Interesting that you’ve got working tape decks at all. Most consumer grade machines that I’ve run across in recent years are kaput.
Most cassette tapes you find are also too degraded to play, unless they’ve been kept in a secure box of some type. I’ve got 25 year old session copies that still play alright, but I no longer have a decent playback machine.
I had most of them transferred to CD a few years ago to preserve my work.
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Its not the cassettes themselves that are degraded, its the capstan wheel whose rubber has oxidized to the point that the tape sticks to them, thus destroying the tape if you attempt to play it.
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I do a lot of books on tape when I travel. Get them at Half Price Books for $2.99, only get unabridged ones.