Hello, new person here and apologies if this has been asked before. I have a pair of B&W CM4 speakers that have been in storage for a long period. It seems that the bass drivers on both speakers have ceased functioning. Prior to going in to storage I recall the rubber edges were replaced. I've validated that the crossover in the speakers appears to work correctly, however when I pass a current through the bass drivers I do not see any movement in the cones. I tried the same test with the mid range cones ans these seem to work fine. Looking at the drivers themselves they look in perfect condition, the cones move freely and there is no obvious signes of damage. Anyone have any ideas, I think I'll pickup a multi meter to check the connection across the wiring...
If nothing happens when you connect a 1.5 volt battery across the bass driver terminals then it suggests the voice coil is open circuit. A check with a meter across the coil should show just a few ohms.
Is it possible to repair, or am I looking at selling the rest of the componenets for sapres/repairs?
If the coil is open circuit and it is nothing obvious such as where the 'tails' connect to the terminals then I would say it (the driver) is beyond any kind of economic repair.
Do the checks first though and sourcing replacement drivers may be an option.
Do the checks first though and sourcing replacement drivers may be an option.
I did once a repair of an open circuit Scan Speak midrange unit (worth new 200 bucks/unit) and after a lot fiddling I found where the area of broken connection was. Resoldering and gluing copper braid back to the cone solved it. I had to detach the whole cone/suspension to be able to repair that. Absolutely a job one would want to avoid.
Perhaps manufacturer will sell you new drivers?
Perhaps manufacturer will sell you new drivers?
Well, not what I expected... So I'm seeing 8 Ohm through the windings... So I scratched my head and then checked and noticed that nothing is sticking to the magnet.... 😕
So I guess, likely beyong repair?
So I guess, likely beyong repair?
Have you connected a 1.5 volt battery across the terminals of the driver to see if the cone moves and you hear a click?
Many speakers have shielded magnets from the days of CRT TV's and CRT PC monitors.
Do the battery test.
(You would need something like the warp coil field from the Enterprise to demagnetise a speaker 😉)
Many speakers have shielded magnets from the days of CRT TV's and CRT PC monitors.
Do the battery test.
(You would need something like the warp coil field from the Enterprise to demagnetise a speaker 😉)
Hmm,I have passed a 1/5v battery across the not working speakeer and no movement. It's definitely the magnets, on the LHS of the image you can see the broken driver and on the RHS is a known working drive. The LHS, piece of wire isn't attracted to the magnet at all, the RHS is immediately attracted...
Adding the image that can be seen...
An externally hosted image should be here but it was not working when we last tested it.
I wonder, someone might have heated one up trying to release the glue. After removing the cone, they might have wanted to align the gap..
It is very strange... so it is looking like these won't work again unfortunately 🙁
(The magnetic field around where the coil goes into the gap in the magnet should be strong enough to rip a screwdriver from your hand... really really powerful at that point. You may not get much of a magnetic field at the edge depending on the design... which might be by design 🙂)
(The magnetic field around where the coil goes into the gap in the magnet should be strong enough to rip a screwdriver from your hand... really really powerful at that point. You may not get much of a magnetic field at the edge depending on the design... which might be by design 🙂)
These magnet structures do not look shielded to me. Shielded structures have an additional ferrite ring glued to the back to cancel the magnetic stray field. Additionally, the structure usually is covered by a mu-metal cap to shield remaining leakage.
Remagnetizing is possible, but it will be a hassle to find someone with the equipment to do it for you. I think it is not worth it.
Remagnetizing is possible, but it will be a hassle to find someone with the equipment to do it for you. I think it is not worth it.
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Sad times, I can't find any sspares for these on Ebay and probably can't afford a new pair. I guess though as they are so old I can find something better cheaper these days..
There is an alternative. Measure TS parameters of the working unit and choose a new one based on those specifications. I am convinced one can find many enough that would fit the specs and bill.
B&W (used) to have a service department that would sell spares, like speaker driver units, if you can find any contact details...
I actually logged a ticket through their website as it seems to be super hard to contact them direct. I've a suspicion that they no longet provide spares for this generation of device (i'm sure I read somewhere this was the case)
TS is an abbreviation for 'Thiele/Small'.
Thiele/Small parameters - Wikipedia
Thiele/Small Parameters Explained. A beginner's guide.