US3706232A

US3706232A - Preset tuner with improved tuning linkage

Info

Publication number: US3706232A
Application number: US159349A
Authority: US
Inventors: Alexander Robertson
Original assignee: Arris Technology Inc
Current assignee: Arris Technology Inc
Priority date: 1971-07-02
Filing date: 1971-07-02
Publication date: 1972-12-19
Anticipated expiration: 1989-12-19

Abstract

A preset tuning mechanism of the type having a channel selector drum including a plurality of adjustable tuning projections is provided with a mechanism adapted to operatively disengage the rotor shaft from the selected tuning projection during channel selection. This mechanism includes a ram which is spring biased into contact with the selected tuning projection and operatively connected to the rotor shaft for converting its axial sliding movement into rotary fine tuning adjustment. The ram is operatively connected to a slide bracket which is provided with a cam follower biased into engagement with a lobe type cam mounted on the channel selector shaft. As the channel selector shaft is rotated the slide bracket is moved downwardly into engagement with the ram thereby in turn to move the ram out of engagement with the selected tuning projection. The drive mechanism for the channel selector drum is also mounted on the channel selector shaft in a manner synchronizing drum rotation with ram actuation. Thus the rotation of the drum occurs only during a dwell stage at which the ram is moved completely clear of the tuning projections, thereby eliminating all relative lateral movement between the tuning projections and the ram. The second embodiment provides an improved indexing mechanism for the drum comprising a pair of molded toothed ratchet wheels spring biased into meshing engagement. One of the ratchet wheels is mounted on the drum for rotation therewith and the other is spring biased into meshing engagement with the drum wheel by spring means mounted on the chassis. The ratchet wheels may be conveniently converted from six to 12 teeth by a simple machining operation.

Description

United States Patent Robertson [54] PRESET TUNER WITH IMPROVED TUNING LINKAGE [72] Inventor: Alexander Robertson, Springfield,

Mass.

[731 Assignees General Instrument Corporation,

Newark, NJ.

[22] Filed: July 2, 1971 [21] Appl. No.: 159,349

[52] US. Cl. ..74/10.6, 74/10.45, 74/1054 [51] Int. Cl ..F16h 35/18 [58] Field of Search ..74/l0. 6, 10.8, 10.54, 10.52,

3,224,282 12/1965 Tyzack ..74/10.15

Primary Examiner-Milton Kaufman Attorney-Maxwell James et al.

[5 7] ABSTRACT A preset tuning mechanism of the type having a channel selector drum including a plurality of adjustable tuning projections is provided with a mechanism adapted to operatively disengage the rotor shaft from [45] 1 Dec. 1 9

the selected tuning projection during channel selection. This-mechanism includes a ram which is spring biased into contact with the selected tuning projection and operatively connected to the rotor shaft for converting its axial sliding movement into rotary fine tuning adjustment.

The ram is operatively connected to a slide bracket which is provided with a cam follower biased into engagement with a lobe type cam mounted on the channel selector shaft. As the channel selector shaft is rotated the slide bracket is moved downwardly into engagement with the ram thereby in turn to move the ram out of engagement with the selected tuning projection. The drive mechanism for the channel selector drum is also mounted on the channel selector shaft in a manner synchronizing drum rotation with ram actuation. Thus the rotation of the drum occurs only during a dwell stage at which the ram is moved completely clear of the tuning projections, thereby eliminating all relative lateral movement between the tuning projec tions and the ram.-

32 Claims, 16 Drawing Figures SHEET 1 UF 7 lm T PATENTED DEC 19 I972 INVENTOR 41614/1/062 ROBERTSOA/ ATTORNEYS PATENT'EUBEB 19 9 3,706,232

SHEET 2 [IF 7 INVENTOR max/W052 paeeersa/v BY -4fzm ATTORNEYS PATENTEI] 05c 19 m2 SHEEI 3 OF 7 INVENTOR ALEX/M/DER RUBERTSfl/L BY ATTO R N EYS PATENTEDnEc 19 I972 3 706. 232

saw u or 7 INVENTOR 4L EX/M/DER R0B 730/11 ATTORNEYS PATENTED nzc 19 I972 SHEET S [If 7 FIG/l INVENTOR AA X4/VOER R0567? 750/11 ATTORNEYS PRESET TUNER WITH IMPROVED TUNING LINKAGE This invention relates to tuning mechanisms and more particularly to UHF tuners of the type having preset tuning devices.

For many years home television receivers have generally included preset detented tuning mechanisms for selecting any one of the 12 channels comprising the very high frequency (VHF) range. With therecent advent of a significant number of channels in the ultra high frequency (UHF) range, home television receivers are today generally adapted to receive such UHF broadcasts. Because the UHF range comprises 70 channels such receivers are generally provided with continuous tuning means, typically of the gang capacitor type.

The many disadvantages of continuous tuning, both from the standpoint of the viewer and the manufacturer, have been recognized. Thus, the use of preset channel selection with a step-by-step detent action as used in the'VHF band has already been suggested for the UHF band. Since it is not practicable to detentall seventy channels, prior art devices of this type and the life of the tuning mechanism is considerably reduced.

A second problem associated with mechanisms of this type is that of accurate detenting. Thus, the rotary drum or turret carryingthe tuning projections must be indexed to a plurality of selected positions accurat ely corresponding to the operative adjustment position of ret or drum is provid edwith a plurality of adjustable I tuning projections spaced about its periphery; The operation length of each tuning projection is preselected by means of the fine tuning adjustment mechanism so that as the'drum is rotated through its several detented positions, the projections of different lengths are adapted to adjust the impedance of a variable impedance tuning element, thereby to tune the oscillator to a preselected frequency. This is generally accomplished by means of a linkage mechanism operatively connected between the adjustable tuning projections and the variable capacitance tuning element, which linkage is effective to transmit the axial movement of the selected projection into a rotary movement of a fine tuning rotor shaft adapted to adjust the imand the projection andthe direction of the tuning force changes slightly. During channel selection the arm must slide from one projection to the next. This process not only causes lateral stress on the tuning projections, it also produces substantial wear which leads to inaccuracy.

each fine tuning projection. Slight variations in this selected position may result in considerable inaccuracies in the fine tuning preset, necessitating frequent adjustment. ln the past .the indexing or detent mechanisms associated with channel selectors have proved somewhat ineffective, particularly with regard to maintaining the initially selected channel' positions within close tolerances over long periods of operation. Thus play often develops in the detent wheel impairing consistent preset tuning positions. Moreover, such play results in lateral movement of the tuning projection during axial adjustmentthereof, which again necessitates frequent fine tuning adjustments.

It is a primary object of the present invention to devise a tuning mechanism which substantially eliminates the aforementioned difficulties.

It is another object of the present invention to design a tuning mechanism of the preset type wherein fine tuning accuracy and reliability is considerably enhanced.

It is yet another object of the present invention to provide a preset tuner characterized by a new and improved fine tuning linkage mechanism adapted to eliminate substantially all wear andlateral stresses on the preset tuning projections.

It is afurther object of the present invention to design a preset tuning mechanism of the type described wherein the fine tuning linkage mechanism employs a straight line timed movement of the tuning projection contact member which is effective to eliminate substantially all relative lateral movement between the selected tuning projection and its associated contact member.

Yet another object of the present invention is to provide an improved detent mechanism for use in a preset tuner for accurately indexing a channel selector memory drum, which mechanism is extremely reliable and adapted to maintain accurate indexed positions consistently over the life of the tuning mechanism.

It is still a further object of the present invention to design a tuning mechanism of the type described having a simple and reliable construction utilizing components which may be inexpensively mass produced.

To these ends, .l have provided a preset tuning mechanism of the variable impedance type having a channel selector adapted to be preset throughout the entire UHF range. The channel selector mechanism includes a channel selector shaft coupled by suitable gearing to a secondary shaft which in turn drives a memory drum having a plurality of adjustable projections for fine tuning. The adjustable projections are preferably in the form of screws mounted along the periphery of the memory drum and extending axially therebelow. A slide mechanism includes a ram which is spring biased into contact with the selected tuning screw and is operatively connected to a fine tuning rotor shaft for converting its axial sliding movement into rotary fine tuning adjustment.

A lobe type cam is mounted on the channel selector shaft for rotation therewith and is adapted to actuate a cam follower fixed on the slide mechanism. As the channel selector shaft is rotated the movement of the cam follower is effective to slide the ram out of engagement with the tuning screw at the tuning station. Further rotation of the channel selector shaft results in a rotation of the memory drum, bringing the next consecutive tuning screw into tuning position, at which time the cam allows the cam follower to return to its original position andthe ram is reseated on the axis of the selected screw. The memory drum is rotated by means of a spoke driver which is designed for synchronous operation with the lobe cam such that rotation of the drum occurs only during a dwell stage at which the ram is moved completely clear of the tuning screws. As a result there is no relative lateral movement between the tuning screws and the ram, thereby eliminating a primary source of wear, misalignment and inaccuracy. The same lobe cam is adapted to detent the channel selector shaft, eliminating the necessity of a separate detent wheel, the spoke driver being adapted to accurately index the memory drum.

In a second embodiment an improved indexing mechanism is provided for indexing the memory drum. This indexing mechanism comprises a-pair of molded toothed ratchet wheels spring biased into meshing en- 'gagement. One of the ratchet wheels is mounted on the drum for movement therewith and the other ratchet wheel is held fixed against axial rotation by a laterally extending detent spring. The ratchet wheels may be conveniently converted from six to 12 teeth by a simple machining operation.

To the accomplishment of the above and to such other objects as may hereinafterappear, the present invention relates to a tuning mechanism as defined in the appended claims and as described in this specification taken together with the accompanying drawings in which:

FIG. 1 is a top plan view, partly in section of a tuner embodying the features of this invention;

FIG. 2 is a cross-sectional view taken along the line 2-2 of FIG. 1;

FIG. 3 is a side elevation taken along the line 3-3 of FIG. 2 showing the gearing between the channel selector shaft and the rotor shaft in broken lines;

FIG. 4 is a cross-sectional view taken along the line 4-4 of FIG. 2;

FIG. 5 is a bottom view of the tuner of FIG. 1 with part of the chassis broken away;

FIG. 6 is a cross-sectional view taken along the line 66 of FIG. 1 showing the operative position of the parts during fine tuning;

FIG. 7 is a cross-sectional view similar to FIG. 6 showing the mechanism during channel selection;

FIG. 8 is a side elevational view of a second embodiment of a tuner constructed in accordance with the present invention;

FIG. 9 is a top plan view of the tuner of FIG. 8;

FIG. 10 is a cross-sectional view showing the gearing mechanisms and the improved detent means of the tuner of FIG. 8;

FIG. 11 is a bottom plan view of the tuner of FIG. 10;

FIG. 12 is an enlarged view of the tooth profile of the six (solid lines) and twelve (broken lines) tooth ratchet wheels of the improved detent mechanism;

FIG. 13 is a cross-sectional view taken along the line 13-13 ofFIG. 10;

FIG. 14 is a cross-sectional view taken along the line l4l4 of FIG. 10;

FIG. 15 is an enlarged perspective view of the slotted ram of the tuner of FIG. 10;

FIG. 16 is an enlarged perspective view of the ram driving collar of the tuner of FIG. 10.

Referring to the drawings, the illustrated tuner cornprises a fixed chassis comprising a linkage housing generally designated 10 enclosing a mechanical tuning drive mechanism generally designated 12 and a circuit housing 14 within which are mounted the various electrical components comprising a variable impedance tuning circuit. The electrical parts within the housing 14 need not be described in any detail as they may be conventional and form no part of the present invention. As best shown in FIG. 2, housing 10 is further provided with a pair of vertically extending

end walls

16 and 18, a vertically extending intermediate wall 20, and a generally horizontal wall 22 extending transversely between end wall 16 and intermediate wall 20. The housing 10 is thus divided into three compartments generally designated A, B and C.

A rotor shaft 24 is journaled in end wall 16 and extends outwardly therefrom into circuit housing 14.

Rotor shaft 24 is operatively connected to the variable impedance tuning elements (such as capacitors or inductors) within housing 14 and provides rotatable adjustment thereof thereby to vary the frequency to which the circuit is tuned in known manner.

The mechanical tuning adjustment or drive mechanism l2provides forboth preset channel selection and adjustable fine tuning for a given channel. To this end a channel selector shaft 26 is journaled in end wall 18 and extends outwardly therefrom to terminate in a flattened end 26a to which a channel selector knob 27 (shown in broken lines in FIG. 1) may beconveniently secured. Within housing 10 shaft 26 is designed to provide for step by step preset channel selection. For this purpose there is provided within housing 10 a turret-like body or memory drum 28 having a plurality of adjustable depending projections 30. Drum 28 is rotatably mounted on horizontal wall 22 by means of a mounting stud 33 depending therefrom (FIG. 6) and is situated therebelow within compartment C and is provided on its upper surface adjacent wall 22. with a spoke wheel 31 having a plurality of radially extending spokes '32, corresponding in number to the number of tuning projections 30 provided. This number corresponds to the number of indexed positions to which the drum 28 may be moved and is here specifically shown as 12.

As best shown in FIG. 1, wall 22 is provided with an opening 34 through which a rotatable spoke driver 36 extends into driving relationship with spokes 32. As best shown in FIG. 4 spoke driver 36 comprises a collar 38 having three equally spaced radial drive projections 40 and is mounted fast on a secondary shaft 29 for rotation therewith. Secondary drive shaft 29 is journalled in intermediate wall generally in line with channel selector shaft 26 and driven thereby via a compound reduction gear train generally designated 130. Gear train 130 comprises a driving gear 132 fast on channel selector shaft 26, a follower gear 134 fast on intermediate shaft 29 adjacent gear 132, and a pair of

intermediate gears

136 and 138 fast on a readout shaft 140 journalled in housing walls 18 and 20 (the readout function of shaft 140 will be described hereinafter). As best shown in FIGS. 1 and 3, shaft 140 is rotatably driven through the meshing engagement of

gears

132 and 136. The rotation of shaft 140 is in turn transmitted to shaft 29 via the meshingengagement of

gears

138 and 134.

Each drive projection 40 on spoke driver 36 as it traverses slot 34 in wall 22 is effective to drivingly engage the spoke 32 disposed in registrationwith said slot therebyto rotate drum 28 to the next operative position. Accordingly, in the illustrated embodiment, four complete rotations of secondary shaft 29 effects one complete rotation of memory drum 28 through its twelve indexed positions.

As best shown in FIGS. 6 and 7, memory drum 28 comprises an inverted receptacle having a base wall 38 and a cylindrical side wall 40 preferably circular in cross section. Base 38'is provided with a central aperture through which mounting stud- 29 extends, the drum being vertically supported by the horizontal flange 29a on stud 29 and freely rotatable thereon. Mounted peripherally on base 38 and extending downwardly therefrom are a plurality of internally threaded receptacles 42, one for each tuning projection 30. Tuning projections 30 are preferably in the form of small tuning screws having a threaded shaft threadedly received within receptacle 42 and a radially extending head or flange 44 having a gear surface 46 on its outer periphery. The length of vertical extension of tuning screws 30 downwardly from drum 28 is thus adjustable by rotation of the screw into or out of its receptacle 42.

The rotary position of rotor shaft 24 and thus the impedance of one or more variable impedance tuning elements within housing 14 is adapted to be controlled by the extension selected tuning screw 30 at a tuning station generally designated 50 at the lower righthand portion of compartment C as viewed in FIGS. 6 and 7. For this purpose there is provided a linkage mechanism generally designated 52 adapted to convert the vertical adjustment of the selected tuning screw 30 into a rotary adjustment of rotor shaft 24.

As best shown in FIGS. 2, 4 and 6, linkage mechanism 52 comprises a vertically extending ram 54 56 formed integral with wall 20. Mounting bracket 56 may be conveniently stamped from wall 20 and comprises two horizontally extending plates 58 having registering slots 60 therein for slidably receiving ram 54. As best shown in FIG. 4 a U-shaped slide bracket generally designated 62 is slidably mounted along wall 20 and comprises a pair of vertically extending legs 64 slidably received withinslots 66 in horizontal wall 22, a cross bar 69 and a base 68 provided with a slot 70 slidably receiving ram 54. A cross pin 72 is mounted on the lower end of ram 54 below base68 of slide 62 and is adapted to engage base 68 as slide 62 is moved downwardly (as viewed in FIG. 4)

As best shown in FIGS. 4 and 6, ram 54 is provided with a vertical .slot receiving a pin 74 fixedly mounted on rotor shaft 24 and radially protruding therefrom. The rotor shaft 24 is spring biased counterclockwise (as viewed in FIG. 4) by means of a torsion spring 76 connected at one end to the housing wall 16 and at its other end to the rotor shaft (see FIG. 2), whereby pin 74 bears against the upper end of slot 75.

As best shown in FIG. 6 ram 54 is in axial engagethem with the selected tuning screw 30 at the tuning station 50 and accordingly is normally biased into engagement with that screw as a result of the spring biasing force of torsion spring 76 transmitted via pin 74 to ram 54. Thus, as best shown in FIG. 6, the rotary posie tion of rotor shaft 24 is determined by the axial extension of the selected tuning screw 30 positionedat the tuning station 50. Movement of that tuning screw downwardly in a direction out of its mounting receptacle 42 effects a corresponding downward movement of ram 54 which in turn effects a counterclockwise rotation of rotor shaft 24 (as viewed in FIG. 4) by virtue of the engagement of pin 74 with the upper edge of slot 75. Conversely, an upward movement of the selected tuning screw 30 in a direction into its mounting receptacle 42 effects a'corresponding upward movement of ram 54 as a result of the spring biased engagement of pin 74 with the upper edge of slot 75 thereby allowing torsion spring 76 to rotate torot shaft 24 in the clockwise direction. The fine tuning mechanism for adjusting tuning screws 30 will be described hereinafter.

Each of the 12 tuning screws 30 is preset to a given axial extension adapted to adjust the variable impedance elements controlled by rotor shaft 24 such that the tuning circuit is tuned to the active channel frequency within the UHF frequency range associated with that tuning screw.

Channel selection may be made by rotating channel selector shaft 26 via knob 27, each one-third of a full rotation being effective to drive spoke wheel 31 and driver 28 an amount sufficient to bring the next consecutive tuning screw into registration with the ram 54 at the tuning station 50.

In order to insure proper registration of tuning screws 30 with ram 54 and to prevent overshooting or undershooting the channel selector mechanism must be detented to all of its 12 positions. In addition, since ram 54 is normally biased into engagement with the selected tuning screw 30, means must be provided to insure a smooth transition from one tuning screw to the next. Thus ram 54 must be lowered away from drum 28 during the rotation thereof to prevent it from being shot upwardly between tuning screws thereby jamming the entire channel selector mechanism. Moreover, as previously noted this ram cocking action should preferably precede the rotation of the drum.

To these ends I have designed an indexing mechanism which is adapted to provide both the required ram cocking and detenting of the channel selector shaft in the desired timed sequence.

This mechanism comprises a lobe type cam 80 mounted fast on channel selector shaft 26 adjacent intermediate wall 20 (on the righthand side thereof in compartment A of housing (See FIGS. 2 and 4). As

best shown in FIG. 4, cam 80 is formed with a peripheral cam surface having three equally spaced projections or hills 80a separated by three depressions or valleys 80b. The cam projections 80a are in precise registry with the drive projections 40 of spoke driver 36. As best shown in FIGS. 4 and 7, intermediate wall is provided with a vertical slot or opening 82 above mounting bracket 58 and slide 62 is provided with a cam follower 84 extending from cross bar 69 through slot 82 and in vertical registration with the operative cam surface of cam 80. As shown in FIG. 4 cam follower 84 is provided with a curved upper surface 84a adapted to matingly engage the valleys 80b on cam 80. As best shown in FIG. 6 slide 62 is spring biased upwardly by means of a tension spring 86 connected at one end to bracket 56 on wall 20 and at-its other end to the base 68 of slide 62, thereby to maintain cam follower 84 biased against lobe type cam 80 in detented relationship. As a result, as channel selector shaft 26 is rotated, cam 80 is effective to force cam follower 84 and slide 62 downwardly against the bias of spring 86 as the curved surface 84 thereof engages one of the radially projecting cam surfaces 80a. Upon further rotation of channel selector shaft, slide 62 is returned to its initial position seated in the next valley or recessed cam surface of cam 80. The mating surfaces 80b and 84a of cam 80 and cam follower 84 respectively provide an accurate detenting of shaft 26 every one-third of a rotation thereof. As slide 62 is moved downwardly at the commencement of each rotation of channel selector shaft 26 from one detented position to the next the base 68 of the slide 54 engages the pin 72 extending laterally from ram (FIG. 4) whereby ram 54 I is carried downwardly against the bias of torsion spring 76 along with slide 62 during the remainder of its downward stroke to the position shown in FIG. 7.

As seen in FIG. 4 the cam surfaces 80a of the cam projections are so shaped that there is v a substantial dwell at the lowermost position of slide 62 before transition to the next valley surface 80b of cam 80. It is during this dwell that a drive projection 40 drivingly engages a spoke 32 on spoke wheel 31 thereby to rotate drum 28 one-twelfth of a complete rotation to bring the next tuning screw 30 into precise axial registration with ram 54. Upon completion of this incremental drum rotation cam follower 84 again seats in a valley in cam 80, releasing ram 54 to reseat on the next tuning screw 30, as shown in FIG. 6.

It will be appreciated that the cam projections 80a must extend radially outwardly sufficiently from the valley 80b to provide a downward stroke of slide 62 of sufficient magnitude to carry ram 54 downwardly to a dwell position in which it will clear the tuning screws 32 when adjusted downwardly to their farthest extent. In addition, pin 72 must be positioned on the lower end of ram 54 sufficiently downwardly spaced from the base 68 of ram 62 to provide for adjustment of tuning screws 32 to their most retracted (upward) positions while ram 54 remains seated thereon.

It will be appreciated from the foregoing that the indexing arrangement herein provided insures that ram 54 is fully retracted or cocked prior to rotation of drum 28 and does not begin its upward return until the next tuning screw is brought into registration at tuning station 50. Consequently there is no lateral movement of the tuning screws 30 during seating and reseating of ram 54 whereby wear and lateral stress is substantially completely eliminated. Moreover, even the straight line impact engagement of the ram 54 with the tuning screw may be minimized by an appropriate design of the cam surface of cam 80. Finally it will be appreciated that the detented positions of channel selector shaft 26 are not critical since the actual alignment of screws 30 with ram 54 is determined by the point of disengagement of driving projections 40 from the driven spoke 32. Preferably the spokes 32 in spoke wheel 31 are arranged in registration with the tuning screws 32 and the length of radial driving projections 40 is such that the spoke wheel is brought to the position shown in FIG. 1 after each incremental rotation. In this regard there is preferably provided a sufficient frictional resistance to the rotation of drum 28 to prevent additional rotation after disengagement of the spoke driver as a result of momentum. A separate detent arrangement (not shown) may be provided for the drum itself if desired. Thus the outer surface of cylindrical wall 41 of the drum 28 might be fluted to provide twelve recesses adapted to matingly engage a detent projection fixedly mounted on the housing 10.

In practice the seventy UHF channels are divided into groups (in the specifically disclosed embodiment there are 12 groups) and the manufacturer presets each tuning screw 30 to an extension corresponding generally to the frequency range defining a particular group of channels. Upon installation in a particular geographical area the tuning screws would be fine tuned to the active channel within that group. In addition, it may be necessary in operation from time to time to make slight fine tuning adjustments as a result of slight variations in the I electrical operating characteristics of the tuning circuit.

For this purpose there is provided a fine tuning adjustment mechanism generally designated within compartment A. As best shown in FIG. 6 that mechanism comprises a hollow fine tuning shaft 92 mounted concentrically of channelselector shaft 26 extending outwardly of housing 10 and terminating short 4 of flatted end 26a in telescoping relationship therewith. Fine tuning shaft 92 is axially and rotatively movable relative to channel selector shaft 26 and may also be provided with a flatted end 92a on which a fine tuning knob 94 (shown in broken lines) may be conveniently secured. Within compartment A shaft 92 is provided with a flange or washer 96 mounted fast thereon and defining the limit of axial movement of the shaft 92 to the right outwardly of housing, 10 by virtue of its enand a washer 104 for axial movementtherewith. Suitable guide means (not shown) may be provided to prevent rotation of bracket 100 during rotation of shaft 92. Mounted fast on the inner (lefthand) end of shaft 92 is a gear 106 adapted to provide adjustment of the selected tuning screw 30 at tuning station 50 via a gear train generally designated 108 mounted on bracket 100.

As best shown in FIG. 6, that gear train comprises a second gear 110 mounted on a shaft 112 journalled in the vertical wall of bracket 100 in meshing relationship with gear 106 and a crown gear 114 also mounted on shaft 112 and spaced from gear 110 for rotation therewith. Crown gear 114 in turn is in meshing relationship with a gear 116 mounted on the horizontal wallof bracket 100 for rotation around a vertical axis. When fine tuning shaft 92 is pushed inwardly to the left as shown by arrow 117 in FIG. 6, bracket 100 and gear train 108 are carried to the left against thebias of spring 98, gear 116 being moved through slot 82 in slide 62 into meshing engagement with the gear surface 41 on the periphery of the head 44 or the selected tuning screw 30 at tuning station 50. Accordingly, to fine tune the circuit the operator merely pushes inwardly on knob 116 to bring fine tuning shaft 92 to the position shown in FIG. 6 whereupon a rotation of knob 116 is transmitted via shaft 92, gear 106, gear 110 and crown gear 114 to gear 116 which in turn rotates the selected tuning screw within its threaded receptacle 42 to adjust the axial extension thereof. That axial adjustment is in turn converted into a rotary adjustment of one or more variable impedance elements within housing 14 via ram 54, pin 74 and rotor shaft 24 in a manner already described. Once the desired adjustment has been made, the operator merely releases knob 116, whereupon fine tuning shaft 92 and bracket 100 return to their normal positions shown in FIG. 7 by virtue of the bias of coil spring 98, thereby disengaging gear 116 from the selected tuning screw 30. It will be apparent that an accidental turning of knob 116 in this position will be ineffective to vary the axial extension of the selected tuning screw 30. Consequently, once all channels are fine tuned, channel selection may be made without upsetting the preset extensions of tuning screws 30 and subsequent fine tuning is rarely necessary.

It will be recalled that a full rotation of secondary shaft 29 covers only three channels. For a typical reduction gear train 130 this would translate into one complete rotation of channel selector shaft 26 for every six channels. Consequently, a separate channel indicator readout dial 118 may be provided having twelve operative positions with means on its periphery to indicate the selected channel. As bestshown in FIG. 2 dial 118 is mounted fast on a readout shaft 120 which in turn is rotatably mounted concentrically on fine tuning shaft 92. Shaft 120 is journalled in end wall 18 of housing 10 and extends therethrough. A collar 122 is mounted fast on shaft 120 adjacent the outside surface of wall 18 and a gear 124 is mounted fast thereon adjacent the inside surface of wall 18 whereby shaft 120 is prevented from moving axially when fine tuning shaft 92 is moved to the left. Gear 118 is in meshing engagement with a gear 126 fast on readout shaft 140. It will be recalled that readout shaft 140 is driven by channel selector shaft 26 by means of meshing

gears

132 and 136. Consequently, the rotation of shaft 140 is transmitted to readout shaft 120 and dial 118 via the meshing engagement of

gears

126 and 124 at a reduced angular velocity. The gear ratios of

gears

122, 124, 134 and 138 are such that for each one-third rotation of secondary shaft 29 (one indexed position of drum 28), readout shaft 120 and dial118 are rotated one-twelfth of a turn whereby a full rotation of drum 28 corresponds to a full rotation of dial 118. Readout dial 118 is generally provided with appropriate indicator on its periphery and if desired may be provided with a lighted window arrangement to more clearly indicate the selected channel.

In the absence of a separate detent or indexing mechanism for memory drum 28, the selected tuning screw may become slightly misaligned with ram 54 during fine tuning. Conventional detent mechanisms such tions, an indexing mechanism which alleviates the above problems. While the structural elements of this second embodiment are somewhat different than those of the embodiment just described, the mode of operation is substantially identical and accordingly only the operationally significant modifications will be described in any detail, like reference numerals designating like parts with the addition of a prime:

As best shown in FIG. 10 the memory drum 28' in this modified embodiment is provided with a novel detent means generally designated 150, in the form of a ratchet mechanism comprising a first toothed wheel 152 mounted fast on drum 28 centrally thereof and extending downwardly therefrom, and a second toothed wheel 154 mounted on the chassis and biased into mating engagement with the drum wheel 152 by a spring means generally designated 156. As best shown in FIGS. 11 and 14 spring means 156 comprises a flat L- shaped metal spring 158 having a base part 159 and an operative resilient spring part 160 extending generally transversely to base part 159. The operative spring part 160 is provided with a convexly curved portion 161 having a longitudinally extending slot 162 therein. The

rotatively fixed wheel 154 is provided with a central opening 163 and two aligned depending projections 164 on opposite sides of aperture 163 (see FIG. 14). Base part 159 of spring 158 is secured on the outer surface of one side wall 166 of the housing by means such as screw 168 and the resilient spring part 160 extends through a slot 170 in housing wall 166, through the housing and outwardly through a slot 1'72 to the opposite housing wall 174. As best shown in FIG. 14 the resilient curved portion 161 of spring part 160 engages the underside of fixed wheel 154, the projections 164 being received in the slot 162 in spring part 160.

The drum wheel 152 may be formed integral with drum 28 in depending relationship therewith. As best shown in FIG. 14, memory drum 28' is mounted fast on a shaft 176 which shaft is in turn rotatably mounted in a slot 178 in the top wall 180 of the tuner housing and retained therein in depending relationship by means of a radially extending flange 182. As shown in FIG. 14 the other end of shaft 176 is received in aperture 163 in fixed wheel 154 and serves to maintain that wheel centered in meshing relationship with drum wheel 158. Spoke wheel 31 is in turn mounted fast on drum 28. The entire drumwheel assembly may be conveniently molded in one piece from any suitable moldable plastic material after which a series of 12 threaded apertures 184 are tapped in the underside thereof equally spaced along its periphery, said apertures receiving the operative tuning screws 30.

In the embodiment illustrated in FIG. 10, drum wheel 152 and fixed wheel 154 are each provided with six teeth 186 in opposing meshing relationship. As best shown in FIG. 12, in the six tooth embodiment the teeth 186 are truncated and comprise angular side surfaces 188 which intersect at the base of the teeth to form notches 189 and a flatted surface 190 at the apex of each tooth. The opposing wheels consequently are adapted to mesh in any one of six rotative positions of drum 28' relative to wheel 154. Thus each time the drum is rotated by spoke driver 36', the angular tooth surface 188 of drum wheel 1S2 ride along the meshing angular tooth surfaces 188 of wheel-154, thereby forcing wheel 154 downwardly in the direction of arrow 190 (FIG. against the bias of resilient spring 158 until the flatted surfaces 190 of

wheels

152 and 154 come into engagement. Further rotation of drum 28' will be effective to shift the teeth of wheel 152 toward the next set of notches 189 in wheel 154 whereby the angular surfaces 188 of the teeth of wheel 152 will ride upwardly on the angular surfaces of the teeth of wheel 154, wheel 154 being snapped back into meshing engagement with drum wheel 152 under the influence of spring 158. In this manner drum 28'is detented to six equally spaced rotative positions. Each shift of the drum from one detented position to the next corresponds to two successive detented positions of the channel selector shaft 26 and two full cycles of the ram 54'. Alternatively, lobe cam 80' and gearing 130 might be modified for a six position channel selector shaft detent and ram actuation. In either case tuning screws 30' are provided only for every other one of the 12 apertures 184 in drum 28.

The six position detent mechanism described above may be conveniently converted to a 12 position detent operation by a simple machining operation. Thus, as shown in FIG. 12, each tooth of the six tooth drum and fixed

wheels

152 and 154, respectively, may be converted into two identical teeth by removing the material shown above the broken lines 194. This may be accomplished by any suitable machining operation and appreciably increases the flexibility of the tuner. Moreover, the use of the detent mechanism described herein insures accurate positioning of the selected tuning screw during channel selection and effectively prevents lateral movement of that screw during fine tuning.

As best shown in FIGS. 10 and 15 a further modification in this second embodiment involves the design of the ram and ram actuator. As there illustrated the ram 54' is provided with a shallow recess 198 adapted to receive a projection 200 mounted fast on a collar 202 and extending radially outwardly therefrom. Collar 202 is in turn mounted fast on rotor shaft 24 by means such as set screw 204 and is provided with an axial slot 206 receiving the free end of torsion spring 76'. Ram 54' is provided with a circular shaft 208 at one end adapted to engage the selected tuning screw and has a flange 210 at is other end adapted to engage the slide bracket 62'. Slide bracket 62' is biased into engagement with lobe cam 80' by tension spring 212 (FIG. 10).

The remaining structural features of the embodiment of FIGS. 8-17 are similar in construction to that of the first described embodiment and substantially identical in operation and will accordingly not be described further.

It will be appreciated from the foregoing that I have designed a preset tuning mechanism having a considerably improved fine tuning accuracy and reliability. As a result of the straight line, geared and timed vertical movement of my novel ram mechanism, lateral biasing of the selected tuningscrew is substantially eliminated thereby considerably enhancing accuracy and operating life of the tuner. Moreover synchronized movement of this linkage in cooperation with the memory drum insures against lateral scraping of the screw during channel selection thereby eliminating a significant source of wear and inaccuracy.

The novel indexing system of the second embodiment of this invention is extremely reliable and provides for consistently accurate indexing positions of the drum for substantially the entire operating life of the tuner. Moreover, this indexing mechanism is simple and inexpensive to construct and may conveniently be converted from a 12-position detent mechanism to a six-position detent mechanism by a simple machining operation thereby considerably enhancing versatility.

While only a limited number of embodiments of the present invention have herein specifically been described, it will be appreciated that many variations may be made therein, all within the scope of the present invention as defined by the following claims.

I claim:

l. A presettable tuner of the type having a fixed chassis and channel selector means rotatably mounted thereon, said channel selector means including a plurality of adjustable fine tuning projections adapted to be selectively moved to a tuning station for fine tuning in response to the rotation'of said channel selector means, the improvement comprising an adjustable tuning means, linkage means at said tuning station operatively connected to said tuning means and adapted to axially engage said fine tuning projections and effective to adjust the position of said tuning means in accordance with the position of the thus engaged fine tuning projection, means biasing said linkage means into engagement with the selected fine tuning projection, and means operatively connected to said channel selector means to move said linkage means out of engagement with said selected fine tuning projection in timed sequential relationship prior to the movement of the next fine tuning projection to said tuning station.

2. The presettable tuner of claim 1, wherein said linkage means comprises a rotor shaft operatively connected to said tuning means, means projecting from said rotor shaft and mounted thereon for rotation therewith, ram means movable into and out of engagement with said selected tuning projection, and means operatively connecting said ram means and said projecting means in driving relationship, whereby movement of said ram means in response to the adjustment of said fine tuning projections is effective to adjust said tuning means.

3. The presettable tuner of claim 2, wherein said channel selector means comprises a rotatable channel selector shaft and a rotatable drum operatively drivingly connected thereto, said tuning projections being mounted on said drum and further comprising cam means operatively connected to said channel selector shaft and to said ram means and effective to move said ram means away from said fine tuning projections in timed relationship with the rotation of said channel selector shaft.

4. The. presettable tuner of claim 3, further comprising intermittent drive means operatively connected to said channel selector shaft and adapted to operatively drivingly engage said drum and to intermittently rotate same in discrete increments in response to a rotation of said channel selector shaft, and wherein said cam means comprises a cam having a plurality of crests and valleys, said valleys serving to detent said channel selector shaft between successive incremental rotations of said drum in timed sequence therewith, said crests being effective to move said linkage means out of engagement with said tuning projections prior to theonset of said incremental rotations of said drum and to maintain said linkage means out of engagement with said tuning projections during said incremental rotations.

5. The-tuner of claim 4, wherein said intermittent drive means comprises a spoke wheel mounted on said drum for rotation therewith and having a plurality of spokes and a spoke driver operatively drivingly connected to said channel selector shaft, said spoke driver having a plurality of drive projections adapted to intermittently successively drivingly engage said spokes on said spoke wheel. a

6. The tuner of claim 3, further comprising means for detenting said drum to a plurality of positions in each of which a tuning projection is operatively positioned at said tuning station.

7. The tuner of claim 6, wherein said detenting means comprises a first toothed ratchet wheel mounted on said drum for rotation therewith and a second toothed ratchet wheel, and means resiliently biasing said second toothed ratchet wheel axially into meshing engagement with said first toothed ratchet wheel and maintaining said second toothed ratchet wheel rotatably stationary with respect to said chassis.

8. The tuner of claim 2, wherein said projecting means comprises a pin, said means operatively connecting said ram means and said projecting means comprising said slot on said ram means receiving said pin, and means urging said pin within said slot into engagement with said ram means in a direction forcing said ram means into engagement with said selected tuning projection.

9. The presettable tuner of claim 8, wherein said channel selector means comprises a rotatable channel selector shaft and a rotatable drum operatively drivingly connected thereto, said tuning projections being mounted on said drum and further comprising cam means operatively connected to said channel selector shaft and to said ram means and effective to move said ram means away from said fine tuning projections in timed relationship with the rotation of said channel selector shaft.

10. The presettable tuner of claim 9, further comprising intermittent drive means operatively connected to said channel selector shaft and adapted to operatively drivingly engage said drum and to intermittently rotate same in discrete increments in response to a rotation of said channel selector shaft, and wherein said cam means comprises a cam having a plurality of crests and valleys, said valleys serving to detent said channel selector shaft between successive incremental rotations of said drum in timed sequence therewith, said crests being effective to move said linkage means out of engagement with said tuning projections prior to the onset of said incremental rotations of said drum and to maintain said linkage means out of engagement with said tuning projections during said incremental rotations.

11. The tuner of claim 10, wherein said intermittent drive means comprises'a spoke wheel mounted on said drum for rotation therewith and having a plurality of spokes and a spoke driver operatively drivingly connected to said channel selector shaft, said spoke driver having a plurality of drive projections adapted to intermittently successively drivingly engage said spokes on said spoke wheel.

12. The tunerof claim 8, further comprising fine tuning adjustment means adapted in a first position to operatively engage said tuning projections for axial adjustment thereof and ma second position being disengaged from said tuning projections, and means for moving said fine tuning means between said first and second positions.

13. The tuner of claim 8, wherein said means urging said pin into engagement with said ram comprises a torsion spring operatively connected between said chassis and said rotor shaft.

14. The presettable tuner of claim 2, wherein said channel selector means comprises a rotatable channel selector shaft and a rotatable drum operatively drivingly connected thereto, said tuning projections being mounted on said drum and further comprising cam means operatively connected to said channel selector shaft and to said ram means and effective to move said ram means away from said fine tuning pro-' jections in timed relationship with the rotation of said channel selector shaft.

15. The presettable tuner of claim 14, further comprising intermittent drive means operatively connected to said channel selector shaft and adapted to operatively drivingly engage said drum and to intermittently rotate same in discrete increments in response to a rotation of said channel selector shaft, and wherein said cam means comprises a cam having a plurality of crests and valleys, said valleys serving to detent said channel selector shaft between successive incremental rotations of said drum in timed sequence therewith, said crests being effective to move said ram means out of engagement with said tuning projections prior to the onset of said incremental rotations of said drum and to maintain said ram means out of engagement with said tuning projections during said incremental rotations.

16. The tuner of claim 15, wherein said intermittent drive means comprises a spoke wheel mounted on said drum for rotation therewith and having a plurality of spokes and a spoke driver operatively drivingly connected to said channel selector shaft, said spoke driver having a plurality of drive projections adapted to intermittently successively drivingly engage said spokes on said spoke wheel.

17. The tuner of claim 14, wherein said ram means comprises a ram, means biasing said ram into engagement with said selected tuning projection, and slide means operatively connected to said cam means on said channel selector shaft and having means adapted to engage said ram in response to the rotation of said channel selector shaft, thereby to move said ram out of engagement with said tuning projections in timed relationship with the rotation of said rotatable drum.

18. The tuner of claim 17, further comprising means biasing said slide means into engagement with said cam means.

19. The tuner of claim 1, further comprising fine tuning adjustment means adapted in a first position to operatively engage said tuning projections'for axial ad justment thereof and in a second position being disengaged from said tuning projections, and means for moving said fine tuning means between said first and second positions.

20. The presettable tuner of claim 19, wherein said linkage means comprises a rotor shaft operatively connected to said tuning means, means projecting from said rotor shaft and mounted thereon for rotation therewith, ram means movable into and out of engagement with said selected tuning projection, and means operatively connecting said ram means and said projecting means in driving relationship, whereby movement of said ram means in response to the adjustment of said fine tuning projections is effective to adjust said tuning means.

21. The presettable tuner of claim 19, wherein said channel selector means comprises a rotatable channel selector shaft and a rotatable drum operatively drivingly connected thereto, said tuning projections being mounted on said drum and further comprising cam means operatively connected to said channel selector shaft and to said ram means and effective to move said ram means away from said fine tuning projections in timed relationship with the rotation of said channel selector shaft.

22. The presettable tuner of claim 21, further comprising intermittent drive means operatively connected to said channel selector shaft and adapted to operatively drivingly engage said drum and to intermittently rotate same in discrete increments in response to a rotation of said channel selector shaft, and wherein said cam means comprises a cam having a plurality of crests and valleys, said valleys serving to detent said channel selector shaft between successive incremental rotations of said drum in timed sequence therewith, said crests being effective to move said linkage means out of engagement with said tuning projections prior to the onset of said incremental rotations of said drum and to maintain said linkage means out of engagement with said tuning projections during said incremental rotations.

23. The tuner of claim 22, wherein said intermittent drive means comprises a spoke wheel mounted on said drum for rotation therewith and having a plurality of spokes and a spoke driver operatively drivingly connected to said channel selector shaft, said spoke driver having a plurality of drive projections adapted to intermittently successively drivingly engage said spokes on said spoke wheel.

24. The tuner of claim 19, wherein said tuning projections comprise threaded shafts threadedly engaged with said drum and adjustable into and out of said drum by rotation of said threaded shafts and wherein said fine tuning adjustment means comprises a fine tuning shaftand gear means operatively connecting said fine tuning shaft and said threaded shafts at said tuning station in driving relationship in said first position.

25. The presettable tuner of claim 24, wherein said linkage means comprises a rotor shaft operatively connected to said tuning means, means projecting from said rotor shaft and mounted thereon for rotation therewith, ram means movable into and out of engagement withsaid selected tuning projection, and means operatively connecting said ram means and said projecting means in driving relationship, whereby movement of said ram means in response to the adjustment of said fine tuning projections is effective to adjust said tuning means.

26. The presettable tuner of claim 24, wherein said channel selector means comprises a rotatable channel selector shaft and a rotatable drum operatively drivingly connected thereto, said tuning projections being mounted on said drum and further comprising cam means operatively connected to said channel selector shaft and to said ram means and effective to move said ram means away from said fine tuning projections in timed relationship with the rotation of said channel selector shaft.

27. The presettable tuner of claim 26, further comprising intermittent drive means operatively connected to said channel selector shaft and adapted to operatively drivingly engage said drum and to intermittently rotate same in discretein'crements in response to a rotation of said channel selector shaft, and wherein said cam means comprises a cam having a plurality of crests and valleys, said valleys serving to detent said channel selector shaft between successive incremental rotations of said drum in timed sequence therewith, said crests being effective to move said linkage means out of engagement with said tuning projections prior to the onset of said incremental rotations of said drum and to maintain said linkage means out of engagement with said tuning projections during said incremental rotations.

28. The tuner of claim 27, wherein said intermittent drive means comprises a spoke wheel mounted on said drum for rotation therewith and having a plurality of spokes and a spoke driver operatively drivingly connected to said channel selector shaft, said spoke driver having a plurality of drive projections adaptedto inter- 30. The tuner of claim 1, further comprising a readout dial rotatably mounted on said chassis and adapted to indicate the selected channel, and gear means operatively connecting said channel selector means with said readout dial.

31. A presettable tuner of the type having a fixed chassis and a channel selector means rotatably mounted thereon, said channel selector means comprising a drum having a plurality of adjustable fine tuning projections extending therefrom and means for rotating said drum to selectively move said tuning projections to a station for fine tuning, the improvement comprising indexing means adapted to detent said drum to a plurality of positions in each, of which a tuning projection is operatively positioned at said tuning station, said indexing means comprising a first toothed ratchet wheel mounted on said drum for rotation therewith, a second toothed ratchet wheel mounted on said chassis and fixed against rotation with respect thereto and resilient means biasing said second ratchet wheel axially into meshing engagement with said first ratchet wheel.

32. The tuner of claim 31 wherein said resilient means comprises a leaf spring mounted on said chassis and engaging said second ratchet wheel and means on said second ratchet wheel engaging said leaf spring and effective to prevent rotation of said second ratchet wheel with respect to said leaf spring.

Claims

1. A presettable tuner of the type having a fixed chassis and channel selector means rotatably mounted thereon, said channel selector means including a plurality of adjustable fine tuning projections adapted to be selectively moved to a tuning station for fine tuning in response to the rotation of said channel selector means, the improvement comprising an adjustable tuning means, linkage means at said tuning station operatively connected to said tuning means and adapted to axially engage said fine tuning projections and effective to adjust the position of said tuning means in accordance with the position of the thus engaged fine tuning projection, means biasing said linkage means into engagement with the selected fine tuning projection, and means operatively connected to said channel selector means to move said linkage means out of engagement with said selected fine tuning projection in timed sequential relationship prior to the movement of the next fine tuning projection to said tuning station.

4. The presettable tuner of claim 3, further comprising intermittent drive means operatively connected to said channel selector shaft and adapted to operatively drivingly engage said drum and to intermittently rotate same in discrete increments in response to a rotation of said channel selector shaft, and wherein said cam means comprises a cam having a plurality of crests and valleys, said valleys serving to detent said channel selector shaft between successive incremental rotations of said drum in timed sequence therewith, said crests being effective to move said linkage means out of engagement with said tuning projections prior to the onset of said incremental rotations of said drum and to maintain said linkage means out of engagement with said tuning projections during said incremental rotations.

5. The tuner of claim 4, wherein said intermittent drive means comprises a spoke wheel mounted on said drum for rotation therewith and having a plurality of spokes and a spoke driver operatively drivingly connected to said channel selector shaft, said spoke driver having a plurality of drive projections adapted to intermittently successively drivingly engage said spokes on said spoke wheel.

11. The tuner of claim 10, wherein said intermittent drive means comprises a spoke wheel mounted on said drum for rotation therewith and having a plurality of spokes and a spoke driver operatively drivingly connected to said channel selector shaft, said spoke driver having a plurality of drive projections adapted to intermittently successively drivingly engage said spokes on said spoke wheel.

12. The tuner of claim 8, further comprising fine tuning adjustment means adapted in a first position to operatively engage said tuning projections for axial adjustment thereof and in a second position being disengaged from said tuning projections, and means for moving said fine tuning means between said first and second positions.

14. The presettable tuner of claim 2, wherein said channel selector means comprises a rotatable channel selector shaft and a rotatable drum operatively drivingly connected thereto, said tuning projections being mounted on said drum and further comprising cam means operatively connected to said channel selector shaft and to said ram means and effective to move said ram means away from said fine tuning projections in timed relationship with the rotation of said channel selector shaft.

19. The tuner of claim 1, further comprising fine tuning adjustment means adapted in a first position to operatively engage said tuning projections for axial adjustment thereof and in a second position being disengaged from said tuning projections, and means for moving said fine tuning means between said first and second positions.

24. The tuner of claim 19, wherein said tuning projections comprise threaded shafts threadedly engaged with said drum and adjustable into and out of said drum by rotation of said threaded shafts and wherein said fine tuning adjustment means comprises a fine tuning shaft and gear means operatively connecting said fine tuning shaft and said threaded shafts at said tuning station in driving relationship in said first position.

25. The presettable tuner of claim 24, wherein said linkage means comprises a rotor shaft operatively connected to said tuning means, means projecting from said rotor shaft and mounted thereon for rotation therewith, ram means movable into and out of engagement with said selected tuning projection, and means operatively connecting said ram means and said projecting means in driving relationship, whereby movement of said ram means in response to the adjustment of said fine tuning projections is effective to adjust said tuning means.

27. The presettable tuner of claim 26, further comprising intermittent drive means operatively connected to said channel selector shaft and adapted to operatively drivingly engage said drum and to intermittently rotate same in discrete increments in response to a rotation of said channel selector shaft, and wherein said cam means comprises a cam having a plurality of crests and valleys, said valleys serving to detent said channel selector shaft between successive incremental rotations of said drum in timed sequence therewith, said crests being effective to move said linkage means out of engagement with said tuning projections prior to the onset of said incremental rotations of said drum and to maintain said linkage means out of engagement with said tuning projections during said incremental rotations.

28. The tuner of claim 27, wherein said intermittent drive means comprises a spoke wheel mounted on said drum for rotation therewith and having a plurality of spokes and a spoke driver operatively drivingly connected to said channel selector shaft, said spoke driver having a plurality of drive projections adapted to intermittently successively drivingly engage said spokes on said spoke wheel.

29. The tuner of claim 24, further comprising means biasing said fine tuning means to said second position.

30. The tuner of claim 1, further comprising a readout dial rotatably mounted on said chassis and adapted to indicate the selected channel, and gear means operatively connecting said channel selector means with said readout dial.

31. A presettable tuner of the type having a fixed chassis and a channel selector means rotatably mounted thereon, said channel selector means comprising a drum having a plurality of adjustable fine tuning projections extending therefrom and means for rotating said drum to selectively move said tuning projections to a station for fine tuning, the improvement comprising indexing means adapted to detent said drum to a plurality of positions in each of which a tuning proJection is operatively positioned at said tuning station, said indexing means comprising a first toothed ratchet wheel mounted on said drum for rotation therewith, a second toothed ratchet wheel mounted on said chassis and fixed against rotation with respect thereto and resilient means biasing said second ratchet wheel axially into meshing engagement with said first ratchet wheel.