My 424mkIII is in really great shape. The other night I was uploading some mixes to the pc. I had an RCA going from the line out to the line in on the PC. I had my headphones in the PC headphone jack. After I was finished with a mix, I plugged the phones back into the 4 track and the monitor trim level blew out. Turning the trim knob has no effect. I've tried de-magnetizing the unit. I tried unplugging the unit for a few days. I've tried every combination of switches to see if it would come back. Still nothing. I just bought the thing and I love it. Am I gonna have to take it in for a repair?