I have an unusual (to me) problem with one of my Studer A807 decks. On both the 3.75" and 15" speed the mid range has dropped out. I get a flat response for, 32Hz-1KHz and 10KHz to 20KHz, but the mid frequencies are around -8db. The 7.5" EQ looks great. I've tried two different MRL tapes and two different head blocks, but the results are consistent. This happened all of the sudden. This machine doesn't have options to choose either NAB or CCIR. Any ideas what might be happening?