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Well, it's not the springs...

Posted by tketcham (A) on March 29, 2014 at 17:28:36

In Reply to: Gyro SE suspension problem... posted by tketcham on March 23, 2014 at 10:40:08:

Something isn't right but it's not the springs...

I had some time today to check the balance of the chassis (with the SME 309 tonearm installed) and found that the chassis is definitely weighted too much at the tonearm location. Something is obviously not right; the chassis shouldn’t be tipping heavily towards the tonearm. I did a quick check using a digital scale (chassis resting on a pivot on the spider at the center of the spindle bearing) and with the chassis sitting level, the tonearm and plate are about 134g too heavy. That is; to keep the chassis level, an upward force of ~134g must be exerted under the tonearm. That's a lot. It’s no wonder that I wasn’t able to have the chassis level without the suspension tower spring at a much higher position to account for the elongation of that spring.

Something's not right. There have been suggestions that I have the wrong mounting plate for the SME 309. But I can't find any specific information about the mounting plates, specifically the correct weight.

So I removed the tonearm from the chassis and took some measurements and here’s what I found:

I disassembled the tonearm from the mounting plate and the mounting plate from the chassis and weighed the plate and all the bolts/nuts/etc. plus the DIN connector. (I weighed the DIN connector by using the attachment point on the spider arm as a pivot.) Here are the results:

Mounting plate - 234g
bolts/nuts/etc - ~23g
DIN connector - ~34g
Subtotal hardware weight - ~291g

SME 309 weight (from SME website) - 717g
Total weight - 1008g

If the correct weight of the tonearm and plate and hardware is supposed to be 1000g then the total weight of the tonearm and hardware is just about perfect. HOWEVER, something isn't adding up! As I noted above, when the tonearm is mounted in place on the chassis, the chassis noticeably tips towards the tonearm position; about 134g too heavy. So all I can conclude is that the tonearm weight is not 717g, but rather about 850g based on my measurements. I have no means of accurately weighing something close to 1kg so I cannot verify my estimate of the actual weight of the tonearm.

My conclusion at this point is that the tonearm weight used to determine the weight for the mounting plate was in error and the plate is thus too heavy. Instead of weighing 234g the plate should weigh 100g. That would put the total weight of tonearm plus mounting plate and hardware at 1000g. Perfect.

So now I'm trying to find out if I do indeed have the correct mounting plate for the SME 309. Or at least if I have the plate that's been sold as the correct plate. Something isn't right, but now I know exactly what I need to fix the problem: A mounting plate for a SME 309 tonearm that weighs 100g.

I'll let you know.

Tom

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