Hi Chris,
Could be the head it's self or asscoiated circuit gone faulty.
But I think it's worth sticking with the cleaning first, as it's 6 years old and if its had normal usage, it needs more than a quick wipe, even if you can't see any dirt.
To be sure, you would need to clean the heads with some sort of alcohol e.g. IPA, or surgical spirit or maybe after shave if you were very stuck. There are special cleaning sticks ( from Maplin ), but I found cotton buds will do the trick. *NB* You need to clean any part that will touch the tape, heads, rollers etc. Play a tape with the cover off and watch what comes in contact with the tape. When you start cleaning, you will see black on the cotton bud, keep aplying more alcohol/fresh buds until the black stops coming. Then you dry it off with fresh cotton buds. leave it with the cover off for a few minutes for it to dry.
Leave off the cover, so you can watch whats happening to the tape, but be carefull of power.
Now power it up. What I found is that no matter how carefull I was cleaning, the tape mechanisem would have to re-adjust itself, so don't put in your best tape, as it may damage it. You may have to switch it on and off a few times before you can play the tape, it will make all sorts of strange sounds, but it should come right.
If the cleaning fails, you could try recording a program with it, and try playing the tape on another machine to rule out a few things.
If it records the program you might be looking at a faulty play head or near by circuit, but if it doesn't record, you could be looking at some thing further on, commomn to both heads.
Good luck.
Noddy
February 2007