OK, first of all, with a little research, you'll see that the REAL reason for all the RA series amp failures are the crappy DONG-WOO relays Alesis chose to use. RA-150, 300, and 500 all have the same problem. Simple solution for the RA-300 (providing you can solder/desolder or have a friend who does):
* Buy one NTE R25-11D10-12 (at $6.24)
* Disassemble the amp (6 screws on top, and four on the side, inside the heat sink fins, the inside screws)
* Carefully unscrew the power supply board (it's the one on the bottom with 4 screws and one screw that holds the rectifier)
* Using a 40w or higher desoldering tool (or solder braid), remove the black relay (8 contacts, total)
* Replace with the NTE one you bought
Voila! One working amp! Total bench time? One hour. Also, check out the 10000uF 63v caps. If they are bulging or "rattle", replace them, too.
-=> Andrew From Concept To Reality, L.L.C.