Adjusting Overhang With a Two-Point Protractor.
Above image: Top view showing the two points of alignment.
The object of the two point protractor is to achieve best possible alignment at 'both' null points. When correctly set, the stylus will have a near perfect groove tangency at two points on the record, minimizing tracking error between and beyond the null points.
The above image shows the two null points in relation to each other. With the platter immobilized, the protractor will be rotated to check adjustment at each null point. For an explanation of the adjustment procedure, see the illustration and text below.
Above pic: not to scale. This two point protractor comes with the HFNRR test record. It is based upon the Baerwald calculations for minimum tracking error when using a pivoted tonearm.
Above image: Showing adjustments to be made. Can you see the slight mis-alignment to the zenith adjustment...? Better fix that. 8-)
Making the Adjustments
To adjust overhang, we must adjust the mounting length between the stylus and tonearm pivot. Typically, the headshell will offer --fore and aft-- adjustment by way of mounting screw slots, as in a Rega tonearm, or perhaps a 'sled-in-a-slide-way', as used in the Thorens TP60 headshell. To adjust mounting length, loosen the fasteners 'just enough' to allow the cartridge to be slid in either direction within the headshell until the stylus is centered over the null point.
(Note: To adjust an SME tonearm for overhang, refer to the SME owners manual. The method is different.)
Zenith angle adjustment:
To adjust Zenith angle, the cartridge body is rotated within the headshell until a parallel alignment exists between the cantilever and the grid lines on the protractor. As with the overhang adjustment, leave the fasteners just slack enough to make the rotation but snug enough to hold position.
If the protractor is a mirrored surface, it will be possible to visually evaluate the parallel alignment between the cantilever and the grid. However, if the protractor is paper, you may have to settle for aligning the cartridge body sides parallel to the grid since it will not be possible to get a visual fix between the grid and the cantilever. Use good lighting.
Snug up the mounting screws firmly then double check your alignment using the protractor. That is our overhang adjustment using a two point protractor. Piece of cake.
1) The object of the two point protractor is to achieve best possible alignment described above at 'both' null points. This will involve some averaging of the zenith angle between the two points, but the stylus should find exact center over both null points.
2) If it becomes apparent that the stylus will not align over both null points by the above method, I can think of two possible causes. The first cause might be inadequate room in the slots. In effect the cartridge can't be moved forward enough to reach the null point. Typically this happens when using a Baerwald based protractor. Baerwald alignments result in longer overhang values. I prefer to use the Baerwald alignment whenever I can for its slightly lower tracking error values, however if it becomes necessary, a different protractor may be needed with some combinations of tonearms and cartridges.
The second cause might be an incorrect pivot-to-spindle mounting distance. Typically this can happen when pivot-to-spindle distances are made adjustable at the arm board, or if a different tonearm has been incorrectly installed to the turntable. I recommend that this very important mounting distance, pivot-to-spindle, is checked and compared to the tonearm manufactures specification. A special gage is required. For one example of a gage that measures 'pivot to spindle', link here. Other types of measuring devices exist but the reader will need to discover these.
3) This page is designed to be a simple operational guide to the use of a 2-point protractor. It is not the purpose of this page to study or attempt to explain the difference between different alignment theories. If the reader would like to wade deep into tonearm alignment theory, this link is to a page that contains the formal papers by Baerwald, Bauer, Lofgren and Stevenson.
For more tonearm alignment tutorials I like the FAQ page at the Audio Asylum. Link here.
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