The transformation into the black goo is an interesting one.  I believe that happens due to the way the Beocord unit was stored.  I think a belt will wear to the point of requiring replacement during regular use inside a normal, home environment. I also believe the belts that deteriorated into goo happened because so many owners stopped using cassette decks (I know I did for quite a while) and put them into storage like attics, basements and storage sheds.  Those environments resulted in the deterioration of the rubber.  That is just my guess of course but I don't think a belt will deteriorate to that extent if the Beocord remains in use and is maintained (service checks). -sonavor

Not so sure John. I’ve recently done a VCR that had always sat under the TV set, until one day it didn’t work. The four belts had turned into goo, unbelievable. After hours of cleaning and belt replacement the VCR worked perfectly again.

Poor storage conditions might accelerate the degradation of the belts, but I don't think that it is the primary cause of goo-ification. I bought my Beocord new and it has spent its entire life in a climate controlled, living space. Regardless, I ended up with goo. This issue is not unique to B&O. I've read similar stories for many other brands of equipment. The common cause has to be in the base material or in the manufacturing process of the belt. Who knows, maybe all of the goo-belts come from the same plant. I'm hoping that they have made progress on the longevity of the belts. I believe that they have improved the formulation for other issue prone parts like foam speaker surrounds. It would be a real shame if the plastics industry reserved the "forever materials" strictly for disposable items like water bottles and soda straws. (yes, I know that they are different classes of plastics) Glitch  

I did a bit more research and found that the correct term for the phenomena is "rubber reversion". However, I think the term "goo-ification" is more fun ;-). There seem to be many theories about what is going on. Many of them are based on the supposition that the polymer stabilizing additives migrate out of the rubber. I'm having a hard time conceptualizing how a working the rubber would slow the process versus keeping it in a state of stasis. There is a group of vintage camera enthusiasts that swear by the "use it or lose it" theory when it comes to rubber parts on cameras. Perhaps the rubber should be considered a viscous mixture and working it keeps everything mixed-up appropriately? Regardless, I've already gone deeper down this rabbit hole than I intended. I'm still hoping that someone can shed some light on the original question. Glitch

I think the use it or lose theory fits here. Thanks for digging into the problem more. -sonavor