I used to be able to unplug the Mini USB cable from my computer after I booted up a patch in the Bela IDE. Now, even if Pepper is powered by my rack, disconnecting the USB connection turns it off. Suggestions for a fix?
Pepper shuts off when USB unplugged, even in presence of Eurorack power
Does the module power on from Eurorack power alone?
The capability of switching from one supply to the other is built into the BBB's power management IC, so nothing should have changed that would affect that. The expected behaviour is that if Eurorack power is available, then it should always power from it and only switch to USB power if Eurorack power is down.
It doesn't power on from Eurorack power alone.
I'm remembering now when this behavior started to happen - it was after I plugged in the Eurorack power connector backwards 
The only part that was visibly damaged was the op-amp, which I replaced, and that restored the original functionality save for this new power issue. Which part(s) do you suggest I replace?
That makes a lot of sense: the reverse polarity probably damaged the 5V power regulator U1. Suitable replacements are: AMSR-7805-NZ (e.g.: https://www.tme.eu/en/details/amsr-7805-nz/dc-dc-converters/aimtec/ )or R-785.0-1.0 (e.g.: https://www.mouser.com/ProductDetail/RECOM-Power/R-7850-10?qs=YWgezujkI1LK5NzKL%2Fc9sg%3D%3D)
15 days later
I went ahead and replaced the 5v regulator, although now I'm not sure it was the problem. Ever since I've had Pepper, I've always had J14 set to "5v straight from rack." If I switch that over to "12v with regulator," Bela powers up just fine.
So I can use it just fine now. But I'm still curious to know where the problem is?
Maybe it's the 5V rail on your PSU that is not workign? With "straight from rack" it should power from it, the jumper simply connects the 16-pin connector's 5V straight to the BeagleBone's 5V input.