Weird cassette problem: speeding up then slowing down!!!

hello all! i recently bought a batch of early 90's cassette demo tapes from a guy in brazil (someone i've bought from in the past with no problems with prior cassette purchases) and all 5 tapes do something i've never experienced in my 40 odd years of listening to cassettes: they intermittently speed up than slow back down to what i believe is normal speed throughout playback on both sides of the tape. all of 'em. upon inspection the actual tape looks pristine as does the felt pad and everything else. i've played them back in both my nakamichi and marantz with exact similar results. my guess is that maybe since they came from brazil and this being summer they were shipped in some very hot and humid conditions. would thisy cause what i'm describing even though the actual tape looks nice and smooth still? any chance at fixing them? i've heard of the baking method. more than anything i'm just curious as to what could have caused the problem.

 

Read this to learn about tape problems which can be addressed by baking: Sticky-shed syndrome - Wikipedia

If the tape doesn't have those symptoms, try fast-forwarding and rewinding the tape from start to end. Sometimes tapes can be unevenly packed (due to playing parts of the tape on different sides/players). A ff/frw of the tape will re-pack it evenly.

One thing which was commonly done in the cassette day is to hold each side of the cassette firmly in each hand, and twist in alternate directions. You may hear a crack in the case, but that is unlikely to be breaking anything (unless you use excessive force).

Another cause could be the plastic sheet that is on top/below of the reels. Try replacing them with those from another tape. Or try putting the tape into another cassette housing.

Good luck.

 

Different decks have different capstan pull strengths and different take up winder tensions and feed roller resistances. If you've played the tape on several good decks to the same effect, the tape is likely proceeding through the capstan at a normal rate; it is the recording that is off.

Speeding up is unlikely in playback, it would be that it was slowing down during recording. The recording cassette deck may be at fault, or the conditions tapes were stored in or uneven windings before they were recorded on were a problem causing drags.

Is the effect a regular wow-like pitch change that seems related to the cassette's reels? Or even larger-scale and random changes? It may be that the cassettes are transfers, and the sticky-shed problem was had on the playback deck, or there was inconsistency in the AC wall power. Regardless, it may take some technology to correct if you are restoring them, or a complaint to the buyer about the quality of his dupes if they are recent.

 

Cassettes are not candidates for baking, that is for reel to reels.

If it's only that set of tapes sounds like the transfers were bad.