People who make analog recording part of their daily routine take for granted that the tape is new and the machine is operable. When called upon to re-master or remix a vintage analog recording, it goes without saying that the machine must be in top form AND that your business should be insured. That said, there is one variable for which you have no control: tape condition. This is especially true for mid-seventies era high-output tapes such as AMPEX 406-407 and 456, 3M 250, and AGFA 468. It does not exclude those made well into the eighties.
Dont attempt to play a "vintage" tape before reading this article! In order to expect full recovery, tapes that have been shelved for an extended period deserve special treatment just like a scuba diver must slowly return to the surface.
All tape consists of three primary components: iron oxide, the "binder" or glue and a plastic carrier. Acetate ? which does not stretch and can be brittle ? was used until the sixties. Though its oxide color is typically reddish-brown, black oxides were also used. Mylar-Polyester eventually replaced Acetate. It handles stress well and never becomes brittle. Sixties-era Mylar tapes with black oxide will be the least problematic.
Over time, the glue that binds the oxide to the plastic will absorb moisture and "break down." The symptoms of "binder breakdown" are immediately obvious even when rewinding. Tearing sounds and sluggish behavior are clues to quit before the oxide comes off. Machines with stationary lifters (Ampex 440-1200, MCI and 3M) will, in many cases, stall well before reaching the halfway point. An older Studer, with its rotating guides, may not reveal any warning signs until the tape is played.
Playing a bad tape is not recommended. Just trying to get through a three-minute pop song will require several cleanings. Once the precious sonic material collects on transport parts it is worthless, not to mention difficult to remove. Do you really want to risk damage to the master for the sake of getting a transfer? There is hope, so be patient.