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In Reply to: RE: Success! Lightweight acrylic plate for Gyro SE... posted by tketcham on December 05, 2014 at 14:10:16
I'm curious, how did you set up your scale to meas. the downward force from the tonearm, armboard, etc.? Did you meas. from the other two positions, i.e, the motor side and the front (center)? Both of those positions have the weight's in the subchassis. I would think in theory that if the subchassis was balanced correctly that the meas. force would be similar at all three points. So if the tonearm side was higher vs. the other two positions you might reduce the weight by the difference between the tonearm side and the approximate average of the others.
Follow Ups:
Hi, Roastaman,
To measure the downward force of the lean I made a pivot point (furniture coaster and an isolation cone) under the center of the platter bearing so that the chassis was no longer resting on the suspension towers but instead balancing on the center pivot. I placed a digital scale under the center of the tonearm pivot point and used a piece of closed-cell foam to raise the tonearm up so the chassis was level. (The piece of foam was conveniently sloped so I could get the tonearm/chassis at just the right height.) The second photo shows the tonearm cable attached but I ended up measuring the weight of the cable separately, disconnected from the DIN connection. (The difference in weight was only a few grams but I wanted to isolate as many variables as I could.)
I made a series of six measurements, resetting the chassis on the pivot point each time, and then averaged the six readings. It averaged out to be about 134g of lean (down force). That's how I figured out the correct weight of the mounting plate I needed (235g stock plate - ~134g down force = ~100g custom plate). When I asked Artech and Michell Engineering about the discrepency they both said that the weight of the stock plate was correct and weren't sure why the chassis leaned that much. They'd never seen this problem before with the stock SME mounting plates. It wasn't the springs so my conclusion is that the chassis wasn't properly weighted for some reason. I asked Michell about a custom plate and they offered a custom acrylic plate that weighs 105g, so I ordered one through Artech-Electronics.
After installing the lighter mounting plate the chassis balanced out fairly well. Not perfectly, but it no longer had an obvious lean towards the tonearm. I think it would require adding small periferal weights around the chassis rim to get it to balance perfectly and I'm not convinced that it would be worth the effort. Getting the chassis to balance out close to level is good enough. The small differences can be corrected by slightly raising/lowering the suspension springs, which is what I did. The final adjustments to get the "Gyro bounce" were easy now that the chassis starts out close to level.
Bottom line: Trying to compensate for a noticeably leaning (unbalanced) chassis by making large adjustments in spring height doesn't allow for an optimum suspension bounce. The fix (a lightweight mounting plate) was simple, but not inexpensive. Still, a worthwhile cost to get an optimal "Gyro bounce".
Regards,
Tom
PS: I want to commend Dave Lang at Artech-Electronics and Steve Rowland at Michell Engineering for their prompt replies to my questions and help in coming up with a solution. My reason for posting all of this is so that other Gyro turntable owners with a similar problem might learn from my experience and come up with a different (and possibly less expensive) solution.
That was a good idea about the pivot point under the chassis. I had other ideas that would have been more involved. Though I hope I would have thought of it eventually!! My Orbe leans a bit to the right, probably more than it should. And I think that many others do as well. My guess is that people have an issue but rely on the spring height adjuster to compensate for it and then figure that the rest is normal. I came across another example of this when looking into putting a Graham Phantom on my Orbe. Michell said no problem and Artech said it wasn't a good idea, too heavy. Many have done it and claim no issues. Few have said it's too heavy. The positive camp out numbererd the negative camp. Some are even using the original armboard and not using the lighter acrylic one that is needed!! I decided not to risk it and went with another TT so I could use the Phantom.
I think I will re-visit this now that the solution for determinig the balance will be easier. I will either add wieght to the other sides or remove weight from the armboard itself or adjust the distb. of weight on the subchassis. THANKS for the pic's!!
You're welcome!
I agree with your hunch that there are more than a few Gyro and Orbe 'tables out there that have a bit of lean. But getting the chassis in better balance before fine-tuning the suspension takes some work and a bit more money. I'm sure many people figure good enough is good enough. My next project will be to find a simple way to do some final balancing of the chassis. The trick isn't so much how to do it, but how to do it so it's attractive and maintains the sonic qualities of the 'table.
Have fun with your project. And let us know what you find.
Tom
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