Hello there. Today I went thrift shopping and picked up a nice 3 head Jvc logic controlled deck and everything seems to work fine, But after a few minutes of playing the tape player it will act like someone's pressing play/ stop really fast and it freaks out engaging and un-engaging the play heads quickly until I turn it off.
That looks like a pretty nice deck, I have a few other silver-faced JVCs and always like the quality of their players. I had one that did that years ago, it might have been a JVC but I really can't remember how I solved it. I'd pull the face off and check the board for the controls and see if everything looks ok, one of the micro-switches may be shorting. There should be a cassette detection switch somewhere that might also be shorting, hopefully it's something easy that just needs some cleaning. The service manual is on HiFi Engine https://www.hifiengine.com/manual_library/jvc/kd-v6.shtml
