Ok, now I’m getting the picture! Let’s start with this:
Well, that's easy: Did you notice the 30 and the 17 in the arrows in the first picture of the Dual document? Those would mean ca. 30 and 17 cm - so one screw adjusts the touchdown point for LP-sized records, the other the touchdown point for single-sized records. If you use the autostart function, you can choose between the two touchdown points with the little selection lever at the tonearm base in front of the lift lever - so the 429 and 529 allow for a speed-selection-independent selection of the record size.Alright, imagine the screws to turn somewhat eggish shaped discs, the eccentrics.
So, I did notice the numbering of the screws, but I did not make the association to disc size; I thought they were just part numbers referring to some larger diagramme, i.e., screw ## 17 and 30.
This is maybe a generational thing. When I was younger, we spoke of records as being 33s or 45s or 78s. No one (in my circle, anyway) ever talked about sizes, they were just implicit: 33s are larger, 45s are smaller. In fact, I was speaking with a young member of a band a few years bag who told me they had put out a 6-inch (or something like that). I had to ask, what’s a 6-inch? And concluded that discussion with, Oh, you mean, a 45….
In fact, when I first set this machine up, I checked that the speed to 33 and was surprised that the tonearm moved to ’the middle’ of the disc and was worried there was something wrong, until I realized there was another dial (which we didn’t have on the CS 308 we were replacing) and when I set it to 30, it was correct (or nearly so, as I found out, giving rise to this thread).
So that means that I have inadvertently moved the start position for the 45s as well, and should potentially take corrective action before playing any of those. In which case, the parallel screwhead slots was a red herring!
A good example would be the antiskating mechanisms of various classic Dual turbtables - like for example the 510, as shown in figure 13 of its service manual - see for example over there:
https://www.hifi-archiv.info/Dual Anleitungen & Kataloge/Dual Plattenspieler & Wechsler CS/Dual CS 510 Service/Dual_510_S08.jpg Works like this: Spring 208 pushes lever 207 against the rim of eccentric disc 65, the position of which is controlled by the antiskating adjustment wheel above on the upper side of the table, not really shown in figure 13. If one turns that wheel, the eccentric disc turn with it, thereby varying the position of both arms of the lever, to the end of the other arm of which one end of the antiskating spring is fixed - while the other end of that spring is fixed to segment 211, which turns with the tonearm. Antiskating wheel in position 0 moves the segment side arm-end of the lever into the centre of rotation of that segment, at which position there's hardly any tension on the antiskating spring, so that it doesn't really pull at the tonearm. But if the wheel is now turned to one side or the other (in case of most Duals each side has a different scale - e.g. one for spherical and the other for elliptical needles), the segment-side arm end moves out of the centre of rotation of the segment and increases the antiskating spring tension - up to its maximum, from which the tension would decrease again (now being on the other side scale of the wheel), reaching the 0 position again after a full 360° turn.
This part I understand in principle, now, though I should study the diagramme you have kindly provided to get the full sense. In the meantime, it suggests that given a 50/50 chance, I happened upon the correct turn of the screw, so to speak
Thanks for taking the time to provide all this illustrative detail. Much appreciated!
