I have a onkyo cassette tape deck ta 2600,when you push play on the cassette,it would play the tape for half of a second and stop

Take it to a local audio repair engineer. There may be a broken drive belt. Or take the case of and take a look.

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Have you check and make sure that the belts are working well?
Is the pinch roller working (spinning)?
A worn out belt will make the mechanism not to operate properly resulting in shutdown to avoid damage to the tapes.

cassette tape stops 5 seconds

I have found a service manual for you that can be downloaded free:

http://elektrotanya.com/onkyo_ta-630m-sm-tape-deck.pdf/download.html

After the page is appeared fully, click on this place:

Get Manual
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Only 2 downloads
allowed a day

and the downloading will begin.
Good luck!
gylacz

Either tape is tight, run it ff the rev all the way back, or transport is failing , motor , belt , pulley etc,

check tape or try another one in deck b, how is deck a

???

i bought some today from **** smith electronic store , i paid $7.97 for a pack of ten assorted sizes

The best cleaner for heads is Servisol Video 40. You can get it from Maplin's UK or Amazon.

Hi Chris..

It's not worth repairing. I would find something else in good condition

Sounds like the tape tension sensor is stuck. The sensor is always telling the mechanism that the tape has reached the end.