Otari MTR90 record head 10k tone fluctuates back and forth.

[arthurdb]

As someone else said, the tape tension seems to be too low. Did you double check the tension setting? Do you have a tentelometer?

Since the fluttering seems to be linked to the rotation of the supply reel, this might not be the only issue:

The bottom flange wobbles a bit. When it comes to two-inch tape I only have deep experience with the JH24, but on those decks, the bottom flange is a crucial part of the transport, and needs to be extremely stable and at the perfect height.

You've mentioned using several new reels, are these from the same batch/source? They could have been a manufacturing defect, or maybe they have been stored improperly for too long by the dealer... Since you have a brand new MRL tape, how does the unwinding from the supply reel look like with the calibration tape?

 

[RoadrunnerOZ]

The lock of the reel tables to the motor shafts is via two Allen grub screws for setting reel table height which must grip to the two flats on the shaft that are at 90° to each other - if these are mis-located or not fully tightened there will be an eccentric action and wobble to the reel as the screws take up and let go.