Hello everyone! I've been lurking here for a couple of weeks now, getting familiar with all this crazy audio stuff. I've finally made my first high-end purchases, an emotiva bpa-1 (a cute little amp that puts out 37w into two channels) and a used pair of B&W P5's, which I got for $250 because a tweeter wouldn't play. I figured it'd be a simple case of replacing the tweeters, putting me at about $400 for an excellent pair of speakers to get started with and giving me a chance to get my feet wet with DIY stuff.
I got them home last night and decided to run a couple of checks to make sure the tweeter was blown before replacing it. This particular model has liquid cooled tweeters that tend to go out after the liquid hardens. I swapped the tweeters between the two to verify that the tweeter was the problem, but to my surprise the tweeter started playing just fine, and the one that I'd verified as working quit. "Uh oh," I thought, "it must be the crossover." After a little while with a screwdriver and multimeter, I established that all the connections seem to be good, leaving a component problem as the only explanation I can come up with.
The crossover circuit is a fairly simple one, looking similar to a standard 18db / octave crossover I found on the internet: it has what I think is a resistor (but could be a small inductor, perhaps to act as a low-pass filter at 22khz or so): after all, it has resistance of about 1 ohm to a DC current, and is boxish and white. This is in series with a 7mfd 70wv Bennic capacitor, which is in series with two things that are in parallel: a capacitor and the tweeter in series, and an inductor. It's difficult to describe all this, so I'll draw a little ascii diagram:
(+)----R----C1-------C2---Tweeter-------(-)
| |
\---------L---------/
Where R is the thing that I think is a one ohm resistor, the C1 is the Bennic 7mfd 70wv capacitor, L is an unmarked inductor, and C2 is marked "B&W loudspeakers C1066".
When I take the resistance across the two terminals with my multimeter, I see a 5 MOhm resitance that slowly climbs (maybe .5MOhms/second, or a little faster). This is the same for both the working and the non-working speakers. Since for DC the circuit looks something like
(+)-----R-----C1------(-)
I'm guessing that the climbing resistance represents the charging of C1. Since the working and the non-working speakers give the same reading, I take this to mean that C1 is probably still working. My guess, then, is that C2 has failed, dropping its capacitance to levels much lower than spec'd, and thus making it a high-pass filter at some ungodly high value like 3ghz, or just breaking the circuit entirely.
My questions, then:
1. Do you agree with my diagnosis?
2. How can I go about finding a capacitor to replace it? Will any decent capacitor off parts express with the same rated capacitance and (at least) the same working voltage be a suitable replacement? Should I replace the matching capacitor in the other speaker as well?
3. Is there a way to determine what the capacitance of c2 is from the value of c1 and the knowledge that the crossover point is at 3khz?
Thanks for reading my post, and thanks in advance for your help! I've learned a lot from these forums over the last couple of weeks, and I look forward to getting to the point where I can contribute more than just questions.
