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Yiangos
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Joined: Sep 7 2005 - 8:41am
A strange problem ?

Guys,i have a friend with a Roksan radius and a Sumiko Blue
point special.He recently bought a Michell Engineering record clamp which consists of a small felt mat designed to be placed on the spindle and underneeth the record and the actual clamp which you screw on top of the record.His setup
is okay (vta antiskating tracking weight etc) yet whenever he plays records with the clamp on,the stylus skips (ie jumps back to the previous groove)on the last couple of tracks of the record,a problem not present when the clamp is not in use.Any ideas as to what's causing this ?I told him to reduce the antiskating force a bit and see what happens and he told me the problem was reduced somehow but not entirely.

Jeff Wong
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Re: A strange problem ?

Maybe I'm picturing this wrong... but, I could almost see it skipping forward to the next groove if the clamp was overtightened, causing the disc to dish and flex (where the centre of the LP is made to be lower.) Maybe a combo of this flex and the antiskating is causing the arm to jump a groove forward and is making the tonearm bounce? He needs to film it in action really close and show us... I have no clue.

Buddha
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Re: A strange problem ?

Hi, Yiangos.

That is most intruiging.

The Michell, like the old Mapleknoll and the SOTA reflex clamp all act by holding the spindle and placing downward pressure on the LP. I know, d'uh. But the Michell uses that spacer under the record to create a bit of a "mound" from which to place pressure at a radius distant to the spindle, achieving downward pressure in a ring a couple of inches away from the center of the record. This is supposed to help hold the playable area of the record more firmly against the platter, in much the same way that a vacuum hold down mechanism. The vacuum uses negative pressure form below, the clamp uses positive pressure from above.

So, the clamp actually depends upon flexing the record slightly in order to accomplish its task.

It could be that the "washer" ring beneath the clamp is too tall for your friend's platter configuration and is causing a flexing of the record in such a way that there is some groove distortion which is leading to easy mistracking. This makes some sense to me, especially considering that the problem is limited to the inner tracks.

Does his platter already have a bit of elevation at the spindle? Is the platter flat all the way to the spindle?

It may be that he would benefit from using a bottom spacer that is not as tall.

I once saw a platter clamp that the owner could screw down so tight, that it actually created enough pressure to make the outer parts of some LP's actually bend upward!

So, perhaps less clamp pressure would help, even with the same spacer.

This is all conjecture, of course, but worth his time to look and see what is occurring.

Monty
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Re: A strange problem ?

I wonder if the clamp could be coupling the platter to the bearing assembly too tightly and transfering a little vibration that prgressively worsens as the arm moves toward the bearing...or even messing with the tolerances...no idea, really.

Yiangos
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Re: A strange problem ?

Jeff,Buddha,Monty thanks for the info.I allready passed your
thoughts to him and waiting for a reply.

Yiangos
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Guys,we fixed the problem.

It was the tonearm rest.It was set half milimeter higher than should be and with the record clamp in place,when the arm reached the last 2 tracks of the record,it skipped the stylus.He lowered the tonearm rest a bit and everything works fine now.Thanks again for the help.

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