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Bandor Woofer Repair - rubbing voice coil

irb

pfm Member
Long post, sorry! I started a thread the other day in the classifieds, just incase anyone had a spare Bandor 100mm woofer lying around. Not likely, I know. But I also thought I'd post here as I'm considering a diy fix in the absence of a spare.

As I mentioned previously, I recently acquired a pair of Pentachord speakers with column bass units. Each bass unit has a pair of the Bandor 100mm woofers, mounted face to face in an isobaric configuration. I realised a little too late that there were problems - probably because I played them so quietly for the first few weeks. Anyway, having dismantled them, I found several problems - two of the four with blown leads, one with a rubbing voice coil. In fact the fourth one also had a rubbing voice coil, but I reckon it was just dirt, and I've managed to get it working fine now. (It's hardly surprising it had dirt in the gap, because the speaker it was in deposited a huge pile of brown dust on my carpet when I pulled the drivers out. I assume the dust had once been damping foam, but it was truly disgusting.)

I've resoldered one blown lead and it's fine, so that's 2 out of 4 working properly. I'm going to have a go at resoldering the other blown lead, although it's a bit trickier.

The one I'm most worried about, though, is the one with the persistent rubbing coil. I've established that if you press in on one side of the cone it is fine, but if you press elsewhere, including the centre, it rubs. So it's maybe just misaligned.

I'm in two minds whether to send it to a repairer or try fixing it myself. I've read about the acetone method, which involves removing the dust cap, shimming the coil to push it towards the position where it doesn't rub, and then putting a few drops of acetone on the spider, which apparently should soften the resins in the spider. Leaving it for 24 hours or so will allow the resins to reset, and hopefully the voice coil will be in a better alignment.

Any experience or thoughts?

And if I do have a go, should I cut the dust cap off or try dissolving the glue with a solvent? I've seen both methods recommended.
 
If you find a reliable competent commercial repairer, post a link here. They will be deluged with work.

Meanwhile Cooky may well be your saviour...……………………………

(Drop him a PM if he does not pick this up)
 
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If you find a reliable competent commercial repairer, post a link here. They will be deluged with work.

Meanwhile Cooky may well be your saviour...……………………………

(Drop him a PM if he does not pick this up)

Cheers for the tip. :)
 
LLLOL

My apologies here to Cooky, Mr. Speaker, font of all information and advice that is actually genuinely useful (and a damned fine bloke).
 
LLLOL

My apologies here to Cooky, Mr. Speaker, font of all information and advice that is actually genuinely useful (and a damned fine bloke).
Gosh, I'm blushing...oh wait it's just a fever...
Ian is all over this tbh, he'll sort it.
 
OK, an update. I managed to remove the dust cap on the dodgy woofer pretty easily, by softening up the glue with some solvent. With the dust cap off, however, I could see that the voice coil former has suffered a bit - a slight kink in it. I'll put some photos up when I get a chance. It's pretty clear that the coil has been driven into the back plate at some point. (The reason why this happened has also become clearer - see my next post.)

However, I had a go anyway at shimming the coil and treating the spider with acetone. The good news is that it made a significant improvement. It hasn't cured the rubbing completely, but it's much easier to move the cone without rubbing. I've not tested with a signal yet - I decided to give it a second go with the shims/acetone first. I'm hopeful that will get it working 100% - I'll know tomorrow.

I'm getting ahead of myself, but does anyone have a recommendation for glue to use when fixing the dust cap back on? (Fabric dust cap to aluminium cone.) The original glue was clear - maybe something like Airfix glue, I'd say.
 
Super Glue comes in numerous flavours, despite what many think and go into print about...……………. Have a look on the Loctite website and there is almost certainly one flavour to suit.

The bond to the fabric will be very largely mechanical/physical - getting into the weave, the bond to the aluminium will ideally be chemical.
 
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I had a feeling right from the start that these Pentachord bass units were a bit fragile. Sure enough, taking them apart suggests they've suffered over the years. The bass drivers in one pair are clearly younger than the other pair - so I think they must be replacements. And one of the older pair of drivers has a different impedance from the other three - 8 ohms as opposed to 5.7 ohms. I suspect it's had its voice coil replaced at some point. (That's not ideal, when they're being used in pairs with isobaric loading.)

I think I've worked out what the source of the problem is. In each unit, the two drivers are bolted together, face to face, with an mdf spacer. But then they're not bolted in place within the cabinets. They slide in until they meet some little stops, glued inside the cabinet, and are held in place by the base panel.

I was puzzled at first how a seal was created between the drivers and cabinet, because the 'stops' don't go all the way round, so the driver frame doesn't seal against them. The short answer is that there hasn't been a seal at all! The intention clearly was that there would be a seal around the outside of the pentagonal mdf spacer, between it and the cabinet wall - but the tolerances just aren't tight enough. There are cloth 'socks' that are supposed to act as gaskets, but they have fallen apart. Even before that, I doubt if they worked. The gap between the mdf spacer and the cabinet is too wide to have been sealed by the socks.

So, in practice, the drivers have been operating in an unsealed box, which I reckon has contributed to them being driven past their maximum excursion. I'm working on a fix.
 
One more thing. I mentioned before that one of the Pentachord bass cabinets deposited a large pile of dust on my carpet when I removed the woofers. I said I thought it was perished foam.

When I took the woofers out of the other speaker, the cabinet was stuffed with long fibre wool, not foam. So, what was the dust? I'm wondering if one or more moths had got into the cabinet (which was not properly sealed - see the post above). I think the dust may have been what moths produce when they eat their way through a large wad of wool. So now I know what a large pile of moth poo looks like.

:D
 
Caterpillar frass.

Adult lepidoptera just need calories and salts, almost always in liquid form - nectar, moisture extracted from droppings, pee etc.

Likely candidate, tube-forming moth, or white-shouldered clothes moth.

:)
 
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Caterpillar frass.

Adult lepidoptera just need calories and salts, almost always in liquid form - nectar, moisture extracted from droppings, pee etc.

Indeed! Thank you Vinny - I didn't know the word frass before.
 
My apologies for that.

The finding and near universal use of the word poo in recent years is a pet hate. It was used only when speaking to young children when I was similarly young.

What is wrong with dropping, turd, even frass? How about scat (although that has taken on a WHOLE new meaning in recent years)? Spraint? Cow pat, rabbit currants, horse chips, and so on....
 
My apologies for that.

The finding and near universal use of the word poo in recent years is a pet hate. It was used only when speaking to young children when I was similarly young.

What is wrong with dropping, turd, even frass? How about scat (although that has taken on a WHOLE new meaning in recent years)? Spraint? Cow pat, rabbit currants, horse chips, and so on....

Excellent. There were a couple of what looked like clothes moths in the pile of dust. I thought nothing of that at the time - it was only later ...

Actually I quite like the word 'poo' - ever since my son many years ago referred to peanut butter as 'poo in a jar'. (You had to be there.)
 
.............Actually I quite like the word 'poo' - ever since my son many years ago referred to peanut butter as 'poo in a jar'. (You had to be there.)

LLLLLLLLLLLLLLLLLLLLLLLLLLLLOL

Excellent, even without being there......

The son of a (Asian) friend of mine always refers to asafoetida as piss powder!!! LLLLOL
 
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Update: I've now got all four woofers working properly. Big relief! Also, contrary to what I said above in post #8, both the older woofers have a DC resistance of 8 ohms, not just one. The younger pair are both 5.7 ohms. So at least each woofer enclosure has a matching pair of drivers, even if the enclosures don't match each other perfectly.

I need to order some long fibre wool, to replace what the moth larvae ate, and I need to cut a new MDF spacer, as one is slightly smaller than the other.

Then I'll be almost set. However, I still have the problem of how to seal the woofers into the cabinets. I'll need to slide the woofers into place and then, with them in place, seal the gap between the outer woofer frame and the cabinet - 1 or 2 mm all round. Any recommendations for what to use? Silicone would create a seal but I'm worried the woofers would be well-nigh impossible to remove, should further maintenance be necessary.

PS Whatever I use, it needs to have some adhesion, as the gap it's sealing will be facing downwards. It needs to stick in the gap well enough not to fall out, over time.
 
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