New And Improved Precision Capacitor

Figure 1. Panel view of the new Type 1422 Precision Capacitor. Note the large, transparent flange on the control knob, which greatly facilitates precise setting.

There are many readers of the Experimenter who will have no difficulty in recalling, in a nostalgic moment, the consistent proud contention of the makers of the Dodge automobile in the 1920's that they made "no yearly models, only continuous improvements." Such a description might with equal truth be applied to the history of precision worm-driven capacitors at General Radio. Until the appearance of the new Type 1422 Capacitor in our current catalog, there had been only two distinctively different catalogued designs in forty years of continuous manufacture, the Types 222 and 722. Yet, these two designs throughout their useful lives were being constantly improved as new methods or materials became available, or when field reports indicated the desirability of modifications. Eventually, of course, the improvements that seemed desirable got beyond the capabilities of the existing design, and then a fresh start had to be made. A review of precision-capacitor manufacture and design (including some specialized designs never catalogued) will help to make more significant the improved features that have been incorporated into the new Type 1422.

Type 222 First Design

When the writer arrived at General Radio in the summer of 1921, the Type 222 Capacitor, then known as a condenser, was already a well-established product of the six-year-old firm (Figure 2).

It was massively and heavily constructed, using cast brass end plates spaced apart by three hexagonal brass posts. Bearings were conical, steel shaft against bronze insert. Only the rotor and stator plates were aluminum. The worm and wheel were stock articles of commerce made available to all and sundry through the commercial foresight of the Boston Gear Works.

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To illustrate the care exerted to reduce backlash, from the very beginning the worm, taper-pinned to its shaft, was spring-pressed into engagement with the worm wheel to control backlash as much as could then be done. The worm shaft was journaled in a long hinged brass casting, as can be clearly seen in Figure 2. After assembly and before calibration, each capacitor was subjected to a running-in period to make the worm and wheel fit more closely to one another. At the same time a similar effect was produced on the conical bearings at the two ends of the main shaft.

The insulation at first was laminated phenolic, at that time the novel material called Bakelite. Even though its losses were appreciably higher than those of hard rubber (the popular good insulation of the day), it was preferred because it was much more stable mechanically.

I mprovements

Almost immediately began a long series of changes and improvements.

Stator insulation progressed from phenolic to porcelain and then to steatite. The floating, spring-pressed, worm and wheel arrangement went through at least four successive designs, each an advance over its predecessors.

Direct-reading scales first appeared 011 the Type 222-M, a capacitor designed for parallel substitution measurements, direct reading in "capacitance removed."

Throughout this process of improvement the deficiencies in the 222 design were being recognized, and, when in 1936 the Type 722 was introduced,1 a number of basic improvements were made. The composite structure of end

'"A New Precision Condenser," General Radio Experimenter, 10. 8, January 193b.

Figure 2. Interior view of an early Type 222 Precision Capacitor, grandfather of all present types. Inset shows cabinet.

plates and spacer rods was replaced by an aluminum frame cast in one piece, with a consequent increase in rigidity and mechanical stability. The conical bearings 011 the main shaft gave way to ball bearings, and the worm drive was again improved, still further reducing the backlash. Finally, all metal parts directly affecting the capacitance were made of aluminum or aluminum alloy to give a low and uniform temperature coefficient of capacitance. Additional capacitance-removed (double-section), high-frequency, and low-capacitance models were listed, and direct-reading scales were used throughout. Linearity over 21, rather than 20, worm turns was achieved, providing small overlap beyond nominal decimal maximum AO's.

Three-Terminal Types

Meanwhile, by the early 1950's, a number of developments in both military and commercial measuring techniques, and demands for high measurement accuracy made it apparent that three-terminal (or insulated-rotor) capacitors should be made available on a catalog basis. Although these were available at first only in high-capacitance models (50-to-1100 pf), the need for

High Capacitance Terminal Capacitors
Figure 3. Panel and interior views of the Type 722

standardization at lower capacitance magnitudes, where three-terminal const met ion is mandatory, dictated the design of units with capacitance (AC) as low as 1.1 pf.

Special Types of Precision Capacitors

Although it was planned to keep in stock versions of the Type 722 for which there was a solid continuing demand, some 150 special variations have been made in the 25 years of its life, a few of them eventually becoming stock items (e.g., Type 722-CB).2 As an example of special designs, several Type 722-CE's have been made having two linear capacitance ranges, respectively 0.05 to 1.1 and 0.005 to 0.11 pf.

However, requirements have arisen with which the Type 722 structure was unable to cope.

Figure 4 illustrates a Type 022 Capacitor having many special features. The rugged 5-sided casting was made to secure rigidity because of the very high frequency-stability requirements for the oscillator which this capacitor tuned». It was used in an instrument for the

-Ivan G. Easton, "A Three-Terminal Precision Condenser," General Radio Experimenter, 23, 4, October 1958.

D Precision Capacitor, successor to the Type 222.

Navy which at sea would be subject to violent tilting as the ship pitched and rolled in bad weather. The rotor plates yielded an end-corrected narrow-range straight-line frequency performance. The extra shaft near the assembler's thumb in Figure 4 actuates a simple computer which averages two readings without calculation.

Figure 5 illustrates small precision worm-driven capacitors, Type 779, which fulfill a requirement for a capacitor of only 210 pf AC to occupy appreciably smaller volume than does the Type 722 and are used in an aircraft fuel-gage calibrator.

Type 1422 Introduced

While the Type 722 was far from obsolete, its design was 25 years old. Most of the improvements that could be made within the framework of the original design had been made. If further progress was to be realized, it would require starting afresh, but retaining the many demonstrated sound features of both the Type 722 and the Type 222, and borrowing from other of ^ the mentioned designs. This has been done in the new Type 1422. Seven dif ferent models are listed, the analogues of the Type 722's previously offered. Figures 1 and (» illustrate external views respectively of typical two-terminal and three-terminal models, while Figures 7 through 1 I are internal views. A number of new features and improvements will be enumerated and described in connection with the illustrations:

1. The cases have been changed to aluminum instead of wood. This gives 2 4 inch more stack room on the shaft with the same case length and a narrower box than the Type 722. The sides of the case are made from a heavy aluminum extrusion embodying decorative flutings (Figures 1 and (>)-

2. For many years users have requested a slow-motion drive for greater ease in fine setting of the worm dial, not that accuracy of reading may be improved but that bridge balancing may be facilitated. Providing the speed reduction is very difficult to do with the spring-pressed worm arrangement. However, the large transparent skirt for the spinner knob (shown in Figures 1 and <i) gives about a 3-to-l increase in diameter versus the fluted portion of the knob. There is even greater improvement in setting ease since one can rest a thumb against both the panel and the edge of the skirt and roll the thumb for fine control.

Figure 5. View of a small precision capacitor, designed for use in fuel gauge calibrator.
Figure 4. Final adjustments on a Type 622 "bathtub" capacitor, a special model designed for mechanical stability under extreme conditions.

3. Figure <i illustrates the use of the new locking Type 874 Connectors 011 three-terminal capacitors. This feature makes it possible to lock connecting cables in place or to fit the capacitor semipermanently and inexpensively with other types of common, coaxial connectors. The customer need merely purchase and attach two locking adaptors to have a capacitor with rigidly fastened coaxial connectors of the type desired.

4. Although there were no obvious places of entry, dust in the cabinet has occasionally been a problem in the past. To control this, 1he hole in the cabinet for the worm shaft has a minimal clearance, the transparent skirt catches the dust when the panel is in the Usual horizontal operating position, and the window through which the scales are read has been changed to gasket ed triplex safety glass.

5. To improve the rigidity and stability with time of the small section of a

Figure 7. Interior view of the CB-model. Note the shielding washers at the extreme left, which prevent the direct electrostatic flux from traversing the solid insulation.

Figure 6. Panel view of the new Type 1422-CB Precision Capacitor, an insulated rotor, 3-terminal model, with a capacitance range of 50 to 110 pf.

two-section capacitor, an additional support for each stator rod is provided by the crescent-shaped aluminum piece, shown in Figure 8. and mounted from an added bridge which is part of the main easting.

<>. Once the small section is 110 longer cantilever supported, its weight may be safely increased. Thus, more plates more widely spaced are used for the small section common to the Types 1 422-D and -Ml).

7. The bridge which supports the small section also serves to make the main casting more rigid. This rigidity is further improved by adding the ribbing on the bottom and filleting the inside corners of the casting.

8. Stator rods have been increased from to 1 4 inch in diameter to stiffen the structure and to reduce the difference in capacitance between horizontal and vertical panel positions.

Í). The adding of the bridge gives the opportunity for a third foot on the bottom, so that the capacitor, when out of its cabinet during manufacture, may be set stably on a flat surface without abrasion of the drum dial.

10. Improvement in the ball-bearing arrangements was desirable to improve

Figure 7. Interior view of the CB-model. Note the shielding washers at the extreme left, which prevent the direct electrostatic flux from traversing the solid insulation.

the backlash situation and stability with time. When the ball-bearing manufacturers recently made available oppositely disposed pairs of ball bearings with any desired preload, the answer to both problems was at hand. The main shaft has a pair of these at each end. At the drum-dial end the outer races are firmly clamped to the main casting and the inner races to a shoulder oil the shaft. At the rear end (visible in Figures 8 and 9) the inner races of the two bearings are clamped tightly to a shaft shoulder by the J4-28 Philips head screw to be seen there. The outer races float in the close-fitting, precision-bored hole through the boss :it the end of the main casting. This arrangement prevents the "climbing" which occurs even in a ball bearing, smaller than that which occurs with a cone bearing, but still observable. Backlash now, if it is detectable, is generally no larger than the width of the engraved line on the dial.

11. It is just as important to give the worm shaft similar preferred treatment to reduce backlash and eliminate non-uniformities of motion that would disturb the linearity of capacitance change. Note that any axial shake in the worm shaft can appear as backlash. To reduce this the new design incorporates a smaller preloaded ball-bearing pair, the outer races of which are tightly clamped within the round housing to be seen in Figures 7 and 10. The inner races are tightly clamped by a nut to a shoulder on the worm shaft. The round housing is supported with sidewise flexibility, but longitudinal integrity by a thin V-shaped beryllium copper member fastened to the housing by four screws and clamped to the main casting. This structure locates the shaft axially. The V-shaped beryllium copper piece provides free parallelogram-type floating motion for the worm shaft. The spring pressure to produce this motion is exerted by a spring-loaded bearing ball contained within the hex-headed housing going through the solid bracket just to the right of the ball-bearing mount. The self-aligning bearing at the dial end of the worm shaft fits nicely over a shaft shoulder and floats without shake wit bin the precision-bored hole in the main casting. The shake inherent in the bearing is taken up by a partially compressed six-fingered Z-washer often used with ball bearings, which bears only against the outer race.

Figure 8. Interior view of the Type 1422-ME, a dual-range unit, calibrated in capacitance removed, 0 to 10.5 and 0 to 105 pf.

12. The nature and disposition of the insulators are greatly different from those of the Type 722. The insulators, instead of being long bars, are short buttons or washers having one face flat and the other spherical in contour. They mate with spherically counterbored holes in the casting or in an aluminum bar. < hie obvious advantage is that these insulators are self-aligning. Another is that they offer a minimum of disturbance to the "all-aluminum" nature of the structure. In the Type 722 the stator plates would, with an increase in capacitor temperature, push the stator rods farther apart than would the steatite insulators (see Figure 3), thus putting a bow in the rods. Thermal behavior with flu1 present design is much more predictable.

13. Since the insulators are made of Rexolite 1422 (the identity of the numbers is pure coincidence), many advantages accrue. Rexolite 1122 is a cross-linked. thermo-setting, modified polystyrene which acquires heat stability as a result of chemical modification without the serious impairment of dielectric

Figure 9. Interior of the Type 1422-N, a high-frequency model whose rotor connection is made through brushes bearing on a disk at the center of the stack in order to reduce residual inductance.

Note the low-inductance lead to the terminal.

Figure 9. Interior of the Type 1422-N, a high-frequency model whose rotor connection is made through brushes bearing on a disk at the center of the stack in order to reduce residual inductance.

Note the low-inductance lead to the terminal.

Inductance Substitution Box

Figure 11. Interior view of the 3-terminal Type 1422-CD, another window type with two ranges, 0.5 to 11 and 0.05 to 1.1 pf.

Figure 10. Interior of the 3-terminal Type 1422-CC Precision Capacitor, in which a window in the rotor plate is rotated between two oppositely poled stator plates to give a range of 5 to 110 pf.

properties usually accompanying such tampering. The dielectric loss of Rexo-lite 1422 is essentially comparable to that of fused quartz and along toward one order of magnitude better than that of steatite. Further, its surface is comparably hydrophobic to that of silicone-coated fused quartz. That is, it is moisture resistant and maintains high leakage resistance without the hazard of the possible disappearance of the silicone. Therefore, there is no need any longer to provide quartz-insulated capacitors for eit her ac or dc uses.

14. As usual, an advantage has its cost. A small price has been paid for the use of Rexolite 1422 insulating buttons. The zero capacitance of each capacitor section is somewhat larger in the Type 1422 than in the Type 722. This difference causes no trouble except in the case of the small section of the Type 1422-D, which is now linear down to only 35 pf versus the 25 pf of the Type 722.

15. Adjustment of the serrated plates for linearizing the capacitance-rotation curve is accomplished by advancing or retracting hex-head screws against the serrations of a spring-tempered phos

Figure 11. Interior view of the 3-terminal Type 1422-CD, another window type with two ranges, 0.5 to 11 and 0.05 to 1.1 pf.

phor-bronze plate. This method yields many advantages over the prior use of hand-bending serrated aluminum plates with pliers. Access to the adjusting screws is through large clearance holes in the ends of the casting.

10. Level-adjusting plates have been moved from the rear to the front crossbar of the main casting so that their adjusting screws may be made accessible through the front panel. This facilitates making that adjustment with the capacitor inside rather than outside its case.

17. In the design of the Type 1422-CB Capacitor, extra shielding has been provided (either cups or washers or both) to keep stray rotor-to-stator field from traversing any of the solid insulation, despite its excellence. Thus the dielectric losses in the Type 1422-CB, as well as in the -CC and -CD, between the two hot electrodes should be only those losses in air and at the air-plate interfaces.

18. An examination of the accuracy specifications for the Type 1422 Capacitors will reveal major differences in format, philosophy, and the specification numbers actually used. Better control of dimensions achieved through changes in mechanical design and improved tooling yield better linearity or initial adjustment figures in some cases. Better Bureau of Standards certifications and more certainty of the precision of our own measurements enable us to give better accuracy figures for calibrations given on the panel charts and on the special 100-point calibrations, which are more significant and useful successors to what used to be called worm-correction calibrations.

Thus the Type 1422 Precision Vari able Capacitors are ready to give the user improved service over the fine record of the predecessor Type 722's. They represent significant improvements in a number of important areas. There is no intention to suggest that they represent the ultimate in precision capacitors, and work is still under way to fhe end that this latest design of precision capacitor shall be the beneficiary of the "continuous improvements" that have made its predecessors the standard of the industry.

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