How can I adjust speed on Ariston Q Deck?

Yes there are a number of avenues to try and the guy over at The Vinyl Engine fixed his speed problem with a regulated supply.The belt suggestion is good and I would check to see there is lubrication in the spindle-well also.A thicker type of oil is recommended here.I recall they stated in the manual you only had to oil it once for life!Anyway let us know how you go.

How about a new drive belt.sounds like its running slow ????

I answered one of these questions over at Vinyl Engine and the cure was to use a regulated not unregulated power supply to power the table.So try that a DC 12V 350Ma at least.