The Extendadot evolved over many years of experimenting with different devices to control the dot speed on telegraph speed keys. I originally made them of brass but the brass reduced the top speed of the keys to less than 30wpm. So I switched to light weight aluminum for the higher speeds. And I found that the aluminum has the perfect amount of give to it in order to enhance the quality of the dots produced. The light aluminum allows the key to be adjusted for a lighter touch which reduces the keys tendency to produce dot stutter. Dot stutter is the tendency of some keys to add an unwanted dot now and then. Some keys suffer from the effect more than others. Dot stutter occurs when the dampening of the dot arm isnt as effective as it should be. Where the dot arm flops around on the damper and adds a dot(s) or half dot to your characters. Dot stutter shouldnt be confused with fuzzy dots. Fuzzy sounding dots can occur if the dot contact spring goes into self oscillation in between dots. Ted McElroy placed an adjustable metal arm around the dot contact spring on some of his early Deluxe Mac keys. I've found the metal arm dot stabilizer used by McElroy causes the dot contact spring to become to rigid. What I've found that works better is winding about 10 turns of dental floss around the dot contact spring to stop the self oscillation in between dots. McElroy stopped using his idea for the metal arm dot stabilizer on his later keys. Ironically, the worst case of dot contact spring self oscillation I've ever found was on a McElroy P500. But winding 10 turns of dental floss around the dot contact spring -- then tying it off -- fixed the problem completely.
I made this double slide Extendadot trombone speed control (pictured above) back in 2007. It has an aluminum body with an added brass sleeve to in effect double the length of the device. The brass sleeve can be easily re-tensioned for a good slip fit. Its a one of a kind experiment which is still in use today.
Pictured above is another experimental speed control designed to fit either round of flat dot arms. You can see the slot for the flat dot arm in the aluminum body. It works well but an inline trombone speed control provides a better feel for the key.
Pictured above are 5 Canadian speed keys that I fine tuned for use in the 2012 SKN ARRL straight key night. From left to right is Xograph, Merrick, Dow,
Tronto, Wilcox. The key with the lightest touch is the Xograph but the one that turned out to be used the most often that night is the Toronto due it produces very snappy dots and is a well balanced small key that is easy to send with.
None of these keys had been used for a considerable amount of time and each needed to be cleaned and fine tuned the day of the event. Which took over 4 hours of hard work.. especially with the Xogrpah -- as it had to be disassembled and the corrosion cleaned off all the fittings. All are equipped with the Extendadot trombone speed control except for the Toronto.. the flat dot arm didnt provide enough clearance to add a speed control. So I used it at one fixed speed that evening. Fine tuning and fixing all the problems on these 5 keys was very time consuming leading up to the SKC event that night.. As the keys hadnt been used for many years ... the biggest problem is poor conductivity between the dot & dash contact to the keys wire terminals. Where the moving dot arm dosnt provide a low resistance ground return. The best fix in providing a proper ground return on the dot arm is to place flexible wire braid from the dot arm to ground as well as from the dash arm to ground.