Magnetic Platter bearing replacement - Sapphire and Si3N4

Re: Magnetic Platter bearing replacement - Sapphire and Si3N

Postby Johnny » Fri Oct 04, 2019 1:04 pm

vienna acoustics wrote:
izk wrote:Ok, second call to Cliffwood today. According to VPI tech, the bottom drive platter is just slid on the shaft and sitting on the metal ring there. Nothing is affixing it to the shaft itself other than weight and friction. It comes right off.

So it appears my issue is that at some point I put it back on backwards and the magnetic grasp of the drive platter onto the slave one is weak, thus the extended rotation I see when the ADS is stopped and the hard stop I see on the bottom platter.


Still can’t understand how you have experienced this improvement you have stated, with all that mess

I was thinking the same thing! It must sound even more awesome now that all the parts are correct.

On a separate note, Kudos to Brf for figuring out that something was not right with Isaac’s table and walking him through a series of steps to diagnosis his problem. Izk, your owe Brent a beer!
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Re: Magnetic Platter bearing replacement - Sapphire and Si3N

Postby izk » Fri Oct 04, 2019 2:59 pm

I have flipped the drive platter to right side (bare metal) down. Relubricated everything once again and cleaned the drive belt so it does not trip @vienna acoustics anymore ;)

Before putting the rim drive back together, I rotated the now clean white rim belt between a table and a hardcover book back and forth for a couple of minutes in an attempt to set it in place as evenly as possible. I always level both the platter and the Rim drive disc. I positioned the Rim disc as far as possible that yields continuous touch confirmed by backlight. Ran it for an hour.

Some observations:

With the Rim Drive off, gently rotating the drive platter back and forth makes the slave platter follow and it feels like the drive platter has a better hold on the top one. Quickly changing directions on the rotation makes the top platter break loose and keep rotating in the original direction.

With the Rim Drive on and in full speed, stopping it does basically what it did before: The drive platter stops immediately and the top platter keeps rotating for a few seconds. It appears to be a bit less pronounced, but it could be my imagination. Furthermore, I do not see how it could possibly be any different. The 2 Rim Drive motors are somewhat stiff when stopped and the magnetic hold is not that strong that it could force the drive platter to overcome the torque of the 2 idle motors.

Soundwise I am not shocked by major improvements. Listening to a record I had been listening to a few times before the fix, there could be a bit more image stability, but it's difficult to tell and if it's there, it's not a drastic difference. So overall, changes are not super dramatic.

It may very well be that there is not a significant difference in magnetic field force with the drive platter facing either way, or it may be that, once the platter is in stable speed, the magnetic force is enough for overcoming friction either way, and specially with these smoother bearing balls.

All in all the gains I described before are still there and maybe a tiny fraction more of them... maybe, but they were certainly not lost or diminished by the change.

Something to note is that I am a very anal speed setter. What is the point of having a $30K TT if the speed is off. I start the ADS 10 to 15 minutes before playing the first record. There around a .2 speed increase between a cold start and 10 minutes later when the grease and oil have warmed up. After this I fine-tune the speed with the RPM Pro app to exactly 33.33 or 45 RPM accordingly and I check after each record side.

Thanks Brent for your support. I think we each learned a couple of things during this process.

Even though @vienna acoustics is skeptical, I am sure I had flipped the bottom platter a while ago and that was not a change introduced by this modification. So it makes sense to me that the improvements I hear are related to the modification itself in spite of the flipped drive. Nevertheless I am considering reverting to the original carbon-steel bearing ball for the top platter. It's lesser smoothness may make it the top platter rotate less on it's own when starting or stopping and it may be interesting to test for some of the issues raised in the paper @@vienna acoustics shared. This bearing is only used during starting and stoping rotation and does not rotate with stable speed and thus has a longer life than the others.
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