Roland has a nifty little thing named “RMP-5”. It’s called a ‘rhythm trainer’ and I like it a lot. It’s something different from the usual stuff I do. And it’s fun.
Less fun is the power supply which is completely missing in the box. While you can use a (supplied) 9V battery, the unit draws about 30mA which lasts only about 6h. The RPM-5 can be powered by a 9V DC power supply, but the one from Roland is kind’a expensive compared with what’s available (e.g.) here.
So of course I looked at the power supply plug at the RPM-5 and it looked like a plain vanilla 5.5mm barrel jack. So I went out, bought a 9V DC unit, connected it, and…nothing happened.
Roland thought it’s more fun to change the polarity. Instead of minus being outside (and thus +9V inside) as usual, they swapped that.
Now the good news is, that the RPM-5 does not break if you try to plug in a ‘wrong’ PSU. It just does not turn on.
The fix is to swap the cables and thus polarity. Et voila, works as expected.