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Weight of the lean...

Posted by tketcham (A) on December 7, 2014 at 07:25:05

In Reply to: RE: Success! Lightweight acrylic plate for Gyro SE... posted by Roastaman on December 6, 2014 at 21:43:20:

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.

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