CED Digest Vol. 5 No. 46

CED Digest Vol. 5 No. 46 • 11/18/2000
Date: Sun, 12 Nov 2000 10:15:26 -0800
To: digest@cedmagic.com
From: Tom Howe <ceds@teleport.com>
Subject: RE: Out of Balance J/K Player Turntable

At 10:36 AM -0800 11/5/00, Michael A. Campellone wrote:
>So, I opened the unit, and checked the turntable for clearance.  It is
>smooth and silent! Then, I tested with my "beat-up test disk", and observed
>that the turntable does indeed wabble, ever so slightly!  You can see the
>reflection of light in the spinning disk "jitter" very slightly in-time to
>the noise.

Usually these turntables get out of balance when the player sustains an impact
while the turntable is spinning. But it can also happen by playing a warped 
disc, wherein the warp of the disc gets transferred to the turntable. A warped 
disc should immediately be removed from the player. You can tell a player is 
playing a warped disc by the generation of a thumping sound from a previously 
quiet player. Warped discs can also be identified by careful visual inspection.

RCA had a machine in the player manufacturing plant that dynamically balanced 
the turntable shortly before it was inserted into the player. There's a picture 
of this at CED Magic:



http://www.cedmagic.com/player-manufacturing/15-direct-drive-turntable.html

This machine operated on the same principle used to balance automobile tires, 
but achieved the end balance via an opposite method. Tires are balanced by 
adding small lead weights to the outer edge of the wheel, while the CED 
turntable was balanced by removing small amounts of plastic from the outer 
edge of the turntable. The arm in that picture with the vacuum hose attached 
is a router that drilled shallow holes into the underside of the turntable.

So it's technically possible to re-balance an out-of-whack turntable by 
drilling additional holes, but I can't think of an easy way to do this short 
of rebuilding that machine RCA used in the factory. A jig to achieve static 
balance could be built rather easily, but it's the dynamic balance that's 
critical. The turntables are interchangeable, so the easiest solution may be 
to get a balanced turntable out of another J/K player that has been set aside 
as a parts machine.

--Tom
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