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Old 27th May 2011, 06:25 AM   #1
llimbo is offline llimbo  United States
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Default Assistance on B&W DM1200 crossover repair?

Hi all, first post here, and I'm up against an issue where I could use some help.

I have a pair of vintage B&W DM1200 speakers that I like a fair bit, where a certain capricious someone turned 'em up and zotted a tweeter. I put 'em in a box and made a note to find some new tweeters, after being told by B&W that I was SOL on brand-new TZ26 tweeters.

Fast-forward to a few weeks ago, where I see a pair of TZ26es from a sequential serial number pair of DM330s that are being parted out; score! Go fetch the speakers and prepare to slap new tweets in, until I realize that one speaker's LF unit isn't operational.

I crack it open to determine whether the connections are good (they are) and confirm that the LF driver works by playing it freerange (it does). I also verify that the HF side of the crossover works fine.

I've tested most of the components on the board that I can - items that I was unable to test were capacitors, but there aren't any shorts present.

At this point, short of retouching every solder joint or replacing caps and/or other components because I'm bored (I'm not, I assure you) I'm not sure what to try next. I'd upload the service manuals from B&W with the schematics and component placement diagrams, but for some reason the website isn't allowing me to.

Any help would be greatly appreciated!

MB
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Old 27th May 2011, 07:08 AM   #2
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B&W crossovers are pretty bombproof. I would be more suspicious of a faulty connection somewhere.

There is a 6 Ohm resistor in series with the HF unit which could fuse if the speaker was left at very high volume for a prolonged period of time. The Caps and Inductors are normally well over specified and are unlikely to fail.

If the LF driver is working then the overload relay is OK.

Check for dirty or faulty connections.
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Old 27th May 2011, 07:14 AM   #3
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Check the PCB for hairline fractures. Because of the heavy components it easily breaks.
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Old 27th May 2011, 05:07 PM   #4
llimbo is offline llimbo  United States
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Quote:
Originally Posted by Andy5112405 View Post
B&W crossovers are pretty bombproof. I would be more suspicious of a faulty connection somewhere.

There is a 6 Ohm resistor in series with the HF unit which could fuse if the speaker was left at very high volume for a prolonged period of time. The Caps and Inductors are normally well over specified and are unlikely to fail.

If the LF driver is working then the overload relay is OK.

Check for dirty or faulty connections.
Andy - Thanks for the feedback. The LF driver works when fed directly from amplifier power, but does not work when attached to the crossover. I've verified operation by swapping LF drivers with the other fully functional crossover that I have; the drivers seem to work normally.

As a precaution, I'll replace the 6 ohm inlines on the HF (which are soldered directly to the positive speaker lead) given the original situation that caused the issue; thanks for the tip.

I've verified all connections with a meter - I have continuity through the wire harness all the way to the PCB. and from the leads through the traces on the board. In addition, I've tested every joint to the trace on the board itself, and to the exposed component leg on the other side. Not sure what other connections might be in play.

Best,
MB
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Old 27th May 2011, 05:09 PM   #5
llimbo is offline llimbo  United States
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Quote:
Originally Posted by QSerraTico_Tico View Post
Check the PCB for hairline fractures. Because of the heavy components it easily breaks.
Thanks for the feedback; I'll go over the board with a magnifier, but it looks single-layer and I've managed to do point-to-point connectivity tests all across it; is there something else lurking that I should be aware of?

Best,
MB
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