Variable Capacitors & Reduction Drive:

Using, Mounting & Wiring.

(updated 12/05/13) This article is made available to assist you in using/installing the air variable capacitors offered in our parts catalog. At this time, three caps are offered: a 365 pf dual-gang, a 365 pf single-gang with 8:1 reduction drive built in the shaft, and a 365 pf single-gang. capacitor lineup

The 365 with 8:1 reduction drive is shown second from left. Physically is appears exactly the same as a 365 without the drive, except for a slight slot in the shaft (barely visible in picture). The reduction drive reduces the rate of rotation of the rotor plates to 1/8th that of the knob shaft, enablng finer tuning.

The two caps on the right are 365s without shaft reduction. A half-rotation of the knob fully turns the capacitor plates. While often installed snug against a front panel - like the second cap - these two are set back 1-inch to accommodate shaft extensions. The white shaft is a combination of two nylon insulators, XS-NS1 & XS-NS2, used to reduce "hand capacitance" if desired. XS-NS-2 is 1 inch long with OD = 0.5 and ID = 0.257. XS-NS1 slips into XS-NS2 and is 1.5 inches long with OD = 0.25. To secure these together and on the shaft of the XS-365, you make two 4-40 tapped holes in the XS-NS2 and all is secured with 4-40 by 1/4 inch screws. The second 365 cap is shown with a 6:1 planetary reduction drive attached. While not shown, it is possible to build an assembly with the 365-8-1 and 6:1 drive combined, creating a whopping 48:1 reduction in cap plate to knob rotation.

Mounting the single-gang capacitors

singe-gang-top-view vertical bracket

The capacitance range of this cap is approximately 15 to 390 pf, as measured with a BK Precision 810C capacitance meter. The cap can be mounted to a front panel, placed on a chassis, with a shaft extending through the panel or mounted above a chassis using our Vertical 365 Bracket. The front panel has two 6-32 tapped holes at top-left and right on its front plate. The bottom has four 6-32 tapped holes for chassis mounting. The frame is U-shaped, thus supplying the front and back of the cap housing. The rotor plates are grounded to the chassis via the shaft. The stator plates connect to two-solder lugs on each side via two small phenolic boards. 

Top, front, and bottom views of the cap are shown in the left three pictures above. Electrical connection to the chassis/rotor plates is accomplished by attaching a wire to a solder lug installed on the front panel or base of the cap frame. For front panel connection, a 6-32 hex nut is added with the lug so that the 6-32 by ¼ screw does not extend into the interior of the cap and thus block full rotation of the rotor plates. Alternatively, you can add the solder lug to the bottom of the cap, thus allowing for panel mounting from the front of the capacitor. Note that the front plate of the cap has a bit of a bulge around the shaft to hold it and the bearings in place. Because of this, you’ll want to drill a third hole in your front panel to accommodate for that.

The third picture above shows the bottom of the cap. Two screws, added diagonally, are sufficient to hold it in place. Of course, for chassis mounting, the solder lug would not be used, just two 6-32 by ¼ screws – from the bottom and thru the chassis – and two washers or a "spacer plate" (see parts list) on top of the chassis but under the cap, so that the screw does not extend too far into the interior of the capacitor body and thereby short the plates.

The last picture above at right displays the Vertical Mounting Bracket, used to mount the capacitor above a chassis and away from the front panel. Use the bottom holes to secure te bracket to the chassis/base and then use the two slits top left and right to accepts 6-32 by 1/4" screws to secure the cap. Adjust to desired height and use nylon parts to extend the shaft to the front panel.

The graphic at right denotes various dimensions of the items in the group picture at the top of the page. The graphic is flipped upside down. A full scale 1-1 graphic pdf file can be downloaded from the articles-index page (at the bottom of that page). The dual-gang, at far right here, accommodates (3) 6-32 screws for mounting, and the front is shown to be 0.1 inches back from the front panel. The panel opening for the shaft is 1.250 inches above its bottom.

The 365s are shown mounted on a gray 060 aluminum shim, listed with the cap in the catalog. The shim holes align with the (4) 6-32 tapped holes in the bottom of the caps. The shaft is 1.020 inches above the cap bottom, including the 060 shim. One can mount the caps on #6 solder lugs instead; these are generally 040 in thickness after tightened down.

Care must be taken in mounting any of these caps to a chassis, in order to not short out or bend the stator plates with the mounting screws. In addition a shim or solder lugs are necessary to protect the small phenolic board that extends down on each side. Without the shim or lugs, tightening the cap to a chassis with screws can crack the boards. The picture of right displays the clearance necessary. The facing phenolic board used to mount the stator has been removed. Note that a 6-32 by 1/4 screw just fits through the 1/8th inch brown hardboard, through the solder lug and the base of the cap. Pick the screw that matches with your board or chassis thickness.

Electrical connections to the stator of all of these capacitors must be via the frame, as noted in the picture at right. One can attach a solder lug as shown or attach it to the bottom side of the board. The rotor connections are always on the sides of the capacitor and are common to both sides. As such the 365s have 4 common (conductive) solder lugs, two on each side. The two capacitors in the dual-gang have a common rotor and are in common with the frame. Each cap has separate stator solder lugs, one on each side.
Mounting/Installing a 6-1 Reduction Drive.

The picture at right shows a 6-1 reduction drive attached to the shaft of a 365 air variable capacitor at left. The front panel has been removed so you can see the knob attached to the shaft of the reduction drive at right.

A good way to approach the installation of this combination is to slip the ¼-in opening of the reduction drive shaft onto the shaft of the capacitor, as noted in the picture, securing it lightly with the two screws provided in the shaft. Then place the combination on the chassis as noted in the picture so the shaft at the far right sticks out enough from the front panel to accommodate a knob. Then slip the L-bracket on the reduction drive at the front so the larger portion of the L-bracket is vertical and flush with the tabs of the drive sticking out horizontally on each side. Secure the bracket and drive together with two 4-40 by 3/8th screws and two hex nuts. Finally, using a pencil, mark the chassis through the two holes on the bottom of the L-bracket and remove the assembly to drill out the holes. Use a 5/32 or 11/64 drill bit to leave a little slack in the holes.   

Now you are ready to secure the combination and reattach the front panel. Use two 6-32 by ¼ inch screws to secure the capacitor, inserting them in a diagonal pattern from the bottom of the chassis, through the chassis, through the 0.080 thick shim, and screw into two of the four holes on the bottom of the capacitor. This takes a bit if jiggling to get started. Then secure the bottom of the L-bracket holding the reduction drive with 4-40 by 3/8 or ¼ screws and secure on the bottom of the chassis with hex nuts. Finally, level the tabs on the reduction drive and tighten the nut and screw combination located on each side.

If you compare the tension on the knob with a 365 capacitor with 8-1 internal reduction drive to that of a 365 with external drive, you will notice that the later will be a bit stiffer. So why use the external drive? If you examine it closely, you’ll note that the front of its shaft has a little flat surface with two small threaded holes for 2-56 size screws to accommodate a behind the front panel dial. We don’t currently have any dials designed but such could be interesting.