Restoring highs - dope?

Hi Everyone, first post but thanks for all the info I've soaked up already.

I have owned a pair of B&W DM7-mkIIs since about 1987. I recently pulled them out of retirement and they still work - quite well - except they have lost their sparkle - the highs aren't quite there, the sound just hits a ceiling (even before my aging ears hit theirs!).

I initially thought I should re-cap the crossovers, but someone very knowledgeable told me that the caps used in these crossovers don't deteriorate with age.

Instead, he said that the tweeter diaphragm has probably got dirty, dried up and become semi-permeable so it is not delivering air as well as it should, and it should be re-doped; it sounds like he means re-coated with dope to "seal" the surface.

Does that sound right? And if it is, what would I use? B&W describes the tweeter as "multi-filament polyester weave dome." And if that isn't right, what should I try? Is re-capping the solution?

If I can recover the highs, these speakers might still compete with the pair of new, pretty good speakers sitting next to them.

Thanks!
 
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The sure way to judge capacitors in a speaker crossover is to measure them with a decent LCR meter that will determine D,Q,ESR and Tan theta. Generally polyester and polypropylene capacitors cause very little trouble and it is the non-polarised electrolytic types that are always suspect, as they deteriorate with age and are of a wide capacitance tolerance.
You can inspect the diaphragm of the tweeter with a magnifying glass and check if there are pinprick size holes in the fabric or if there are patches of the sealant( coating) missing. The protective wire mesh may provide a degree of difficulty in this perusal. If you find that the sealant has failed, you may be able to obtain a replacement diaphragm from B & W or an agent. If not then a thin coating of a flexible PVA solution such as Aileene's Fabric Adhesive can be painted onto the dome and its' surround. Some tweeters use ferrofluid and this material does dry out over a long period time. I cannot recall if this applies to the tweeter used in the DM7 speaker.
 
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