Though AudioKarma has plenty of
opinions on this topic, perhaps Danny has some alternate solutions.
Just picked up a pair of C40s that have some sort of gold/bronze doping compound on the tweeter dome's outermost layer. First I thought it may be nicotine or grime. Googling for images of this speaker, it seems the orig soft-dome tweeter did have some of "gold" coating or doping. And they all seem to be crumbling away (as in new-old-stock eBay replacements) . On mine, I can take a fine-hair art brush and delicately flick it away. It seems to be orig doping that is de-cohering This stuff may be losing integrity after all these decades (the C40 was intro'd 1987).
I believe these tweeters were made by Tonegen for many manufacturers and the dope has or is failing by now. Some similarly treated domes from other manufacturers are also doing this. It is suggested: "Without the dope film the mesh dome can’t create any significant air pressure and the output suffers dramatically."
Search for tweeter re-doping and you’ll find many threads including suggestions for repair, but I’ve yet to see the old dope compound identified. Presumably as well as providing air resistance the dope also has an effect on mechanics of the dome at high frequencies and the performance of the suspension given the dope covers this area too. I have read on other forums that this can cause HF loss because the dome no longer suspends air. I'm new to these speakers, and they are vintage. Indeed they do lack top-end sparkle. But not lot.
Danny/All ... what are my options ?
Thx!
