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Bose 901 distortion. Replace surrounds?

M

melcam

New Member
I'm a newbie and have never repaired speakers before and know little about them. I'm looking to acquire and build a vintage system.

I found some Bose 901 Series IV speakers at a garage sale. The foam surrounds are toast - some of them are completely gone, and I originally planned to replace them. The speakers look good except for the foam - no other visible damage. When testing the speakers, there is significant distortion even at low volume levels. I don't have the original EQ (if I'm able to fix the speakers, I planned to buy one on eBay) so I just tested by hooking directly to my Denon amp. But, but I'm wondering if replacing the surrounds would even fix the distortion issues. They really do sound terrible with buzzing, cracking etc, and I don't know if it's worth the time and money to replace the surrounds. Is there another way to test them or determine if they are worth fixing such as measuring the ohms? Would corroded and missing surrounds cause significant distortion?
 
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Mel DO NOT test speakers with bad/missing surrounds, even at low volume; especially without the EQ. You'll have to ohm out the speakers, there could be a multitude of causes for your distortion. The fact that speakers are missing surrounds I would not be surprised if they were ran that way for a while and loud enough to cause permanent damage.
 
So I need to refoam all the drivers before knowing if any of them are bad? I'm pretty sure the previous owner had been using these speakers recently, with the damaged/missing surrounds. Maybe rebuilding would be a waste of time.
 
Bose drivers are high quality. Not very likely they're toast. If you lay the speaker on it's front face, 8 speakers facing up, you can run a low signal and determine if any of the drivers are bad. Running them in normal position causes the cone to sag and draw on the VC and place it out of alignment. This causes rubbing/scratching and if run long enough VC damage. These speakers are wired in Series (0.9 ohm each) and as such if you had an open in ANY VC there would be NO sound at all. So, we know, at least, that you have no opens. If you have a multi-meter test the impedance, should be in the neighborhood of 8-ohms (at the speaker terminals). You can post readings here and we can chime in. I own Series I and II, which are cloth. Series III and later drivers are readily available, so even if you have to replace drivers it should not cost too much.
 
Thanks for the help here, much appreciated.

I used a mutli-meter (set to 200) on the main speaker terminals to check the impedance. It read 5.7 ohms on one speaker, and 5.3 ohms on the other. From what I've ready this seems to be in the normal range. Does the difference between the two suggest any issues?

I also removed a couple of the drivers and checked impedance on them individually, and I got .7 ohms on each one, which would seem to be ok if each driver is .9 ohms as you stated Copa. Should I check all of them?

My multi-meter is a cheap one, and when touching the leads together it reads .2, so I understand the measured impedance would be off by that much.
 
I say go for it! Refoam those drivers. When you buy the foam, ask for a couple extras, just in case you end up with a bad driver or two like I did.
 
There's no crossover inside to cause problems.

We've determined that the drivers are not "open circuit".

About all that leaves is cabinet condition which you say is good, refoaming the drivers, and buying (and reconditioning) an appropriate equalizer.

I say "go for it!" If it does turn out that one or more drivers isn't salvageable due to spyder problems or voice-coil alignment, you'd still be able to replace those drivers and salvage the others. This seems low-risk to me.
 
Thanks. I'm going to proceed with the refoaming.

Do I have to get a series IV eq or will any of the 901 series equalizers work with these speakers?
 
As best I can tell, 901 equalizers kind of "pair up" across the six series of speakers.

Series 1 and 2 can interchange equalizers

Series 3 and 4 can interchange equalizers (The equalizers built-into the Bose receivers are of this sort, so if you found a Bose receiver in good condition, you'd be all set.)

Series 5 and 6 can interchange equalizers. <<< This is actually listed on the Bose web site.

The original equalizers shipped with each series of speakers may vary cosmetically from the series before or after, but the curves are the same--or similar enough--that they can be interchanged as listed above.

Be aware that, just like speaker crossovers or any other electronic equipment, the electrolytic caps are likely going to be out-of-spec on an older equalizer. Some upgrades can be made to the resistors as well.--tighter tolerance, lower noise. It might be that the integrated circuits (where used) can be replaced with more-modern ones, but I haven't explored that AT ALL.
 
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