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Back to TD150 Mk II index page

1)

Above photo #1 shows the underside of the table, just prior starting the stage 2 sessions.  The object being to set the table up in near stock configuration, align it and then just listen for a few days to get some sense of the deck's signature sound.  Springs and rubbers have been cleaned and powdered (talc). An arm board has been cut from 3/8 inch thick grade LE phenolic.  The SME 3009 S2i tonearm has been mounted.  The suspension has been leveled and tuned for vertical bounce action.  

It is at this point that I notice, when looking at this view from underneath, the three very different spring adjustments.  It is very clear that the spring nearest the tonearm is taking more weight than the other two springs.  In another project, I had weighed this tonearm and compared the weight to the Thorens TP16 tonearm.  The SME was heavier by a few ounces.  However the TD150 Mk II does not use the TP16.  It used the TP13A.  I don't know the weight of that arm but assume it must be substantially lighter.

2)

above illustration #2.  A cad drawing of the subchassis that shows the spring locations relative to the spindle bearing.  It appears that the two arm board springs are located closer to the bearing than the far spring.  I think this spacing will result in an uneven spring loading even when the standard arm and board is being carried.

In spite of the uneven spring loading, it is possible to get a "mainly" vertical bounce action with careful tuning of the springs, their height adjustment and by centering the spring around the stud that holds it.  However, it isn't possible to get a true "piston" action.  Instead, the bounce action of the armboard and platter may be observed as relatively even but with some difference in timing.  The tonearm end will crest just slightly behind the unloaded end of the arm board.  The platter crests at a slightly different time.  This results in a slight jiggle.  The jiggle can be reduced substantially thru careful tuning as described above, but not completely eliminated.  

My take is that a true piston action will not occur until the mass of the suspended elements is centered between the three springs.  I can think of two ways to achieve this.  

1) The first, and easiest, is to fix additional weights to the subchassis next to 2 of the lighter loaded springs.  The object being to load each of the three springs with equal weight.

2) The second, and more elaborate, would not only balance the weight load between the three springs, but place the mass center lower within the vertical dimensions of the box.  I can envision this as a second frame attached under the upper existing frame.  Provision could be made for a moveable weight to help find a precise mass center.  The lower the center of gravity, the more control will be had over bounce action.  Do this right and the under frame could also serve to stiffen the upper frame.

However, I'm thinking ahead.  I have seen similar spring loading issues when tuning my TD160.  The spring nearest the arm board seems to be taking a heavier load.  I feel that this may be inherent in the design.  Call it a weakness of the table. We have already seen another weakness with regard to subchassis rigidity.  I will accept the above limitation for the time being, but down the road, if significant sonic improvements are to be achieved, a re-design of the subchassis needs to be done so that each of the three springs are loaded equally and that the mass center is as low as is possible within the confines of the cabinet interior.