Jvc hr-d750u

[lost audio]

A $5.00 Goodwill find and it almost looks new.It has HIFI,HQ,manual tracking and DA 4 heads plus little audio meters.It has a good picture even on a LCD tv but there's a switch on the front that say's AC ON LINE on/off does anyone know what it does?. And can this unit use the meters to adjust tracking or do you just have to kinda guess it's tracking good?Like adjusting the tracking till the hifi goes off then go the other way while counting how many times I push the button till the hifi goes off and then set it in the middle. Is there any common problems with it like the early 90's JVC's that had guide post problems,this one was probably late 80's.