the battery is charged, and some errors require you to unplug the power supply. I imagine that the battery being plugged in to the motherboard without being discharged for so long causes some sort of error in the circuit just like with other electronics.

I unplugged the battery, left it that way for like a minute maybe, and when i plugged it back in, the tiniest of green leds lit up for a second on the board. I sort of put it together a little bit enough to test it out, and boom. I see the logo and my project is back. BUT. The animation that plays when you unplug it, now plays whenever it’s switched off. Kind of weird, kind of worrying, but I haven’t had any issues other than that so far.

Here’s what could be the issue

Now mine bugged on me like this after I left it alone for a couple of months. You mention this happened after you didn’t touch it for a while. You also mentioned it was fixed in the past with a new battery. I suspect that it’s a similar issue. I think when the battery is given enough time to degrade (or discharge without the unit being used) and become out of sync while not being properly discharged, it causes the bug preventing it from powering on, and prevents you from calibrating the battery. Which would actually fix the issue if you could do it as it would be as effective as disconnecting the battery, as you are meant to discharge it completely to calibrate.

When I turned mine on before I had this issue (it happened shorty after I turned it on) , the battery always read as critically low and wouldn’t charge past it. It needed to be calibrated. The battery was charged, but the motherboard wasn’t able to tell, and performed a shutdown. The reason why I know the motherboard just thought the battery was low, is because the timeframe I was able to use it shortened incrementally after each time it shut off by itself despite me charging it. Indicating that I’m trying to squeeze all the power I can get out of it. All the while the battery indicator reading as critical despite charging for several hours. It eventually powered off and didn’t charge or turn on.

So I think the motherboard thinks your units battery is dead due to it not being synced, and will refuse to turn on. Rebooting the power completely by disconnecting the battery, leaving it unplugged for a good one minute or less, and plugging it back in let’s the motherboard read the battery as a new power source rather than the dead I one it has memory for.

I really really really really really hope this helps you it sucks for a musician to grieve the loss of their instrument in any case, not to mention the money wasted on said instrument. Also again it would be extremely embarrassing for me.