On Wednesday, 14 November 2018 01:28:51 UTC, John-Del wrote:
On Tuesday, November 13, 2018 at 6:14:47 PM UTC-5, Dave Platt wrote:
In article ,
John-Del wrote:

This is a favor for a friend of a friend. I've never done a refoam before but I realize it's not rocket
science but there's probably a learning curve. I won't have the option of do a trial run.

These drivers come out of a pair of Infinity Crescendos, and the plastic dust caps seem well bonded to the
cone. I'd much prefer to shim the cone before bonding the foams, but I don't know if the dust caps can be
removed somehow from the cones without damaging them.

Any advice appreciated.

SpeakerWorks sells re-foaming kits for a lot of sizes (both foam, and
rubber surrounds).

There are at least three ways to make sure that the cone is centered,
when you do the final glue-down of the surround:

(1) Remove the dust cap, shim, and then re-glue the dust cap
afterwards. Apparently the dust caps can usually be removed
without too much damage by cutting them carefully near the base
with a single-edged razor blade or a sharp box cutter or etc. One
must be careful not to cut the voice coil, damage the voice coil
former, or get debris into the gap between magnet and voice coil.
Lots easier with a paper voice coil than with plastic, I don't doubt!

The instructions which SpeakerWorks provides with their re-foaming
show how to do this, but it is _not_ their recommended approach.

(2) Do it "unassisted". SpeakerWorks shows a procedure where you
press gently down on the dust cap, and carefully manipulate the
dust cap and cone side to side slightly so you can feel the
positions where the voice coil moves in and out freely without
touching the magnet assembly. This is their recommended approach,
and they claim that most customers have no problem with it.

(3) Use electro-mechanical centering. If you apply a small DC voltage
(a single AA cell is sufficient - even 1 volt works in many cases)
it will energize the voice coil, move the speaker inwards or
outwards by a fair percentage of its Xmax, and the force of the
"spider" and the magnetic field interaction between voice coil and
magnet will tend to center the voice coil in the gap.

I used method (3) when re-foaming a pair of Minimus 77 drivers, and it
worked perfectly - no scraping was evident after the repair was done.

I did it again, last night, when gluing new rubber surrounds onto a
pair of Minimus 7 drivers, and it seems to have worked just as well.
After the glue dried I disconnected everything, and gently pressed the
dust caps, and the cones moved smoothly without any sign of rubbing.

The method I used was: apply a positive voltage to the coil, moving
the cone in the "outwards" direction for several millimeters. This
makes it convenient to glue the new surround to the cone, as the outer
edge of the surround will be up above the frame. Let the glue dry.
Lift up the outer edge of the surround, spread glue on the frame,
relax the surround, then turn voltage down to zero (returning the cone
to its rest position), reverse polarity, and apply a small negative DC
voltage. This pulls the cone "inward" slightly and snugs the edge of
the surround down onto the glued frame.

You can do this with just a penlight cell - when I did the Minimus 77
drivers I found that 1.5 volts was plenty, even with the two drivers
wired in series. I took a more delicate approach with the Minimus 7
drivers last night, using an adjustable DC supply (with the current
limit set to about 100 mA just to be safe!).


Thanks Dave - I really like the idea of adding a bit of DC to the coil. I do have a programmable power supply so I'll be sure not to cook that VC with too much DC.

I was thinking a nice, *slow* setting, syrupy although not too viscous adhesive would allow the cone to self center under the new foam. The kit the guy gave my brother has a small bottle of adhesive (that leaked a bit!). Hopefully it'll be suitable.

The coil is centred by the spider 100% of the time. What you want to centre is the edge of the speaker at the foam surround. There is nothing in the standard moving coil speaker mechanism that will do this by itself. Method 2 works IME.


NT