TD124 pitch control

[analog_sa]

Hmm, this ain't gonna be easy Sorry, i'll only give a few more opinions; there is lots to read on the web.

I guess that means disassemble the 3 small screws at the bottom right? How do I tell the steel ball is worn out?

No idea what screws are you talking about. The ones that hold the spindle to the platter? These obviously have to go. The ball turns and you can inspect all sides for wear. It is only held in place by a washer.

I thought the armboard needs to be coupled with the bearing platter so that the arm/cartridge don't vibrate with respect to the record. Especially in a spring suspended design, for example, decoupling the armboard doesn't make sense to me. Can you explain?

Absolutely right about suspended design. In a non-suspended adding enough mass to the arm-pod seems to do the job well enough. Yeah, i know all about Linn's theories about some magical closed loop between bearing/platter/arm/cartridge and the free flow of energy. I think it's pretty much bull. You can also overdo the mass/damping thing - there always is an optimum but it requires some trial and error to establish.
For a nice example of a separate arm pod design look at Redpoint Audio.

Wht is it difficult. I thought with a spring suspension you just choose a natural frequency for the system (4-5hz), and choose springs that will yield that given the mass they bear. Then you need to apply some damping and you're done. Where am I wrong?

You are not wrong. It's just a lot more difficult to build properly. Have you ever tried setting up the suspension on a Linn? Of course even a child can do better than that but it's still not easy.

Why is that? I thought mass was important.

For non-suspended decks mass is important. But there is still no reason for large size. Minimum surface area will reduce sensitivity to airborne vibrations and hopefully simplify the natural resonance modes.

I'm not sure how it's achievable given the square Thorens aluminum chassis

I was assuming you won't require the services of the Thorens chasis any more. From what i understand it is hardly presentable at this stage.

So, which is recommended. I was thinkink light and rigid, if coupled to the main platter chassis with the 3 given screws.

Again, i was assuming a new chasis design. The original armboard mounting is probably largely responsible for the 124 lack of deep bass. If i was to build a new plinth for an unmodified 124 i'd make sure the horrible thin extension of the chasis is not used for an armboard.

I've seen that VPI do it on their TNT

You are probably refering to the flywheel. It is maybe a few hundred times heavier than the 'stepped pulley' you mention. This really gives it a purpose in life.

DC or AC?

It is possible to achieve excellent results with AC synchronous motors but only if they get a really nice PS. Pure sine, separate phases, etc.
With DC motors the PS is very simple. Only problem (annoyance?) is that you need to periodically set the speed.

[Tee-Rex]

OK, I get it. Thanks for your patience and the great advice.
I guess I'll make some progress by myself.

Cheers,
-T