If you want to ground it, attach a wire to the metal subchassis, and put that to your ground lug.
I don't know about the platter thing.
I don't know about the platter thing.
If it is a typical Thorens "TD" design there should be a single tiny screw located towards the center of the bottom. Removing this screw will release a pot metal brace along with the inner platter.
This screw is also "generally" what the external ground wire is attached to. The internal ground is most likely linked to the one channel channel of the phono IC. If you remove the plastic cover you should be able to see the barrier strip @ which the tonearm leads meet the phono IC and the internal ground will be the odd (fifth) wire located @ one end of the barrier strip.
If it does not hum, then don't worry about an external ground.
As someone educated in physics, and who has studied turntable designs quite a lot, I have a lot of respect for the Thorens TD-145 and other similar models from this range.
The TD-145 incorporates a suspended sub-chassis design, which in my view is the best design route to take where it comes to making turntables, so that the base helps to isolate the phono pickup from acoustical vibrations. The platter and tonearm are de-coupled from their surroundings in the most effective, and economical way possible. This way, the vibrations from the speakers, as well as footfalls, are mechanically filtered out.
You can test this hypothesis out very easily yourself. On any direct-drive turntable, place one index finger on the platter, and then knock the turntable base with two knuckles from the other hand. The vibrations are directly transmitted to the platter. Same with a non- suspended belt OR idler -drive design. You can confirm the same thing with marble or slate; knock a little harder and you will still feel the shock wave. However, with the Thorens design you will not feel the transmitted knock.
From the beginning, phono pickups have been amplifiers of vibration, and thus it makes sense to eliminate vibration at the point of contact, other than from the record groove.
Now, for skipping stability, the suspended chassis design must have a massive enough platter, so that the rotational angular momentum of the platter will contribute to planar stability. If you have ever tried to turn the front forks of a bicycle, where the wheel is spinning suspended in the air, the resistance you feel is from the directional conservation of angular momentum, a basic physics principle. Many cheaper suspended chassis designs went with a lighter weight platter than the Thorens has, and were more prone to skipping. In the TD-145, the balance between platter mass and subchassis suspension was optimal. (They can skip if jiggled however, and if your floors are springy then this may not be the best design to install.)
I have one of these, with an up-to-date Grado Sonata Platinum cartridge, and I couldn't be happier with the results for a 'vinyl' source in my system.. I also replaced the original tiny rubber feet with 1" adjustable solid brass spike cones, to let me precisely adjust the leveling.
With mine I also performed a conversion ( available in old Thorens literature online - see http://www.theanalogdept.com/ ) to give the tonearm wiring the 'US' grounding scheme, so that it does not hum when plugged in to my receiver the usual way. I also had to repair the phono cable ends, by shearing off the last 1 inch of them and replacing the ends with new switchcraft style rca plugs. Now it works better than new, with even channel balance. It's a job, but the fine-tuned suspension in the original base helps produce a golden tone.
I've noticed things have finally come full circle - Thorens has reintroduced the suspended chassis turntable in the TD 350 & 550. Another similar turntable based on the old TD series is the Linn LP12; and Pro-Ject makes one of these with a pure magnetic suspension. But at nearly $4K for the new Thorens TD 350, which is quite similar to the TD 145, you would do well to keep your 'classique' 1970's model TD 124, 126, 145, 160 etc.