i have a number of old cassette tape's some play well and other play slow my question is there any type repair i can do to the cassette ? other then slow play they  look in good shape. and im sure it is not the deck because i tryed them on other device's

thank you for your input

Do you notice that longer tapes are giving you issues primarily? If not it's possible the storage position has compressed the reels against the case and now there is more friction.

Just some ideas?

Maintenance for your Cassette's:

1.  Slap the cassette on it's face against your thigh to flatten the reel-cake & wake yourself up

2.  Have you tried Winding the cassette completely from beginning to end (side #1) and then reverse direction (side #2 ) back to the beginning of side One and then,

Play in the machine that is giving you the problem & See if that help's Airing out your cassette album's.

3.  If first ( 1 & 2 ) dont work then you may consider replaceing or removing the plastic lubricating plastic on each side of the cake inside the cassette case.

4.  Or move your cassette cake to a New Cassette shell.

5.  the Idler's for your cassete may be binding or need lube - there's 2 inside each cassette shell - though in cheaper ( really cheep ) I've seen NO Idler's in those .

Hope these suggustion's help