Vintage DD turntables. Are we living dangerously?

Dear Richard, For further clarification and to expound on your post above, the current Artisan Fidelity Sp10Mk2 Technics Sp10Mk2 replacement platter (~5kg.) is comprised of a magnesium alloy based lower segment and pure Copper upper section. The platter's internal cross section features concentric damping provisions to help tame stray resonances. Initially, the platters used a slightly heavier (+.4kg.) aircraft aluminum lower section but was eventually replaced by magnesium alloy possessing superior damping characteristics.

Pryso, Acute observations regarding the robustness of the factory Technics Sp10Mk2 A's DC based motor, indeed it (including supporting bearing architecture) is easily capable of accommodating a heavier platter and/or mat over stock. For reference, the factory Mk2 platter weight = 2.9kg. and a respected replacement platter mat, say for example the well known vintage Micro Seiki CU-180 weighs in around 1.4kg.

Chris74.
Thanks for the expansion of details on your SP10MK2 platter.

Further, I agree on the robust nature of the bearing assembly. Both it and the MK3 are substantial indeed.

Geoff.
I see two main structural paths in a TT... One from the record upper surface, thru the platter, bearing, plinth (or shelf for those of us who prefer au naturel ), arm, cartridge and stylus. "The loop path"
The other, from the record surface to Mother Earth "The ground path".
For now, ignoring suspended TTs

I have used lead extensively in my TT designs, but it has always been to " laminate" these two paths and has not been inserted into either of them.
I have found this methodology to work well.

Cheers.

Last month Lew brought up an aspect of performance that we read little about, speed correction timing.

"It's also the case that the L07D servo was deliberately designed to exert a much looser control on the speed than does the Technics servo, for one example. I think it only activates when there is +/-3% speed error."

Maybe it's my imagination, but I've heard this difference between my Kenny and other direct drives. Any thoughts on the timing of speed error control?
Can properly functioning non quartz servo controls outperform some quartz locked counterparts?
Regards,

Fleib.
To add a little more to your question. I took a lot of notes when upgrading a customers LO7D. One surprising observation is that the motor stator is rubber mounted via grommets. Such that it is possible for the stator to twist a little backwards in an anticlockwise direction when applying torque. This would seem to be counterintuitive but it is there in the design.

I don't know about the 3% servo thing, but it has been quoted many times in various posts. If this is how it is built, it would be a reasonable assumption that it is effectively open loop once up to speed. In this way it is relying on the synchronous motor's innate speed accuracy and the platters inertia to maintain the correct RPM.

07-13-15: Richardkrebs
Fleib.
To add a little more to your question. I took a lot of notes when upgrading a customers LO7D. One surprising observation is that the motor stator is rubber mounted via grommets. Such that it is possible for the stator to twist a little backwards in an anticlockwise direction when applying torque. This would seem to be counterintuitive but it is there in the design.

I don't know about the 3% servo thing, but it has been quoted many times in various posts. If this is how it is built, it would be a reasonable assumption that it is effectively open loop once up to speed. In this way it is relying on the synchronous motor's innate speed accuracy and the platters inertia to maintain the correct RPM.

Richardkrebs,
It is staggering that you would be performing turntable modifications without a thorough understanding how the TT works. It is fanciful to speculate that the Kenwood engineers designed "slippage" when torque is applied by using rubber grommets. It is more likely that they employed the grommets to reduce mechanical and/or electric noise within the suspended board and coils.

To help you understand the L07D servo operation, here is a link to the owners manual - http://www.vinylengine.com/library/kenwood/l-07d.shtml

Page 4 gives you an overview - the L07D uses a dual mode speed control system - if the speed error is below 3% the phase is controlled with a wide lock range and large phase gain. If the speed error exceeds 3% the servo controls speed rather than phase, and applies higher torque. When the servo mode switches from phase to speed control, the coupling changes from DC to AC to minimise influences from the motor drive circuit and motor offset.

Clearly the L07D relies more on platter inertia and phase locking for coping with stylus drag unless the speed error is very high. This is quite a different design to the Technics SP10 solution which relies on a much more aggressive servo speed control system with the DJ market in mind – the ability to start and stop on a dime was more important to the Technics engineers than the Kenwood engineers. I have listened to many L07D’s and SP10mk3’s at length and in my view the difference in servo design and implementation forms a significant difference in presentation, particularly in timing and coherency.