Time for this week's update.
I've done another midrange driver (well, partly).
So, Friday evening at work - everything's done, it's after hours, everyone else has gone home. Time to re-ferrofluid another midrange.
I examined its movement prior to disassembly, and noticed a clicking sound when manipulating the cone.
Close inspection revealed that, just like the first one, the glued join between the voice-coil former and the cone was starting to fail. A little more poking and prodding and the two separated.
OK. Having closely watched when the first driver got fixed, I now know how to go about it, and happened to have the correct solvents and adhesives to hand. On the one hand, I'd have preferred it if the thing hadn't fallen apart. On the other, if it's going to break, now is by far the best time for it to do so. Fine... I'll fix it.
So, I made up some plastic shims to hold the coil former central in the magnet gap. I cleaned the mating surfaces of the cone's neck and the coil former with isopropanol and applied the primer (heptane) for the adhesive. There's a 5 to 8 minute wait for the primer to do its thing, and then you assemble the two parts and apply a little cyanoacrylate adhesive by means of a syringe and needle. Simples...
And this is where things went wrong. I'm adding micrograms of adhesive at a time, watching it wick into the joint. That's nice.
And then I spotted that not all of it was going where I wanted. The edge of one of the coil's shims was in contact with the inside of the top of the voice-coil former, and I watched in horror as a drop of cyanoacrylate ran down the shim and into the space where the voice-coil former entered the magnet's gap. Disaster - I've just superglued the coil former to the magnet. 'Oh dear, I seem to have messed this up a bit' I said to myself (well, not those words exactly, but you get the picture).
Hmmm... I have very little time do something about this (seconds). OK, I've got another syringe and a bottle of methyl ethyl ketone (death-strength solvent) to hand. If I chuck a millilitre of it into the gluey mess I've created, perhaps it'll stop it from bonding all of the moving parts together. No better idea came to mind, so I did. It appears this was my master-stroke of luck.
In record time, I freed the front surround and rear suspension from the chassis of the loudspeaker and wiggled the cone and coil assembly free, with no obvious damage. A moment of great relief, one might say.
Of course, now I need to clean out the drying cyanoacrylate (and old congealed ferrofluid) off everything.
Although I was unaware of this, it seems that applying methyl ethyl ketone to superglue (whilst using language that would kill a priest) turns it from rock-hard indestructible nastiness into a gooey rubber-like substance that peels cleanly off. It was at this moment that my train of thought went from 'I've just wrecked this' to 'I can fix this', and my pulse rate and blood pressure started to return to normal.
Twenty minutes of q-tip and MEK action cleaned the voice coil and former to perfection, and some MEK and strips of cardboard seem to have cleared the magnet gap completely based on inspection with a microscope.
Things I learned in the process:
Cyanoacrylate dissolves in MEK.
Razor blades will cut through the adhesive bond of the front surround just as well as they will cut a chunk out of your thumb.
Methyl ethyl ketone in a cut hurts a lot more than isopropanol does.
You can clear the magnet gap of solvent and old ferrofluid using compressed air remarkably quickly.
If you clear the magnet gap with compressed air, everything in a ten foot radius gets sprayed with ferrofluid.
Ferrofluid is very hard to remove from walls, floor coverings, computer keyboards, toolboxes and skin.
The driver's cone and coil are attached. They're not back in the chassis of the speaker yet, but I see no reason why they won't reassemble without difficulty. The alignment and bonding of the voice-coil former and cone looks good.
Of course, now we're two-for-two on the voice-coil formers detaching from the cone neck, I expect the other two mids to go the same way. If they don't immediately fall apart, I'll gently torture them until they do and then re-glue, but hopefully more straightforwardly than this one...
Progress....