I Modified This Vibroplex Bug For Slower CW Speeds

Here is the key I got used from Ebay. It’s in perfect condition.

Years ago, I only used a straight key for CW. But this time, I decided to treat myself to a “Bug” – a semiautomatic key. This kind of key uses an oscillating pendulum to generate clean, well formed “dots” but the longer “dashes” are still fully manual. The Bug was originally designed for expert Morse operators, to reduce the Repetitive Motion injuries from operating a key all day, and to make it easier to send at speeds over 20 words per minute. However, there is a real need for the key to work reliably at much slower speeds too. The original key included an optional “swing weight” which you can see as an arm connected to the pendulum, which can be moved to adjust the speed up and down, but using the swing weight alone the key speed is 20-50 WPM, still far too fast. It’s not too difficult to see that, being a simple pendulum, adding even more weight should slow it down. The question is, how to add that weight?

I started by cutting a short piece from a copper bar, and drilling a hole to fit it onto the pendulum shaft. This allowed speeds down to about 15 WPM. However I wanted to go really low. So I cut a short piece of 0.195 OD brass tubing and inserted it over the original pendulum shaft. It’s held in place by light friction only, with a piece of wire insulation chosen to provide the right fit. A piece of 10-32 brass threaded shaft was pressed into the tube making a permanent piece. Brass nuts then can be added or subtracted. In addition, the original copper weight was given 10-32 tapped threads allowing it to be added as well. This brought the slow speed down to about 7 or 8 WPM. The entire assembly is now capable of speeds from about 7 to over 45 WPM depending on what weight is present and where the swing weight is positioned. Its 4 configurations are: original with swing weight (20-50 WPM), with extension (12-22 WPM), with big weight on pendulum shaft (15-25 wpm), or with extension and weight both (7-16 WPM).

The key is a delight to use! I’ve been warned by some other hams that adding this much weight to the pendulum might stress the bearing assembly, but I’m not too worried about that. The force needed to hold the pendulum horizontal is indeed higher, and that means the friction is higher, but I think a tiny spot of heavy lithium grease might insure that wear on the bearing is tolerable.