Black residue stuck on drum
 

i just picked up a JVC that has some black residue stuck on the drum.
i tried 91% alcohol with a chamois stick and it wont come off.

i have some lacquer thinner, but im afraid that my f-up the head

the unit seems to work just fine, but i know it is there.
any idea what it might be and how to clean it?

It's likely a piece of oxide shed from a bad tape. Alcohol will just make it adhere more. Get a piece of Scotch tape, and gently try to pull it off that way. It must be 3M Scotch branded tape, too! That off-brand garbage uses gooey slime for the adhesive, and will destroy your heads. Even with Scotch tape, you want to be gentle.

Slightly off-topic, in response to your using a chamois stick: Remember to always clean with the rotation of the drum -- never up/down against it.

scotch tape didn't work
but lacquer thinner sure did!!
ruined my chamois stick too.
had to be real careful to avoid the heads and just clean the drum.

went over the whole head with 91% alcohol and a new chamois afterwards.