Variable Caps & Mounting
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.
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 to 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.
The graphic at right denotes various dimensions of the items in the picture. The graphic is flipped upside down. A full scale 1-1 graphic pdf file can be downloaded from the articles-index page. The dual-gang, at far right, 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.