TD124 pitch control

[Tee-Rex]

Hi,

I've seen some threads on this topic, but could not find a straight answer to my question. I'm experimenting with a TD124 that has it's motor and stepped pulley externalized. Most of the other internals of the deck were just taken out. Among them is the eddy pitch control.
With the motor now in a seperate box, driving the stepped pulley (in yet another seperate box), driving a high mass platter, I was wondering how I will be able to control fine pitch variations around the desired frequency. I was thinking a controlled frquency PS.
Any ideas on how to build one ar get one? Other techniques?

TNX
Tee-Rex

 

[Netlist]

Are you talking about Thorens?
Picture perhaps?

/Hugo

 

[analog_sa]

Hi Tee-Rex

This sounds like a very interesting project indeed. Are you still using the idler to drive the platter? Pictures will indeed help.

The magnetic brake may offer some sonic advantages by virtue of smoothing the motor's torque. Is it impossible for you to still use it?

Frequency generator is indeed one solution. If you build a standard Wien bridge keep in mind that a stereo pot with very good tracking between the channels is required. Any channel imbalance will result distortion in the oscillator output.

My preferred solution is a microcontroller-based oscillator. You can either use a separate ADC to read a pot or choose a controller with an ADC such as 90S8535.

Of course you'll need some way to boost the osc output to around 100v.

 

[Tee-Rex]

Yes...

It is a Thorens 124, MK I (I believe).
I've taken some pictures of the parts and will post them soon (maybe it's time for me to go for a digital camera - but what can I say - I'm an analogue sort of guy;-). However, everything I described is still in design phase (the boxes, plinth etc.), so actually there's nothing much to show (yet).
I thought to take this step by step with the help of people on this forum.
Anyway - a disclaimer for the 124 purists - this will be using the 124 in a very unorthodox manner. I apologize in advance if it upstets anyone. On the other hand, I would very much like to utilize the advantages of the original as much as possible, so advice will be more than welcome.


-T

 

[Tee-Rex]

I apologize

Are you still using the idler to drive the platter? Pictures will indeed help.

Since I'm sort of new-ish to the forum, I'm still being moderated - meaning that my posts are being read by someone before everyone sees them, and that takes some time. I've already posted one answer (to Hugo) so will try not to repeat too much of it as it is not yet posted while I'm writing this one.
As I was saying in my previous post, I will attach some photos as soon as I get the camera film developed (analogue technology, you know...). However, it's mostly in design stages, so nothing much to show but a dust covered pile of stuff.
I was hoping to share this project with the forum as I go forward. I hope it will be interesting.

The idler wheel is not used here (I don't even have it). The subplatter will be belt driven directly from the stepped pulley.

I can't use the magnetic brake because I don't have it as well, and anyway, I'm need to think how it would fit into the pulley box.

Is the output of the oscilator a sine wave?

And I'll finish with a small prayer:

Oh, ye gods of Moderation, release me from thy bonds, and I shall become a true believer!

-T

 

[Netlist]

Re: I apologize

And I'll finish with a small prayer:

Oh, ye gods of Moderation, release me from thy bonds, and I shall become a true believer!

-T

With pleasure.

Please go on with that project. I recently disassembled and reassembled a TD124 with white gloves.

/Hugo

 

[analog_sa]

The subplatter will be belt driven directly from the stepped pulley.

Ok. So, that's not a 124 anymore then. You are building a new TT using the main bearing, platter and motor, right? Nothing wrong with this; i guess i am also not a 124 'purist'.

My first suggestion in this case is to get rid of the original motor. It's pretty pathetic even in the original design but in a belt drive design there is simply no excuse to use it. There are lots of nice low noise DC motors which will be really preferable to it.

An interesting related story: a friend of mine with a 124 went through an amusing path. He first bought a VPI platter, put it on top of the Thorens platter and drove it by belt from a VPI motor. He seemed to like it for a while. Eventually he bought a VPI bearing as well and removed the Thorens from the picture. The sound was no longer to his liking. After a lot of trials he ended up using the Thorens chasis with a surgically attached VPI bearing/platter driven by the VPI motor. There are also springs involved in the suspension but this is not so important. The interesing bit is his claim that a lot of the magic is due to the Thorens cast alloy chasis.

 

[Tee-Rex]

So here we go (long)

OK, great!
Let's take a few steps backwards (BTW - I did find the magnet brake - but we'll get back to that a little later).

Some background. This is a Thorens 124 Mk1 (I'm almost sure - what's the best way to tell?). My cousin modified it about 5 years ago, really quick and dirty job - he took out the motor and the stepped pulley and out them into seperate enclosures (very flaky ones). One belt connects the motor to the pulley, and one from the pulley to the platter. The Aluminum chassis was very roughly cut with a disk to allow the belt to be connected externally. The aluminum top platter is gone, instead there is a ~2" thick PVC platter weighing around 10kg. The original (I think) 12" plastic arm board was raised on spacers in order to accomodate the high platter.
This was sitting in a pile in the basement for a couple of years, and he called me up a couple of weeks ago asking if I want it - because his wife said she's going to throw it away. So I drove over and took it (along with 8 cartons of LPs). There's also an SME3012 arm in the package.

The idea is to use the same constellation, but house the different parts in well thought of and much more aesthetic enclosures. So, this is not really a TD124 restoration, but rather a DIY table based on this classic. What I thought is to take this one step at a time - each part needs quite a lot of thinking and advice will be most welcome.

The phases I was thinking of are the following:

a) Make sure the parts used are in a usable condition (motor + shaft and pulley, stepped pulley and it's bearing, main platter bearing, tonearm etc.)
b) Design of the arm board - material, size, strcture, locations for arms (9", 12", preferably both together)
c) main plinth - material and structure, suspension etc.
d) Stepped pulley housing - material, structure, breaking mechanism, suspension. hight and level control, etc.
e) Motor and it's housing - same as above
f) Misc: new stroboscope circuit, choosing belts etc.

Sound OK?

My cousin has access to the best presicion machanics around and my best friend has a huge carpentry factory - so I think almost anything is achievable mechanically speaking.

I'm really excited about this...

-T

 

[analog_sa]

My take on your project:

a) Make sure the parts used are in a usable condition (motor + shaft and pulley, stepped pulley and it's bearing, main platter bearing, tonearm etc.)

The main bearing will make or break the project. If the spindle is bent (very rare) there is no cure. Have a look at the captive stainless steel ball. It often shows signs of wear and it's very easy to replace. A tungsten ball may be a great upgrade.

b) Design of the arm board - material, size, strcture, locations for arms (9", 12", preferably both together)

Two schools of thought here. The armboard may be an integral part of the bearing/platter or a standalone pod. I always thought the standalone was a compromise. Now i am not so sure. It is certainly a lot more flexible.

c) main plinth - material and structure, suspension etc.

A suspended design seems a lot more difficult to implement successfully. I would rather choose a well isolated platform (symposium clone, air-isolation, etc).

An easy and good sounding solution is drilled and lead shot filled multilayer baltic birch. A constrained layer of PVC/Alu/Wood is likely even better but a lot more difficult and expensive.

A minimal in size plinth is also beneficial - either teardrop shape or just round if you have a separate armpod.

From all material choices the most important sound-wise seems to be the armboard/pod material.

d) Stepped pulley housing - material, structure, breaking mechanism, suspension. hight and level control, etc.

I have no idea why would you need that pulley. A flywheel may be a much more sensible addition.

e) Motor and it's housing - same as above

You must choose a low-noise motor if you really want to improve upon the Thorens. The original has a very high torque which seems essential for a rim drive or agricultural equipment but is quite unnecessary for a belt drive.

 

[Tee-Rex]

analog_sa, thank you for the guidance. Some questions (sorry if some are trivial):

Have a look at the captive stainless steel ball

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

The armboard may be an integral part of the bearing/platter or a standalone pod. I always thought the standalone was a compromise. Now i am not so sure.

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?

A suspended design seems a lot more difficult to implement successfully

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?

I would rather choose a well isolated platform (symposium clone, air-isolation, etc).

Can you explain a little more about these 2 solutions?

A minimal in size plinth is also beneficial

Why is that? I thought mass was important.

either teardrop shape or just round if you have a separate armpod

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

the most important sound-wise seems to be the armboard/pod material

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

I have no idea why would you need that pulley

The idea is to further isolate motor vibrations transmitted through the belts from the platter. I've seen that VPI do it on their TNT, and it also allows tou to use 2 motors (I don't know if that's good)

A flywheel may be a much more sensible addition

Where would that go?

You must choose a low-noise motor if you really want to improve upon the Thorens

So, which one, DC or AC? I thought AC would allow me to decrease the torque once the platter is spinning full speed by lowering the voltage. Is that doable with a DC motor?

Again, thank's for sharing your knowledge and exerience.

-T

 

[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