An Update of the Original Parkwood Pup QRP Straight Key
The first Parkwood Pup was a small straight that was designed for QRP operation, either in the shack or in the field. It used ball bearings for the lever pivots, and tension was supplied by opposed magnets. Connection to the rig was made by a monaural or stereo audio cable. The little key proved to be rugged and reliable, but not being one to let well enough alone, I recently decided to come up with a new version.
Actually, the Pup II was intended to be a straight key test bed for a new kind of pivot system that I am developing. For want of a simpler name, I am calling it the "rocker-plate ball pivot" mechanism. So far I have used it in several miniature semiautomatic "bug" keys, and it is part of the design for planned iambic and single paddles and for a miniaturized rotary bug. (Some earlier paddles used a variant of the mechanism.) So it seemed that a small straight key would be a good vehicle for further development of the idea. The size and shape of the original Pup had proved to be ideal for a QRP straight key, so this experiment became a successor to the Parkwood Pup.
The Pivot Mechanism
The idea behind the pivot mechanism is simple but effective. A compression spring holds the key lever against a flat bar, and the lever pivots on the edge of the bar. Varying the spring compression changes the force required to depress the lever. However, at this point there is nothing to hold the parts in register. For this purpose two polished brass balls are secured to the underside of the key lever, spaced as far apart as possible. These balls fit into two corresponding holes in the flat bar that are reamed out 0.001" larger than the ball diameter. This provides lateral and fore-and-aft stability as well giving the key a smooth and precise feel. The holes in the key lever that are used to attach the brass balls are drilled slightly oversized, and there is a turned flat on the mating surface of each ball. This allows the ball spacing to be adjusted slightly to eliminate binding - it is not hard to drill a hole to the 0.001 inch, but such precision in spacing is quite difficult to achieve. The resting (key up) position is fixed by the flat surfaces of the parts involved (which must be free of burrs and high spots), and the key-down position is determined by the contact spacing. Even without lubrication, the action is smooth, and with the large bearing surfaces and small movements involved, friction and wear should be negligible. See the drawing below for details of the mechanism.
The next illustration shows the parts of the key. The connections to this version are via binding posts rather than by a phone jack. The knob is also the means of adjustment of the contact spacing (and the key travel), and the "Navy" knob skirt is also used to lock the spacing adjustment. Resting position of the key (not adjustable) is set by the flat bar into which the lever balls insert. Tension adjustment is provided by the knurled nut above the spring, which changes how tightly the lever is pressed against the flat bar.
Some Details of the Construction – the Lever
Details of the key lever are shown in the next illustration. The pivot bar is set into the base to reduce the overall height of the key and to allow its attachment with the single screw that also carries the tension adjustment nut. At the upper left are the key lever with the pivot balls
attached, and the flat bar with the matching sockets; the upper right shows a bottom view of these parts assembled. A top view of the assembly is at the lower left, and the compression spring and adjustment nut are at the lower right .
Assembly of the Lever and the Base
The screw that holds the flat bar with the ball sockets also holds the compression spring adjustment nut. The sequence below shows these parts assembled.
All parts are shown at the upper left. At the upper right, the lower contact
parts have been installed and the knob is assembled. The upper portion of the knob bears a brass screw that is threaded through the knob skirt, and then through lever itself. This allows the contact spacing to be changed by turning the aluminum knob,while the knob skirt serves as a lock for the setting. The fully assembled lever (lower left) is ready to be installed, and the assembled key is shown at the lower right.
Two binding posts at the front of the key allow connection to the transmitter (upper left). A channel is milled into the base to allow connection without increasing the height of the key (upper right). An insulated connection runs from the lower contact point to the “hot” binding post. The ground terminal is connected directly to the key base (lower views).
An Alternative Tensioning System
The key described so far used a coil spring to provide the operating tension. To extend the design a little further, a magnetic tensioning system was devised. For simplicity, it was decided to make it compatible with the components of the spring system – that is, with the adjusting knob and the key lever. The magnets used were of the rare earth type; they were cylindrical, with axial holes. As the figure below shows, they were mounted, in place of the spring, on the upright threaded rod with facing polarities. The lower magnet pressed on the lever, while the upper one pressed against the adjusting knob. The key seemed a bit “crisper” with the magnetic system, but that was only a subjective impression. The range of tension adjustment was similar, but the distance of the magnetic interaction limited the travel of the lever somewhat, but a very comfortable contact spacing was easily set and maintained.
The key is sized for portable operation with typical QRP gear (left). At the right it is shown in an aluminum version, alongside its predecessor, the first Parkwood Pup. So how does it compare with the original?
In the aluminum version, the Pup II weighs 87 grams, compared to 102 grams for the original. The brass Pup II weighs in at 188 grams. Not surprisingly, the brass version has a little more “big key” feel than its aluminum counterpart, but the difference is not significant. The overall touch for the old and new versions is quite similar, with the Pup II having a little slower return stroke because of its different type of pivoting. However, either version can produce good CW at up to 15 words per minute with little trouble. The speed is limited by the ability of the operator (me), not by the key performance.
From the point of view of its construction, the Pup II has a lower parts count than its predecessor, but more precision is required in the machining of the pivot system. Its spring (or magnetic) tension adjustment is a bit quicker and more convenient to use than the magnetic system in the Pup I. Because it doesn’t have conventional ball bearings, the Pup II is less easily affected by dirt and moisture in the field.
From the point of view of the objectives of this project, the Pup II was a successful test bed for a straight-key version of the rocker-plate ball-pivot mechanism. It also gives me another little key to play radio with the next time I go camping.
Richard A. Meiss, WB9LPU
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