Using the TAD Arc Protractor
Using the TAD Arc Protractor. The Jelco SA370H shares its alignment geometry with Rega RB250 thru RB1000 tonearms. TAD arc protractors are drawn for the individual tonearm and are --not-- interchangeable for other tonearms....... unless the tonearm shares its mounting and alignment geometry with that of another. It happens. Just be sure that the arc protractor in use is for your make model tonearm.
Null points and overhang settings on this protractor are per the Baerwald calculations (also known as Lofgren A)
 | Effective Length: 239mm |
| Overhang: 17.3mm |
| Mounting distance = Eff. Length - Overhang |
| inner null: 66mm |
| outer null: 120.89mm |
| Set Vertical Tracking force to the recommended setting using an appropriate scale |
| First, place a wedge under the platter so that it may not rotate. Note also that a "disposable record" is used as a "work surface" beneath the protractor. Secure the protractor so that it will not rotate when you don't want it to. In this session I used 3 quarters to weigh down the protractor over the record surface. |
| Adjust the protractor and arm so that point zero and point 1 at the outer boundaries of arc travel match to the stylus without moving the protractor. |
| From this point forward the protractor remains stationary and it is the cartridge that is adjusted within its headshell so that it may precisely hit all the alignment points of the protractor. In fact the stylus should rest on top of the arc line at any point on it once correct adjustment is done. |
| At the inner and outer nulls adjust "zenith angle" of the cartridge by swiveling it one direction or the other so that the alignment grids of the protractor run parallel to the cantilever. |
hint: click on thumbnail image to view the image full size
point zero
point 1
point 2 (outer null) Cantilever is in line with grid marks
point 3 (inner null) Cantilever is in line with grid marks.
wide sequence shot of the 6 alignment stages
Once satisfied with the overhang and zenith adjustment, double check vertical tracking force.
