Originally Posted by
Barry
The Jelco 750D arm has an effective length (stylus to pivot) of 229mm, with an overhang of 15mm. Thus the TT spigot to arm pivot distance is 214mm (229 - 15 = 214mm). The installation protractor is marked with the effective length of the arm you are wanting to install, hence in your case '229'.
The null radii (on the alignement protractor) will be 59.1mm and 112.5mm.
Where you drill the hole for the pickup arm won't matter, as long as the distance from the TT spigot is 214mm. Choose anywhere on the arc that doesn't cause the counterweight end of the arm to foul the lid of the plinth, but is not too far in a clockwise sense that makes the pickup difficult to cue.
Technically speaking that protractor is bollocks. It's not for setting the pivot distance but to set the overhang if the headshell has slotted holes to mount the cart. But how can you accurately set it when the tone arm base is fixed to the plinth.
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