Vintage DD turntables. Are we living dangerously?

I was incorrect about the SP-20 not having quartz lock. The motor is also brushless.

http://audio-heritage.jp/TECHNICS/etc/sp-20.html
Model Direct drive turntable Turntable 32cm aluminum die-cast, 2.5kg Inertial mass 345kg · cm 2 motor
Quartz phase locked control
Brushless DC motor Drive system Direct Drive
Rotational speed 33 1 / 3,45rpm
Wow and flutter 0.025% (JIS C5521)
WRMS SN ratio 60dB (IEC17B) 73dB (DIN45539B)
Starting torque 1.5kg · cm
Start-up time 0.7 seconds (90 °) within (33 1 / 3rpm) brake
Pure electronic brake Load fluctuation Within 1.5kg · cm 0%
Rotation speed deviation Within ± 0.002%
Power supply AC100V, 50Hz / 60Hz power consumption 4W
External dimensions Width 368.5 × height 99 × depth 368.5mm weight 8.0kg

Hi Peter (pbnaudio),

I'm defintely interested in converting my DP-80 into a similar version of your Groovemaster DP8. How do you compare your DP-8 to your SP10Mk3? Also can you make a 2 arm version of your DP8? I'll send you a pm via your website.

Lewm, I used a Micro Cu-180 on my TT-101 and it sounded wonderful.
Trouble was....the brake-action when the stop button was pressed couldn't cope and at the time I didn't know that there was a variable resistor to adjust this brake action.
I, like you....was concerned by the weight on the bearing and also I doubted that the bi-directional servo control could possibly work as designed...🤔
I thus began a search for the platter mat that would sound the closest to the Cu-180 and believe it or not......the gossamer weighted Victor Pigskin came within a whisker of the same sound.
What's the science of this...❓‼️
Go figure...

Agree and concur with numerous posters above, the Technics Sp10Mk2/Mk2A bearing structural architecture indeed can safely support the additional weight of a Micro Seiki CU-180 or the rarer CU-500 series Gunmetal Copper alloy platter mat provided the spindle's thrust pad is in good order, of course, thrust pads can also be changed and improved, if desired.  Clients have mentioned over the years rumors of Pass Labs using a Stainless Steel based replacement platter with reported success directly in place of factory Sp10Mk2 platters although I have yet to personally listen to one or actually examine one myself.  I believe it was roughly 2.5 - 3 years ago when we sat down and designed, prototyped then began to manufacture a suitable direct oem replacement Copper and Magnesium/Aluminum alloy based platter for the Technics Sp10Mk2/Mk2A models.  This is the same platter found on the AF Sp10Mk2 NG model.  If anyone is curious as to how it performs in comparison to a stock setup, please feel free to PM me directly for further insight.  As many are aware, the Sp10Mk2 design incorporates an electronic braking system, in addition to the mechanical brake belt facilities, which will tolerate the added weight of a heavy platter mat or higher mass replacement platter but the subsequent braking affect is slightly diminished in stock form.  When it comes to actual platter-to-record interfaces, naturally, each individual users system to a degree will react uniquely through difference playback characteristics, this applies when changing Sp10Mk2 platters entirely and/or platter mats and the reality is with all things high end audio related, no "one size fits all" solution exists.  This is the subjective and sometimes perplexing nature of the hobby and something I for one believe makes things all the more interesting in our niche world.   

Just for kicks, below are some additional weight specifications gathered over the years.

- Technics Sp10Mk2 Factory platter weight - 2.9kgs  = 6.4 Lbs.
- Technics Sp10Mk2 Factory Rubber Mat - .54kg = 1.2lbs
- Misc. Outer platter periphery rings - (Various manufacturers measured over the years) ranges - .5kgs - 1.4kgs / 1.1 lbs. - 3.2lbs. 
- Oyaide MJ-12 Aluminum record mat - .68 kg = 1.5lbs. 
- Artisan Fidelity Copper/Magnesium/Aluminum alloy replacement Technics Sp10Mk2/Mk2A platter - 5.4kgs = 12lbs. (designed to be used without a record mat)
- Record Clamps - too many to list - typically ranging from 6oz - 4lbs.+  

I'll have my MK3 back on the bench in a week or two, time permitting.  If someone wants to lend a heavy platter mat to the cause, I can perform some measurements to try and determine the impact to motor performance. 

My intuition says it's less about PLL alignment, and more about drive gain.