Some time ago I bought a Roland M-240 24 channel line mixer. It looks like a nice and compact mixer, but the seller never got around to delivering the power supply / wall wart for it. I have tried googling to figure out what kind of power supply I could use, but have not found any info whatsoever.
So, today I opened the mixer and had a look at the power input board. It is quite simple - no bridge recitifier and no circuit breaker. It has a few huge caps though.
The connector on the back says +/-21.5V DC 500mA. The internal voltage regulators however - AN7812F / AN7815F / AN7912F / AN7915F - can all handle +/- 35V input, so anything between 21.5V and 35V should be fine - it might even work with 18V but this is not guaranteed. Beware that a different voltage than specified may make the voltage regulators run hotter than intended though!
I made a schema of the circuit, it looks like this:
Looking at the circut makes me think a bit - the two diodes look very much like they form a half-wave rectifier in conjunction with the 2200uF electrolytic capacitors. I do however take no responsibility if you connect an AC power supply to the mixer and it burns out!