how to use LDA quartz regenerator??? help

Is the speed lock engaged in the back? Disengage the toggle for speed lock and try again. I presume you have the 401 plugged into the back of the LDA and the LDA plugged in. It is hardly complicated. If this does not work, than there is either a defect with your Garrard or the LDA. 

To the poster between the OP and the undersigned, are you just on a quest to get your post number up by bloviating on everything that you know nothing about?

Simple, You bought the wrong turntable. I suggest you sell it and buy a modern turntable. You will be much happier.

Are there no adjustments that affect voltage or current output from the LDA? If so, have you fiddled with them?

Jason, nearly all TT motor controllers are in essence AC power regenerators. They work by affecting voltage and current out put to the TT. And they can be very effective at stabilizing TT speed and improving SQ. The Garrard 301 and 401 can benefit from a good one. I don’t know whether the LDA is a good one to mate with the induction type motor of a Garrard.

Well OP, was it the back toggle speed lock engaged?

If that does not solve the problem, plug the 401 directly into the wall and check for speed. Perhaps your motor needs a rebuild?

How are you monitoring the speed? The strobe markings on the 401 platter are not reliable and nothing beats the Phoenix products now being sold by Sota 

And last, what is the provenance for your 401? Are you sure you have the original motor and not a replacement motor? That is something of a long-shot thought as the LDA should control and change speed regardless of whether the motor is AC synchronous or induction. 

Are there no adjustments that affect voltage or current output from the LDA? If so, have you fiddled with them?

http://www.turntablepsu.com/lda.html