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Discussion Starter #1
Hello,
I have a question about my Bose 501 series II's.
Miraculously the surrounds on the woofers are pristine, and sound pretty good, but the high end just disappeared on one of the cab's.
Is there any good guide for an amateur to try to sort out what's wrong with an old 70's xover, or should I just assume it's junk and swap it out?
(yeah, I checked internal wiring as well as I can, and see no obvious defects...)
Thanks very much!
 

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Hello,
I have a question about my Bose 501 series II's.
Miraculously the surrounds on the woofers are pristine, and sound pretty good, but the high end just disappeared on one of the cab's.
Is there any good guide for an amateur to try to sort out what's wrong with an old 70's xover, or should I just assume it's junk and swap it out?
(yeah, I checked internal wiring as well as I can, and see no obvious defects...)
Thanks very much!
Bose (for good reason) chooses to publish little information on their products. However I can make some guesses. I'm assuming your version of the 501's has two cone mid/tweeters and you hear little or nothing from either. This suggests an issue with the crossover. Without the advantage of published data, I'm going to assume they used electrolytic caps in their crossover. This is common in many commercial designs due to the low cost. Electrolytic caps tend to dry out and lose their ability to pass signals as they age. With a vintage product such as the 501's, this seems the most likely potential cause. I'd suggest you locate the crossover and replace the mid/tweeter capacitor(s) with equivalent values of polypropylene capacitors. Capacitors can be generally identified by verbiage indicating the value in microfarads and their working voltage. For example a 10uF 100V would indicate a 10 microfarad capacitor rated for a maximum of 100 volts. You certainly can replace them with electrolytic caps again if you wish, however the poly caps are almost universally accepted as having superior sonic performance.
Curt
Speaker Design Works
 

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Actually the 501s sounded ok -- as did most Bose speakers. High end not, but... A neighbor had a pair of original 501s and we lighted to watch the "light bulb" light up on peaks, when we were teens. I always thought it was stingy of Bose to go from 4 tweeters in first edition to 2 in later revs, if I recall.
:)
Your troubleshooting advice is good, but wouldn't it be better to check individual drivers? In my experience, a tweeter is far likelier to fail than a capacitor.
 

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Discussion Starter #4
Thanks very much for getting back to me!
I managed to nick a tear into the foam surround on the trouble speaker, so (since it's now a re-foaming job, too) the priority of this project has been decreased, but I'll take some good photos and remove the existing crossover components, source some caps and try too get it working again that way.
I was just hoping there was a way to test with some degree of certainty that the crossover is the culprit.
Since I see no new wiring defects, and since both tweeters would've needed to die together, I assumed the hi range drivers themselves are fine.
Anyway, thanks for your help!
I'll let you know if I learn anything of interest.
Best,
~C
 
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