Can an old Thorens Table be "Over Dampend" to the point where sound is adversely effected?

Your experiences should be a salutary lesson to some, about the dangers of 'over-damping'.
Here is a quote from a learned dissertation on vibrations:-

Damping is always present to some degree in any real system. Without it, there would be no way to limit the amplitude of a resonance. The materials employed to provide damping, the ratio of these materials to those elements that need damping, and the method in which they are applied, all determine how effective any scheme is at reducing broad-band vibrations. However, you cannot mechanically "over-damp" a structure or component that is not designed to be a transducer. You can over-damp some circuits electrically; you can over-damp the "Q" of a speaker as well. You can also over-damp structures associated with a phono cartridge: a tonearm, plinth, etc. But you cannot over-damp a preamp's chassis or an equipment rack.

Note the 'over-damping'revelations concerning phono cartridge, tonearm and plinth.
Despite what some here have been propagating for years....over-damping of turntables, arms etc is not only possible, but can deleteriously affect the sound.

And on that note...May we ALL learn something here...Thank you "halcro".

A lot of people like to dampen these older Thornes tables either on the interior of the wooden plinth or all over the inner metal spring board, and arm board, they use things like "Blue Tac" or that Dynamat stuff in car doors. It will kill the sound and rob the life out of the music.

Despite what some here have been propagating for years....over-damping of turntables, arms etc is not only possible, but can deleteriously affect the sound.

You do have to be careful. But damping a plinth usually dosen't get you in trouble, unless that plinth is so floppy to begin with that all you really did was to change the resonant frequency!

One area where people get in trouble is damping the arm board. The arm board has to be rigidly coupled to the plinth to insure that it moves in whatever plane the plinth moves (a corollary is that if the arm board is of a different material from the plinth it can introduce a coloration all by itself). If you've damped it incorrectly, that may not happen and the result is that the pickup will interpret the difference between the plinth and the base of the arm as a coloration. 

The other problem you have to really be keen on is to make sure that you are not treating the turntable as a sort of tone control to synergize with weaknesses elsewhere in the system.

Dear @atmasphere : """  not using damping materials on the arm in such a way that the performance is compromised. For the most part, common sense. """

it's clear that we can't overdamp a TT plinth, as you said maybe only change its frequency resonance.

The key word in all this critical main analog audio damping subject is: " compromised ".

If the damping impede the " free " horizontal/vertical tonearm motion or increment its friction movements then we are in serious trouble but if damping in a tonearm does not contribute against its task then we can't overdamp it.

Same for a cartridge, we can't overdamp as a fact we need and want an innert cartridge other than the stylus/cantilever precise movements ridding the LP grooves.

The arm board mated to the tonearm is of vital importance and here what we need is " serious " coupling in between.

A TT platter can't be overdamped neither, what we can have here is a bad damping but can't overdamp it. We need here an innert TT platter with out compromising its function.

All these damping theories and facts move on according to quality performance at each single link in the audio system chain and what the owner is accustom to or according his music/sound priorities and knowledge levels.

For many of us maybe to much damping could means words as: dull, reduced stage, lower transparency and the like but for other of us this is what we are looking fr because could be nearer to the live event in a near field listen  scenario.

Regards and enjoy the music,
R.

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