Need help with my B&W CDM1NT - diyAudio
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Old 13th February 2014, 05:31 PM   #1
ron1999 is offline ron1999  Israel
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Default Need help with my B&W CDM1NT

My power amp got damaged, sent DC to the speakers
i want to find out how can i test them to verify what damage they have suffered?

i know almost for sure that the tweeters are fried, but i want to check the X over and the base unit

any ideas?
what measures should i take?
what numbers should i look for?
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Old 13th February 2014, 05:35 PM   #2
abcdmku is offline abcdmku  United States
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I'd say first check impedance, just to make sure you didn't burn the voice coil at all.
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Old 13th February 2014, 05:36 PM   #3
ron1999 is offline ron1999  Israel
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VC of the tweeters is dead for sure -
they make no sound when i connect the speakers

the mid-base units do make sound

what impedance levels should i expect?

how can i know im not measuring the Xover?
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Old 13th February 2014, 06:17 PM   #4
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Unscrew the binding posts plate screws if the xover is behind it check resistors aren't burnt and capacitors haven't swelled. Sounds like tweeters are fried though.
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Old 13th February 2014, 06:19 PM   #5
ron1999 is offline ron1999  Israel
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Some more info:

just check impedance in both speakers:

Numbers are First / Second (refering to L and R speakers)
At speaker posts (cross over)
Mid-base: 4.40 / 4.45 ohm
Tweeter: No reading / No reading

At drivers (separated from cross over):
Mid-base: 3.91 / 3.92 ohm
Tweeter: no reading, no reading

i think this verifies that tweeters are dead on both of the speakers, right?

in addition, the mid-base drivers has a sticker on them saying they should show 8ohm load, they only show 3.91 and 3.92, does this mean they're also fried?
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Old 13th February 2014, 06:31 PM   #6
Lojzek is offline Lojzek  Croatia
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ron,

manufacturers often install 4 ohm bassmid drivers,
so your measurement is probably right. Try to push
gently the cone and see if it makes any kind of noise.
Should be silent if mechanically OK.
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Old 13th February 2014, 06:38 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

The close matching of the midbasses indicates IMO they are fine.

DC usually blows midbasses and does not affect tweeters.
Tweeters get blown typically by rampant h.f. oscillation.

However your tweeters do seem to be both dead.

rgds, sreten.
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Old 13th February 2014, 08:27 PM   #8
ron1999 is offline ron1999  Israel
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thanks for the replies,

Tested the mid-base mechanically - moving the membrane, they move without any sound - i guess this means that mechanically they're ok.
i hope that they're also good on the electrical side - wish i could reach the true figures of a normal mid-base unit like this

i guess next step would be orderdering some new tweeters

Last edited by ron1999; 13th February 2014 at 08:29 PM.
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Old 15th February 2014, 03:38 AM   #9
AllenB is offline AllenB  Australia
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Oscillation in an amplifier sometimes causes the kind of damage which then sends DC to the speakers so it is possible it blew the tweeters. In addition to this, imperfect connections somewhere in the system may have played a part. Sometimes a (faulty) amp will perform normally with a speaker connected but will oscillate if this connection is interrupted.
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Old 15th February 2014, 04:24 AM   #10
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usually it's a party that causes such damage....
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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