Picked up a QD-5500 with unknown history with about 20 reels with material already on them. Not sure if they were recorded on this machine. Anyway, I'm trying to figure out why it has a muffled sound. Head alignment, or worn out? I've already cleaned and demagnetized the heads. Could the tapes be wasting away? Also it shows output on three of the four meters, the right front channel comes and goes.