Hi,
I finally managed to overload a few sub drivers during a show, despite having set the proper factory settings (including limiter).
While one coil completely destroyed itself, one voice coil exhibits the classic bubbling on the vc former, leading it to rub in the rather tight magnetic gap.
The drivers are old Adamson AW18 (which are based on McCauley drivers afaik), where you can separate the motor from the basket and can therefore gain access to the cone and vc without destruction. As original replacement baskets are ridiculous expensive for a vintage pa (costs even more than a brand new 18sw115 for just the basket/recone), I wanted to try to save at least this cone.
Does anyone have a tip to flatten or get rid of the bubbles? My current ideas range from injecting a tiny bit of epoxy to just cutting them out with a razor...
Thanks in advance!
I finally managed to overload a few sub drivers during a show, despite having set the proper factory settings (including limiter).
While one coil completely destroyed itself, one voice coil exhibits the classic bubbling on the vc former, leading it to rub in the rather tight magnetic gap.
The drivers are old Adamson AW18 (which are based on McCauley drivers afaik), where you can separate the motor from the basket and can therefore gain access to the cone and vc without destruction. As original replacement baskets are ridiculous expensive for a vintage pa (costs even more than a brand new 18sw115 for just the basket/recone), I wanted to try to save at least this cone.
Does anyone have a tip to flatten or get rid of the bubbles? My current ideas range from injecting a tiny bit of epoxy to just cutting them out with a razor...
Thanks in advance!
I'm going to wish you luck on that but who knows how successful it will be . What is the DC resistance of that VC? Zooming in it looks like it got pretty hot.
An experience reconer might be able to put new coils on those drivers.. given the access afforded by the design. Might be something to look into.
An experience reconer might be able to put new coils on those drivers.. given the access afforded by the design. Might be something to look into.
The coil itself seems to be fine, I'm reading 6.0 ohm, close enough to 5.8 ohm in the data sheet considering contact resistance and semi accurate handheld meters.
It did get hot enough to emit typical burnt vc smell, but not all drivers hat the same damage. One could shorted out and completely annihilated itself, this one bubbled up and another one got a bit black towards one end, probably indicating a sagged spider. The other three coils look fine. Really unfortunate that I forgot to put an additional rms limiter in place. It was a bass music event and right when I was checking another room, the dj turned up the system a lot and landed some semi continuous sines right at the tuning frequency. Factory peak limiter of 37dBu/54V (rms) seemed conservative enough at first, but apparently it was not 🥲
I also considered having just the VC replaced by a shop, just haven't found one yet (located in GER). I also wound't know where to get one with similar enough dimensions/parameters
It did get hot enough to emit typical burnt vc smell, but not all drivers hat the same damage. One could shorted out and completely annihilated itself, this one bubbled up and another one got a bit black towards one end, probably indicating a sagged spider. The other three coils look fine. Really unfortunate that I forgot to put an additional rms limiter in place. It was a bass music event and right when I was checking another room, the dj turned up the system a lot and landed some semi continuous sines right at the tuning frequency. Factory peak limiter of 37dBu/54V (rms) seemed conservative enough at first, but apparently it was not 🥲
I also considered having just the VC replaced by a shop, just haven't found one yet (located in GER). I also wound't know where to get one with similar enough dimensions/parameters
It is conservative enough. But the damage accumulates. Once it suffers from some damage, it does not 'heal'. The initial damage may have occurred long ago. The problem with the bubble is, it is a thermal insulation, so these spots always get hotter.
Filling with epoxy doesn't solve the problem, it expands at a different rate. And it becomes soft (yes, some cured 2K materials can do that) at much lower temperatures than the kapton can withstand, the VC movement will likely spread it to where you can't get it out from at all.
To find a replacement VC isn't that complicated as you might think. Look for drivers with the exact same VC diameter and close enough other parameters and get a recone set. The real task is then to separate the cones from the VCs. It's expensive and risky.
Tbh, cutting the bubbles off is the cheapest and safest option.
I'd probably replace the drivers though. You will likely not find any Kevlar drivers with similar parameters and to mix different types of drivers isn't likely going to work well.
mh, ok, thanks. Cutting it is then as a first measure.
There's not much more to loose here unless I find someone who is able to swap a coil
There's not much more to loose here unless I find someone who is able to swap a coil
Can't tell how deep yours are, but I was able to sand some Kapton bubbles flat enough to make no noise using very fine sandpaper.
Would have been fine if the music had 12/1 crest factor like in the olden days 😉
thanks, that might do for the smaller bubbles 🙂
a few will have to be cut out though I'm afraid
still, around 400w per driver with a 4" coil doesn't sound excessive, they are rated for 600w free air (derated to 450w insider the B318).
But sure, for bass music I should have set an additional thermo limiter to maybe 150w. I wanted do to that before the show (new controller), but my bass amp broke down so I had to get a last minute replacement, which robbed me of the time to set the second limiter (unfortunately not possible on the front panel, just via network/pc). Voltage gain was all correct though (verified with a multimeter). Just a series of unfortunate circumstances 🙁