Just registered so this is my first post, so be kind please as I am pretty ignorant about all things electronic but I do like fixing stuff.
Anyway, I've got some B&W CM4 speakers I bought used and then didn't listen to for six months. Now that I finally set them up I realized that one speaker isn't really getting much bass - I don't know if it's the driver or the crossover or whatever. I also learned I should always test stuff before buying it. Oh well, live and learn.
I've switched cables, checked the phase, switched sources, switched amps and have determined it is the one speaker. I've been using Coltrane's Atlantic Years in Mono as a test as I know it well and the mono makes comparisons between the two speakers easy.
I figured I would poke around the speakers to see if there were any loose connections or anything so popped out the midrange, bass, and tweeter and disconnected and reconnected them. I popped off the speaker terminal and did the same thing. I can see the crossover but can't figure out how to reach it.
The one weird thing I did discover was a random screw magnetically sticking to the back of the bass unit - it appears to be the same type of screw used to hold in the tweeter and the speaker terminal plate, but none of those are missing. I have no idea where it came from and if it's related to the lack of bass. As best I can tell the bass unit and the midrange aren't really getting much bass at all in comparison to the other speaker, which makes me think it's not the bass unit/driver/whatever the best word is. Anyone know where such a screw would come from?
In doing this two of the foam surrounds have cracked, so I imagine I should replace those - which brings up a question - is there a recommended place to get surrounds (if I am using the right word - the foam around the midrange and bass)? It all seems dry so I figure I should probably replace all four if replacing any.
Next question is does anyone have any suggestions for figuring out next steps to get the speaker to have bass again? Of course it's an older speaker (circa 2001) so B&W doesn't carry the relevant parts anymore. I can't really afford to spend much on them, so am hopeful I can do it myself. I have a relative with much better technical skills and tools if it involves anything like soldering.
Thanks for your thoughts/advice.
Anyway, I've got some B&W CM4 speakers I bought used and then didn't listen to for six months. Now that I finally set them up I realized that one speaker isn't really getting much bass - I don't know if it's the driver or the crossover or whatever. I also learned I should always test stuff before buying it. Oh well, live and learn.
I've switched cables, checked the phase, switched sources, switched amps and have determined it is the one speaker. I've been using Coltrane's Atlantic Years in Mono as a test as I know it well and the mono makes comparisons between the two speakers easy.
I figured I would poke around the speakers to see if there were any loose connections or anything so popped out the midrange, bass, and tweeter and disconnected and reconnected them. I popped off the speaker terminal and did the same thing. I can see the crossover but can't figure out how to reach it.
The one weird thing I did discover was a random screw magnetically sticking to the back of the bass unit - it appears to be the same type of screw used to hold in the tweeter and the speaker terminal plate, but none of those are missing. I have no idea where it came from and if it's related to the lack of bass. As best I can tell the bass unit and the midrange aren't really getting much bass at all in comparison to the other speaker, which makes me think it's not the bass unit/driver/whatever the best word is. Anyone know where such a screw would come from?
In doing this two of the foam surrounds have cracked, so I imagine I should replace those - which brings up a question - is there a recommended place to get surrounds (if I am using the right word - the foam around the midrange and bass)? It all seems dry so I figure I should probably replace all four if replacing any.
Next question is does anyone have any suggestions for figuring out next steps to get the speaker to have bass again? Of course it's an older speaker (circa 2001) so B&W doesn't carry the relevant parts anymore. I can't really afford to spend much on them, so am hopeful I can do it myself. I have a relative with much better technical skills and tools if it involves anything like soldering.
Thanks for your thoughts/advice.
Where in the world are you Narkopop? Helps to know when suggesting sources for parts.
The screw is probably just an extra screw which got attracted to the speaker magnet during factory assembly. This will not be responsible for the lack of bass.
Speaker refoam kits are widely available, it's just a matter of obtaining the correct size. Various sizes are available from this source:
Universal Speaker Surround Repair Foam Woofer Edge Repair Parts UK | eBay
The screw is probably just an extra screw which got attracted to the speaker magnet during factory assembly. This will not be responsible for the lack of bass.
Speaker refoam kits are widely available, it's just a matter of obtaining the correct size. Various sizes are available from this source:
Universal Speaker Surround Repair Foam Woofer Edge Repair Parts UK | eBay
This source has the correct woofer and midrange driver rubber surrounds:
Special rubber ring for Bowers & Wilkins CM4 woofer
Special rubber ring for Bowers & Wilkins CM4 midrange
Special rubber ring for Bowers & Wilkins CM4 woofer
Special rubber ring for Bowers & Wilkins CM4 midrange
Regarding the lack of bass in one speaker cabinet. First of all, the midrange driver does not produce bass, so lack of bass may be attributed to the bass speaker.
Holes/cracks in the rubber surround of the bass driver will reduce bass output. Were the bass surrounds bad in the faulty cabinet but not in the other?
It is unlikely that there is a fault in the crossover unit, but you can check it by swapping over the bass drivers between the two cabinets. If the good bass driver becomes 'faulty' in the other cabinet then the crossover circuit in that cabinet would be suspect.
The faulty bass driver may be open circuit. If you don't have a multimeter to check the resistance of the voice coil (should be around 5 ohm) then connect a 1.5V battery across the bass driver terminals to see if that gets the cone to move. If the cone doesn't move you have a damaged woofer. (Don't do the battery test on the tweeter though!)
EDIT: I should add that the woofer should be disconnected from the crossover before testing it.
Holes/cracks in the rubber surround of the bass driver will reduce bass output. Were the bass surrounds bad in the faulty cabinet but not in the other?
It is unlikely that there is a fault in the crossover unit, but you can check it by swapping over the bass drivers between the two cabinets. If the good bass driver becomes 'faulty' in the other cabinet then the crossover circuit in that cabinet would be suspect.
The faulty bass driver may be open circuit. If you don't have a multimeter to check the resistance of the voice coil (should be around 5 ohm) then connect a 1.5V battery across the bass driver terminals to see if that gets the cone to move. If the cone doesn't move you have a damaged woofer. (Don't do the battery test on the tweeter though!)
EDIT: I should add that the woofer should be disconnected from the crossover before testing it.
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So, You switched cables, but did You try each of the speakers on the same amp channel with the same source ?
Lack of bass can be much thing. A simple bad solder is sufficient.
The next thing to do is to measure the speaker coils. Do You get the same result ?
Now disconnect the crossover and try each woofer directly to the same amp channel (Feed the wire thru the went for example) Is there a difference ?
Try also the midbass but with low level.
The crossover is probably glued to the wood, but it should come out with patience for inspection. If there are electrolytics it's time to replace them.
Lack of bass can be much thing. A simple bad solder is sufficient.
The next thing to do is to measure the speaker coils. Do You get the same result ?
Now disconnect the crossover and try each woofer directly to the same amp channel (Feed the wire thru the went for example) Is there a difference ?
Try also the midbass but with low level.
The crossover is probably glued to the wood, but it should come out with patience for inspection. If there are electrolytics it's time to replace them.
I'm in the northeast United States. Thanks for the links and your insights - much appreciated!
Thanks - the woofer surround in the bad one is actually fine - the good one cracked but still sounds much better. I'll try the tests you mentioned.
Thanks! I did try with same source/cable/channel. It's definitely the bass of the speaker. I'm going to attempt to measure the coil if I can figure out how to do it. I have a multimeter so hopefully I can figure it out.
Plug your test leads in as shown in the attached picture. Select the 200Ω resistance range on the dial (if your multimeter does not autorange).
Connect your test leads across the woofer terminals. If the woofer is OK, you should get a reading of around 5Ω.
Attachments
Awesome! Thank you so much for the detailed instructions - exactly what I need. I need to find the multimeter amongst my junk and once I do I will use it. I have a toddler so I have to find time when she's not around to poke her finger in but will update this post as soon as I am able to.
So I managed to find the multimeter and I think I used it properly - basically regardless of which terminal I had the contacts on - I wasn't sure which was + and which was - so I tried both ways - the resistance went to OΩ. The first couple tries involved static sound coming through the woofer. So does this mean the woofer is shot? Any idea what my options are since they don't sell this unit anymore?
And thanks again for all your help. It is much appreciated.
Check that you are on the 200 range and not the 200k range (where the display would read zero).
The fact that you are getting a crackle on connecting the test leads is a good sign.
You must test both woofers so we can compare them.
P.S. The positive terminal of a woofer is often marked with a red dot or is connected to by a red wire. When you reconnect the wires to the driver you must make sure that they are the same way round as they were originally. Taking photographs or making sketches before removing wires would keep you right.
The fact that you are getting a crackle on connecting the test leads is a good sign.
You must test both woofers so we can compare them.
P.S. The positive terminal of a woofer is often marked with a red dot or is connected to by a red wire. When you reconnect the wires to the driver you must make sure that they are the same way round as they were originally. Taking photographs or making sketches before removing wires would keep you right.
Thanks! I will double check the multimeter setting. I can't really see one that says 200 but will try to figure it out. In terms of +/- the colors of the wires are blue and yellow, which confused me. And I did take photos before touching anything because I've learned the hard way in the past that I won't remember the details I think I will.
Thanks - just took at the rules and guidelines and will follow them.
The multimeter I am using is this one:
https://www.homedepot.com/p/Commercial-Electric-Analogue-Multimeter-M1015B/202353292
I think I don't know how to use it as both drivers measured the same after I removed them (i.e. around 0) and I know that one of them works fine. The measurement where I would at least get static was in the lower left corner with the ohm symbol and "x10"
I swapped the bass drivers and the problem followed the driver - so now the "good" speaker is the bad speaker. So seems like it must be something with the driver? If you put your hand near the driver on a bass heavy song you can feel pressure/air being moved. On the "bad" one you can't feel anything except maybe minor vibration but definitely not the same bass.
Any ideas what my next steps should be? Thanks
The multimeter I am using is this one:
https://www.homedepot.com/p/Commercial-Electric-Analogue-Multimeter-M1015B/202353292
I think I don't know how to use it as both drivers measured the same after I removed them (i.e. around 0) and I know that one of them works fine. The measurement where I would at least get static was in the lower left corner with the ohm symbol and "x10"
I swapped the bass drivers and the problem followed the driver - so now the "good" speaker is the bad speaker. So seems like it must be something with the driver? If you put your hand near the driver on a bass heavy song you can feel pressure/air being moved. On the "bad" one you can't feel anything except maybe minor vibration but definitely not the same bass.
Any ideas what my next steps should be? Thanks
No - I meant the opposite. The non-working woofer continues to be non-working in the other cabinet, and the working woofer continues to be working in the cabinet that previously housed the non-working unit.
And I don't really hear any bass from the non-working unit, but I can feel a minimal vibration with my hand, but I don't know if that's coming from the midrange above it. The working woofer goes out and in I can feel air being moved which I definitely cannot with the non-working unit.
'The Speaker Exchange' in Tampa offer a repair service for B&W bass units.
If we can't figure out a fix, you can contact them on their web page regarding estimated cost of repair etc.
B&W Repairs - Speaker Exchange
Your bass unit will be either a ZZ12246 (new dustcap) or an LF00620 (old dustcap).
If we can't figure out a fix, you can contact them on their web page regarding estimated cost of repair etc.
B&W Repairs - Speaker Exchange
Your bass unit will be either a ZZ12246 (new dustcap) or an LF00620 (old dustcap).
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