the heads have small slots in them, one for each channel. Look closely for a buildup of gunk inside the slots. Clean with denatured alcohol and a q-tip. Also, if the transport system is misaligned, it could cause problems. The tape has to be 90 degrees perpendicular to the heads for best results. Another problem could be the bias, which could be an internal component problem. If I didn't see any obvious dirt or misalignment issues, I'd send it out to a qualified technician.