Jvc hr-d750u

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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.

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Ok the AC online just shuts off the outlet on the back when the power is turned off.

It had loading issues but after taking the loading transport(to clean the idler) out a small screw fell out,don't no where it came from but the loading has been fine now.

One issue is the HIFI playback is sensitive and turns off and on with every little glitch on a tape.I saw on Ebay there's a HR-S5000U and the person said the hifi is stable and a correction was performed as it's a common issue.Different model but was there a simple adjustment to correct this and what would it be?.