Hi Tom, if the Ampex tapes that squeal are backcoated, it's probably sticky shed. It's not the backcoating but the era of tape manufacture as far as anyone can tell at this point (there is some debate though). I've pretty much found the same thing that you have regarding squealing (luckily I only have one that's bad but it's a real favorite of mine). After various treatments (drying with silica gel, Last) it worked great for just a couple of passes and then reverted to it's old tricks. For me, one of the worst things about sticky shed is that what makes it squeal. The binder that holds the oxide on the backing is mixing with moisture (hydrolysis) and changing to a tar like substance. This gets on the guides and heads and the friction of the tape moving across it makes it stickier. Since this stuff is now on your machine, it makes tapes that don't squeal squeal. That stuff is
real tough to get off and I use the strongest head cleaner I have. It gets into the inside edges of the guides (since the tape edge is where the binder is most exposed to moisture and friction) so you really have to get a magnifier and good light to be sure you've gotten it all.
So, is there a practical solution to this problem for us listeners. There's baking of course (like Larry's dehydrator). That was the
cure that Ampex came up with when the problem reared it's ugly head. Unfortunately, it's only temporary but you can almost always get a pass or two to transfer the contents. The Last tape preservative, as you found, doesn't work for true sticky shed tapes. My experiment (which is still ongoing) with silica gel gave me a couple of days worth of clean playback but I returned the
treated tape to my shelves and when I tried playing it after a few days, it started squealing again. I've since returned it to the sealed bag with several lb.s of gel and haven't tried it again.
What I'm thinking is that a combination of all the above might work the best. The process I have in mind would be as follows;
1. Bake the tape as outlined by eddie Ciletti
http://www.tangible-technology.com/tape/baking1.html2. Treat with Last tape treatment
3. Store in a sealed bag with a few silica gel packs
You can forgo step one and dry the tape with the gel alone, but it takes a very long time (months) and each time you pull the tape out to check it , it's going to begin absorbing moisture. I just sealed mine up and forgot about it for almost a year! (which was cool with me since I really did
forget about it!.
The Last will reduce the friction that exacerbates the problem.
And hopefully, storing it with the silica gel will keep the tape from going bad.
This might seem like a lot to go through but it really depends on how many tapes with this problem you have, and how much you want to listen to them on the original tape.