Touble With My Trolling Motor

Last season I purchased a MinnKota PowerDrive 55lb. It was a Craigslist purchase, but it was basically new, not a scratch or mark on it. The end of the season, it got stuck on full speed. Everything else worked, but it wouldn't slow down. I called MK today & explained the situation. I also included the fact that I'm a hardcore advocate and loyal customer. So they took 25% off the circuit board and & the express shipping. I ended up paying $67 but I also wasted $70 on a pedal thinking that was the issue. I'm gonna fix the motor, but I'm really uncomfortable keeping the motor. I want to go back to cable steering. My local Wal-Mart has the 50lb edge for $450. I would love to buy a more powerful 24V motor but my boat doesn't have any more room for batteries. Any ideas gentlemen??