Wearing Tefillin For Musaf On Roush Choudesh

The Mekubolim gave two reasons not to keep the Tefillin on for Musaf. The first is the one you mentioned, that it has a semi YT status, and the second is because one says the Kedusho of "Kesser", and therefore one shouldn't have his Tefillin on.

These two reasons were not accepted in Ashkenaz. The second reason doesn't have any appliance to Ashkenaz, since we don't say Kedushas "Kesser", and the first reason also wasn't accepted, since removing of the Tefillin is considered bizayon Hatefilloh, which was considered more important of an issue.

We first find in the 16th century that some started taking off the Tefillin then in Ashkenaz, and at the 18th century it was the Minhag of the majority.

Michael FRBSH

Rav Hamburger told me there isn't too much data about this question written by Poskei Ashkenaz (since the Minhag in the past was to keep the Tefillin on at Musaf too).

Michael