Cleaning Nak tape heads...
You can use some Q-Tips with some alcohol on them and gently rub the metal oxides off the heads. Clean the gap on the heads, but don't jam the Q-Tip against the heads with a lot of force, just enough to clean them. Change direction and you should see the buildup being removed if there is a lot present.
I also rub the capstans, those spinning metal rods that move the tape along the pinch rollers. I clean the rollers too, but try and dry them off with the other dry end of the Q-Tip quickly. Rubber and alcohol probably shouldn't be mixed for too long.
I have a MAXELL tape head demagnatizer that is shaped like a cassette and I pop that into my Nak RX 505 for 10 seconds and it supposedly does the trick. You can get those hand held tape demagers from maybe Radio Shack? They look like an electric toothbrush and are pretty easy to use.
Good luck, Chris