SEAS 25F-EW repair and hack

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Disco Volante

Well-known member
Joined
Jun 20, 2021
Messages
276
Location
Southern Denmark, Fyn
The classic 10" driver from Seas, sold truckloads in the 70's. Still reasonably easy to get hold of, at least here in Scandinavia.
A lot of them suffer from loose magnets due to moist storage, the zinc plating on the steel parts of the motor expands and cracks the old (sloppily applied) epoxy.
Well, what better fun on a rainy sunday, let's have a look at what's inside and possibly a way out of the misery;-)
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The cone and suspension is in the way. After removing the loose parts of the motor, (no longer clamping the VC tight), scalpel and fingers do a great job of removing the soft parts.
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While I'm at it, why not convert these two regular 25F-EW to a pair of unobtanium 25F-EBWX? (Seas for extra magnet and foam surround)...
This is where the machine shop comes in handy.
New larger magnets (120x18x60), watercut steel discs (designed in FreeCad). The center piece can be recycled, with a 2mm shim to account for the thicker magnet.
Did I mention I *love* my lathe?

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Threaded holes in the front plate and magnet gap adjusted. I was able to gain .4mm on the outside diameter after measuring against VC. Hope I can still get the thing centered properly, come that day. But the higher efficiency is worth the extra trouble.
I increased the thickness from to 8mm; the VC is 14mm high which will give an Xmax of 3mm, approximately, and a significant increase in efficiency.
 
After coffee break, turned a centering plug from POM. Love this plastic, it cuts perfectly. A bit wasteful turning it from solid stock, but it's Sunday after all...
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Ready for assembly:

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TIG-welded the pole piece to the back plate. It was a press fit, but it needs to be secure. Seas used a sort of riveting technique, but now that I've cut it out of the old backplate, I have to weld it.

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Glued it up with epoxy, put a little bit of a rather firm polyester felt in the space inside the magnet. These magnets are made for 2" VCs, so this little 38mm 1.5" thing leaves a big gap inside.
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It's looking up, but the hardest part is still ahead...

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Got the basket on the motor, cut three screws to exact length, used a drop of Loctite to keep the screws in place and a little SMP sealant between the parts to prevent resonances.
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The old rubber edge was a royal pain to get off. These drivers are very well made (well, except for the glue on the magnet). Used several sharp blades in my little cutter, strong glasses and just very fiddly business getting the rubber and glue off without cutting into the delicate paper cone.
Foam surround mounted, carefully centered. I use a German brand "Auro" of a less smelly/toxic and somewhat adjustable contact-glue.
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For future hacks on these units, I'll probably just leave the rubber surround on. The idea was to lighten the outer edge of the cone to lessen the inevitable HF breakups associated with the bigger motor system vs a very light membrane. Measurements will show, I might have to coat the membrane with something to stiffen it up a bit, moving the breakups above the point of crossover.

Dust cap needs to go, to be able to center properly. I use various thicknesses of paper. This one took three layers of ordinary office paper to get a tight fit. They need to not overlap, so cut to length. We don't want any excentricities in the VC and centering needs to be as good as possible.DSC01768.JPG
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Ok, it's beginning to look like a speaker again. Smeared glue on the edge of the spider to bond it to the edge where I cut it a while ago. Not too pretty, but it'll work. I use a glue called Rubberfix made by Casco for this. It's also (mainly?) for fixing shoes, and it's the most durable "liquid rubber" I've tried (I don't work for Casco).
It takes a while to harden though, up to 48hrs depending on thickness.
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The second magnesium basket had some patches where oxidation had broken through the paint, so it got a partial blasting with the neighbour's glass-bead blaster, and a fresh coat of matte black paint. Also glued up the second motor system..IMG_3788.JPGIMG_3783.JPG
 
Resurrecting this old thread just to add a few measurements of the finished drivers. Somehow not too impressive after all that work ;-/
I'd expected a little higher sensitivity with a significantly larger magnet...

Measured outdoors on the parking lot on a quiet day, to get a better idea of the "actual" LF response.
Closed box 35l, Umik-1 0.5m from driver, 1.44V on the terminals @1000Hz.

Red line is a known-good specimen, 1975 vintage..
Green has the standard rubber surround and the larger motor.
Blue is the foam surround, large motor version. As expected, some phase funk going on above 700Hz, and a little extra SPL possibly from the lighter weight. The foam surround weighs 11g, the rubber 33.
Second picture is the same graphs with a little spacing.
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I love this.
Adding magnetic flux in the gap will increase the sensitivity at higher frequencies, but typically won't do much below 100 Hertz. Most woofers choose to use smaller magnets to decrease the sensitivity at higher frequencies to boost the apparent bass. Increasing the length of the voice coil or decreasing the thickness of the top plate (reduces flux density, increases linearity) will help with the bass (and reduce mids and treble)
Reducing moving mass by changing the rubber to foam will increase the sensitivity and the resonant frequency, (which will move the impedance peak up, reducing useful bass). Adding mass to the cone (or trying weights on the dust cap) can counteract that (and reduce sensitivity).
Speakers are a giant can of worms; More power to you, and good luck with this project.
 
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