Repair of displaced pole piece in early AR-3 mid-range

[mvboland]

As part of my AR-3 project, I was able to repair a particular issue with an early (serial number < 1000) AR-3 mid-range driver and wanted to share it in case it helps others keep theirs in service.

As many here have done before, I sent my hi and mid drivers off for refurbishment. After removing the dome, the pole piece was noted to be displaced relative to the hole in the faceplate:

After some helpful feedback from @tysontom regarding how those drivers are put together, I loosened (did NOT remove) the central bolt on the back of the unit which secures the pole piece to the backplate. Note that the glue used to secure those fasteners was thin and easily broken:

Once that was done, I confirmed I could move the pole piece using a small screw driver.  As I was thinking about what tools/shims to use to center it in the faceplate, I pulled out my set of nail setting tools, one of which fit perfectly into the notch in the faceplate. With some tapping (gently so I did not ruin the magnet!), I was able to center the pole piece:

I then re-tightened the center bolt and the unit was ready to have its coil and dome re-placed:

I believe the root cause here was loosening over time of all of the fasteners as none were tight when removed from the enclosure. The glue applied to the back of the driver at the factory would be expected to hold the nuts on the 3 bolts securing the faceplate to the backplate in place but I am not clear it kept the bolts themselves from rotating over time. I did tighten everything before re-gluing the nuts and bolt on the backplate.  I would have preferred to use a thread locker (Loctite) but did not want to remove any of those fasteners out of fear I would not be able to get things back together.

Thanks again to @tysontom and @RoyC for their time and sharing of knowledge on this project!