[ARC5] Parts needed for ARC-5/274-N - splines...

[Mike Hanz]

On 5/26/2014 1:55 PM, hwhall at compuserve.com wrote:
> Tom Kneitel's Surplus Conversion Handbook, 1970 edition, had a short note on an ARC-5 RX tuning adapter that seemed the best I've seen. It provides mechanical support for the coupling joint, which seems to be key to getting a good feeling control and one that places minimal strain on the coupling material.
>
> It requires making a short (3/4 inch) cylindrical fitting of brass or aluminum (could probably do it in modern hard plastics or composites, too), drilled & tapped 1/2 - 28 at one end to screw onto the RX tuning ring, with a 1/4 inch hole continuing through the other end. Making that is the hard part.

Trudat.  :-)   Tom must have misread his thread gauge as well, since the thread is actually 1/2-27.  1/2-28 makes an interference fit, and with fresh aluminum to aluminum contact may weld the two parts together through galling action.  I'm not sure how the powers that be, or were, came up with the odd thread count per inch, as I discovered for the flex cable article on my website that the 27 T.P.I. thread came from a 19th century standard for gas light fittings! It persisted in electronic hardware in a number of places, including coax microphone connectors with the center soldered button.  You can buy a tap with that thread, but it counts as a "special thread" that is more expensive.


> A 1/4 inch dia short shaft extension goes in with enough length to touch the spline shaft when installed, and a piece of plastic tubing makes the coupling from shaft to spline (could use brass or copper, too, I suppose). Then just attach your favorite knob to the 1/4 shaft.

In a way, it is simply a home brew variation of the retention nut part 
of the command receiver local control at the bottom of 
http://aafradio.org/garajmahal/flex_drive_cables.html

It is certainly a more sound way to make the connection than having the 
shaft cantilevered out from the tuning capacitor.

     73,
  - Mike