SEAS 25F-EW Motor Repair: Fixing Loose Magnets and the Centering Jig That Makes or Breaks It
The 25F-EW’s chronic problem — and what actually causes it
Loose magnets are the most common failure on surviving SEAS 25F-EW drivers. The 10-inch woofer moved through a lot of Scandinavian living rooms in the 1970s, and a substantial number of them spent subsequent decades in damp garages or basements. That storage history is what does the damage.
The motor structure uses mild steel for the back plate and front plate, both zinc-plated. In dry storage, zinc plating is reliable protection. In humid conditions, the zinc oxidizes. Zinc oxide takes up considerably more volume than the original metal, and that expansion presses outward at the bond line between the magnet and the plate. The original epoxy on many of these drivers was applied thin and unevenly — the factories were building commodity product, not museum pieces — so the bond cracks under the pressure.
Once cracked, the magnet can shift by several millimeters. In mild cases the driver still produces sound but distorts or buzzes at certain frequencies. In worse cases the voice coil rubs against the pole piece at rest. Either way the underlying repair is the same.
Before you start: assess what you actually have
Check the surround before disassembling anything. Foam surrounds from this era typically have not survived. If the surround is gone or crumbling, you will be doing a full recone anyway, which changes the sequence of work slightly.
Also check the cone for tears and the voice coil for rubbing by moving it slowly in and out of the gap by hand. Rubbing confirms the magnet has shifted. A coil that moves freely but the driver still sounds wrong — distorted at high levels, say — points to the magnet having shifted slightly rather than dramatically.
What the teardown looks like
Getting clean access to the motor joint requires removing the cone assembly. The surround and spider need to come free, the voice coil former slides out, and the cone comes away. This is the correct order even if the cone is in good condition — you need the pole piece clear and the gap fully accessible.
With the cone assembly removed, the motor structure can be lifted from the basket once the basket screws are out. The magnet and plates may already be partially separated. Handle the ferrite magnet carefully: it chips if struck sharply against metal.
Separate the plates from the magnet using a thin putty knife or chisel worked around the perimeter. Go slowly. The goal is clean separation, not speed.
Cleaning the mating surfaces
Old epoxy residue must come off completely. A rotary tool with a sanding drum works well on the steel plates. On the magnet face, use an abrasive pad by hand — the ferrite is brittle enough that an aggressive power tool can chip the edges. Work until the surface feels uniformly rough to the touch, not polished.
Pay close attention to zinc oxide deposits. They appear as powdery white or light grey patches and tend to accumulate in recessed areas and along edges. Brush them off before the final clean. They do not bond well to epoxy and act as a weak layer if left in place.
Wipe all surfaces with isopropyl alcohol — 90% concentration or higher — and let them dry completely before mixing any adhesive. Skin oil alone is enough to compromise a thin bond line.
The centering jig — the part most repairs get wrong
This is where most home repairs fail. The magnet must land in the correct position so the gap around the pole piece is uniform on all sides. Off by half a millimeter and the voice coil will bind against one wall at some point in its travel.
The voice coil former itself is the most reliable centering reference. Before disassembly, if you can, measure the gap width so you know what thickness of shim stock to cut. Cut strips of thin cardstock to match that width. Slide them uniformly around the pole piece at three or four evenly spaced points. Lower the voice coil former over the pole piece — it will now sit centered, held by the shims. This is your reference for correct magnet placement.
Apply a consistent bead of slow-cure epoxy to the mating face of the back plate, position the magnet, and bring the assembly together while the centered former is in place. Do not use 5-minute epoxy. The working time is too short to get alignment right before it starts to set. A two-part mix with 10 to 15 minutes of open time gives enough room to work carefully.
Apply clamping pressure along the axis and leave it for a full 24 hours. Then remove the cardstock shims and check that the former moves in and out of the gap smoothly with no binding anywhere in its travel range. Any binding means the alignment is off. Better to find it now than after the cone is re-glued.
Reassembly
Once the cured motor passes the gap check, reassembly follows the reverse of teardown: spider and cone re-glued into the basket, surround fitted, voice coil lowered into the gap using fresh centering shims, leads re-attached.
If the surround had rotted, fit the new one at this stage. Foam surrounds are still available for these drivers, and the repair is a natural moment to do that job properly rather than patching around existing deterioration.
The hack angle: what to do while the motor is open
With the motor already on the bench, a few modifications are worth considering.
The pole vent in the 25F-EW is modest. Some restorers enlarge it slightly with a drill to improve thermal performance — air circulates through the gap as the coil moves, pulling heat away from the former. This only matters at higher power levels and is entirely optional.
If you are reconing anyway, check any aftermarket recone kit against the original Thiele-Small parameters before ordering. Some kits use a different cone mass or surround compliance, which shifts the resonant frequency. Whether that suits your enclosure depends on the original box alignment.
If the driver is beyond saving — cracked cone, damaged coil, shattered magnet — the SEAS A26RE4 (H1411-08) is commonly cited as the closest current-production equivalent. The T-S parameters are not identical to the original, so a direct drop-in swap into an existing cabinet will need some enclosure adjustment to restore the original tuning.