B&W DM7 MK II Test and Repair

lunchbox22

Well-Known Member
Alright guys, looking for some assistance here. I thought there was a fuse somewhere inside these things, but I can't seem to find one.

Issue: Very little/no output from this speaker.

Testing so far:
Used preamp to switch output to only L and only R to determine whether the issue was at preamp stage. Reversed RCA cables between preamp and amp, issue stayed with this particular speaker. Switched speaker cables (left channel driving right speaker, etc.) to determine whether particular amp channel had blown, problem stayed with this speaker.

Actions taken:
I opened the case, removed the woofer and passive radiator, and crossover assembly. Pics are attached. Found some interesting gunk on the big cap - anyone know what that is?

Questions:
1. Does anyone know if there is a fuse in this speaker?
2. If not, what components, if inoperative, would result in the no/little sound phenomenon described above?
3. While I've got this out, what, if anything, should be replaced?

Thanks!

2013_08_26_16_30_41.jpg

2013_08_26_16_30_47.jpg

2013_08_26_16_30_51.jpg

2013_08_26_16_31_04.jpg
 
They don't have a fuse but automatic overload protection, using the relay you see on crossover.

You can go here for the schematic:

http://www.bwgroupsupport.com/manuals/service

Thanks for the direction. I've downloaded the file they offer; it's a schematic. Could I have fried the relay, resulting in little/no sound?

Maybe I should back up a sec. I have a few more questions. Before disassembly I would only hear the music faintly from the tweeter, and that was it.

How can I determine if the woofer is fried, or the tweeter, or both?
Is it possible to have fried the relay?

Can someone take a look at the schematic and give me some ideas on which components might cause this issue?

Untitled.jpg
 
You can check each driver with a DMM they should have roughly the resistance printed on them.

I take it you have a pair of these, and one is working?

If so why not use the crossover from the working one to see if it's either the crossover or drivers in the not working one?

At least that would narrow it down to either drivers or crossover.
 
You can check each driver with a DMM they should have roughly the resistance printed on them.

I take it you have a pair of these, and one is working?

If so why not use the crossover from the working one to see if it's either the crossover or drivers in the not working one?

At least that would narrow it down to either drivers or crossover.

Good idea!. I'll do both of these things this weekend. Thanks for your help!
 
Alright, so I pulled out the DMM and tested the woofer. Its testing open when I connect the two leads. Does this mean the woofer is dead and beyond repair?



Pics:
2013_08_27_22_16_55.jpg

2013_08_27_22_17_07.jpg

2013_08_27_22_17_16.jpg
 
Last edited:
First off, I don't like the looks of the 1000uf cap in your third pic in the OP. Looks like it's leaking....

Second, let's double check the settings on you meter and make sure the reading is correct. Test something you know is good, check the resistors on the board...

Look at the schematic, the 1000uf cap is in the woofer line, if it's bad it's you problem, assuming the woofer is not open too...

look at the schematic again, both drivers go through the relay...if the tweeter works, so does the relay...
 
Last edited:
First off, I don't like the looks of the 1000uf cap in your third pic in the OP. Looks like it's leaking....

Second, let's double check the settings on you meter and make sure the reading is correct. Test something you know is good, check the resistors on the board...

Look at the schematic, the 1000uf cap is in the woofer line, if it's bad it's you problem, assuming the woofer is not open too...

look at the schematic again, both drivers go through the relay...if the tweeter works, so does the relay...

1000µf/100 bi-polar cap

Holy snikies
http://www.mouser.com/ProductDetail/Cornell-Dubilier/PSU100015A/?qs=9I6i58cghutuMrsRYFH2KQ==
 
Last edited:
Second, let's double check the settings on you meter and make sure the reading is correct. Test something you know is good, check the resistors on the board...

I just tested the other woofer - it isn't testing open, it tests at 5.5 ohms.

look at the schematic again, both drivers go through the relay...if the tweeter works, so does the relay...

The tweeter wasn't working either. Could this be the relay? How do I test the relay?


Woof. But, its cheaper than a new pair of speakers! Thanks for the info on the part, avionic. What about the other cap on the board - should I replace that too? Any suggestions for which part to buy?
 
Last edited:
2013_08_30_20_39_02.jpg


Is this bipolar? Can i just get another cap that is 11uf and 50V?
Edit: looks like caps these days come in 12 or 10 uf. And I can't find a 12uf 50V aluminium (is this the issue?) cap with axial leads at mouser, digikey, or arrow. Thoughts?
 
Last edited:
So, all of the electroltyic caps will need to be replaced in both speakers, these are now 30 years old and one has failed and taken out the woofer and maybe the relay. I see four electroltyics on the board, the two nonpolars and the two smaller polarzied ones. The woofer voice coil may be reparable or a replacement may be found, there are compaines that do this, check the web. Have you tested the tweeter to see if it is good?

The schematic does not seem to match your board exactly, this is not uncommon and may be close enought.

If you cannot get the same values you can go to the next higher rating in both voltage and uf, but stay as close as possible. You should test all the other passive componets on your boards and replace any that may be too far out of spec as well, this will be mostly the resistors, the inductors are usually fine.

The good thing here is that once you get everything upgraded your speakers will be good for another 20 years and should sound much better than before.

Parts Express and Antique Electronic Supply may have what you need, as well as others.
 
Last edited:
To add to the above, if you have any other older electronics, say 20 years or so it is a good idea to replace the electroltyic's before they can cause issues, I don't like any over 10 years or so, and replacement them as soon as I get a new (old) piece of equipment. I have seen them work fine after 50 years or more, but it's not worth the risk.
 
If you cannot get the same values you can go to the next higher rating in both voltage and uf, but stay as close as possible.
Or parallel two or more to obtain the correct value.

Is this bipolar?

I believe so.

50 VDC (REV) --SWAG REV stands for reversible ie non-polar or bi-polar

Use something like these two in parallel for 11µf/50vdc. You can always up the capacitor voltage rating without any ill effects..

http://www.parts-express.com/pe/showdetl.cfm?partnumber=027-340
http://www.parts-express.com/pe/showdetl.cfm?partnumber=027-320

2 of these in parallel for the 1000µf/100vdc a wee bit cheaper than the CDE can from Mouser.
http://www.parts-express.com/pe/showdetl.cfm?partnumber=027-378
 
Last edited:
If the relay circuit (protection circuit)is activated and the relay contacts are opened up. Then the signal by-pass's the relay and is routed through a 390Ω resistor reducing the amount of current applied to the HF driver, also attenuating the signal.
The relay contacts are normally closed. So if the protect circuit is malfunctioning it would still allow a signal to the tweeter at a reduced volume level.

If the relay contacts were corroded or tarnished it would definitely reduce the signal being applied to the tweeter. To test you could simply clip a jumper wire across the 390Ω resistor.
 
Last edited:
Anyone? Should I get in the market for a used woofer?

When measuring the resistance of your woofer make sure you have a good electrical contact with the connections on the woofer and your meter probes. Doesn't take much oxidation to give you a false reading.
 
Would this be an acceptable replacement? It's tantalum - Not sure what the difference is between that and aluminium.
http://www.mouser.com/ProductDetail/...X8LdFcrlV5U=
I would not use tantalum. They are not bi-polar. Do you need 500 of them at $2330.00 plus shipping.







I would stick with 11 µf.. Besides,those are also not bi-polar and I don't think you need 25 of them at $207.50


Stick with film or bi -/ non-polar electrolytic.
 
Last edited:
Back
Top Bottom