I have been following this thread with interest. The one thing Dual manuals don't disclose is the overhang figure. The 'Ebay protractor' that was shown on the previous page seems to have it though. It says it's for a variety of Dual models, that supposedly have the same arm length. I looked at a Test Report from an old German hifi magazine (check the link here
— sorry, it's in German) for a Dual 701, that has the same effective length and offset angle as the 510. The overhang figure of 18.9 mm, and spindle to pivot distance of 203.1 mm are mentioned there too. Can we assume they are correct? Who knows... in that same test report it is said that the null points are at 61.5 and 124 mm though. That doesn't correspond with the null points of the Ebay protractor. Interesting...
So let's assume the overhang figure is right. We now have these parameters:
Effective length: 222 mm
Overhang: 18.9 mm
Spindle to pivot: 203.1 mm
Offset angle: 25° 20'
Is it possible to make a custom arc protractor using Conrad Hoffman's program? Let's see...
Regular alignment methods and groove radius sizes all give a different outcome. The only way to make it so that it corresponds with the specs given by Dual is to use Baerwald / Lofgren A alignment, and a set of custom groove radius sizes.
Being the obsessed fool that I am, I worked the numbers and came up with these parameters, to be used in Conrad Hoffman's program:
Spindle to Pivot Distance: 203.1 mm
Inner Groove Radius: 57.643 mm
Outer Groove Radius: 158.885 mm
Alignment method: Lofgren A
The result looks like this:
So this is the same protractor as the 'Ebay protractor'. The position of the null points is a little bit different, but we're talking less than a hundredth of a millimeter here. Even more fiddling with the groove radius numbers might correct that. The numbers are weird anyway, especially the Outer Groove Radius which is way out of the actual disc size. What's up with that?
I guess the only way to figure out if it is really
correct, is to very accurately measure the spindle to pivot distance yourself. Not an easy task...