COLLINS F250Z4 3Khz USB MECHANICAL FILTER 250Khz IF F 250 Z 4: $45. COLLINS F250Z4 3Khz USB MECHANICAL FILTER 250Khz IF F 250 Z 4 This listing is for a Collins 3KHz USB Mechanical Filter for the 250Khz IF radios; Tested on my service monitor and working perfectly. You will receive the EXACT item shown in the pictures. Collins Mechanical Filter Catalog. Collins Bulletin 1031A, Mechanical Filters (200 DPI, Color PDF, 4.7MB) Collins Quality Standards Manual. This manual was used internally at Collins Radio to specify the standards of workmanship to be used in their equipment. It contains all sorts of useful information including, but by no means limited to.

THE COLLINS 51J-4 RECEIVER

The Collins 51J-4Receiver is designed for communication applicationswhere stability and dial accuracy of the highestorder are the prime requisites. Under normaloperating conditions, the receiver operates in therange of 540 kc to 30.5 mc with a total setting errorand drift of less than 1 kc at any frequency withinits range. The receiver is designed for amplitude-modulated and continuous wave reception, althoughits accuracy and stability make it suitable for manyapplications where it is desired to receive or setdefinite frequencies without searching or makingfrequent adjustments. This receiver incorporatesthe new mechanical filter in the intermediatefrequency range to obtain the desirable rectangular-shaped passband.

MECHANICAL

The 51J-4 Receiver isavailable in two styles. One is a panel and shelfassembly suitable for mounting in a standard rackcabinet. Over-all panel dimensions are: width,19 inches; height, 10-1/2 inches; and depth behindpanel, 13-1/2 inches. A dust cover that fits overthe top of the chassis is removable from the rear.The other assembly is in a cabinet suitable fortable-mounting. Outside cabinet dimensions are:width, 21-1/S inches; height, 12-3/8 inches anddepth, 13-1/8 inches. Available on special orderis a speaker that matches this cabinet. The speaker'sdimensions are: width. 13 inches; height, 11 inches;depth, 7 inches. The speaker, the cabinet of thetable-mounting assembly, and the front panel of therack-mounting receiver are finished in St. JamesGray wrinkle.

The following controls are located on thefront panel:

R-F GAIN - CRYSTAL FILTER -AUDIO GAIN - PHASING -BFO ON-OFF - OFF-ON-STANDBY -CALIBRATE - MEGACYCLE TUNING -ON-OFF - BFO PITCH - KILOCYCLE TUNING -AVC ON-OFF - ZERO ADJ -LIMITER OUT-IN - METER OUTPUT-INPUT -ANT. TRIM - CAL (100 KC ADJUSTMENTS) -CRYSTAL FILTER SELECTIVITY - FILTER SELECTOR

The operating range of 540 kc to 30.5 mc iscovered by 30 one-megacycle bands that are selectedby the band switch knob and indicated by a slide ruledial having graduations of one-tenth megacycle (100kc)intervals. The main tuning control covers eachof these megacycle ranges with 10 turns of a 100division dial calibrated at one-kilocycle intervals.The receiver's frequency stability is consistent withthis finely divided calibration even at the highestfrequencies .

A four-ohm headphone jack and a 600-ohm speakerjack are provided on the front panel. The antennaconnector, 50-ohm i-f output connector, breakingrelay terminals and four-ohm and 600-ohm audiooutput terminals are provided-on the rear. A heavyduty a-c power cord extends from the rear of thechassis .

ELECTRICAL

When advantageous, theModel 51J-4 Communications Receiver uses single,double, or triple conversion in tuning the entirefrequency spectrum of 540 kc to 30.5 mc. Nineteentubes, three of which are dual, are employed in thereceiver. With the exception of the rectifier tube,all are of the miniature type.

The receiver r-f circuits tune from .5 to 30.5mc. thus Band 1 is referred to as covering the range.5 to 1.5 mc. However, the lower end of theoperating range is considered to be 540 kc ratherthan 500 kc because of the questionable operation inthe extreme low end of the band where frequenciesapproach the receiver i-f frequency of 500 kc.Limited operation at the extreme low end is possiblewith somewhat reduced performance.

The frequency range of the 51J-4 Receiver,.5 to 30.5 mc, is divided into 30 one-megacyclebands by a system of switches and coils which formthe r-f amplifier and first mixer circuits. Bandchanging consists of moving powdered iron 'slugs'into the coils in one megacycle steps until inductancelimits of the coils are reached, then changing coilsand repeating. Injection voltage for the first mixeris obtained from the fundamental or harmonic outputof an oscillator, the frequency of which is controlledby one of ten quartz crystals selected by theMEGACYCLE band switch. The main tuning controlis a vernier dial calibrated in 100 one-kilocycledivisions. This control operates through a differentialmechanism to move the band change 'slugs'in the coils enough to cover the range between theone-megacycle band change steps. Thus the BandSwitch selects coils and crystals and also roughlypositions the tuning slugs. At the same time one ofthe two ranges (1.5 to 2.5 mc or 2.5 to 3.5 mc) ofthe variable i-f channel is selected and tuned alongwith the r-f coils.

The crystal frequencies for the first mixerinjection are so chosen that the frequency producedby the first mixer will always fall in the 1.5 to 2.5mc or 2.5 to 3.5 mc range of the variable i-f channel.

Exceptions to the operation just described arebands 1, 2, and 3. Band 1 (.5 to 1.5 mc) uses anintermediate mixer between the first mixer and thevariable i-f coils. This mixer accepts frequenciesin the range 10.5 to 11.5 mc from the first mixer.A 12-mc signal developed by the crystal controlledoscillator is applied to the first mixer to determinethese frequencies. The crystal controlled oscillatoralso applies an 8-mc voltage to the intermediatefrequency mixer to produce a signal within therange of the variable frequency i-f coils which tunethe 2.5 to 3.5 mc spectrum. Bands 2 and 3, whichcover 1.5 to 2.5 mc and 2.5 to 3.5 mc respectively,are identical in span to each band of the variable i-fcoils and thus feed through to the second mixerwithout utilizing the first mixer.

Following the variable i-f and the second mixerare the crystal filter and a four stage fixed intermediatefrequency amplifier containing mechanicalfilters. Conversion to the fixed i-f of 500 kc isaccomplished by injecting a 2 to 3 mc signal from aCollins 70E-15 oscillator to produce a difference of500 kc from the frequency existing in either band ofthe variable i-f amplifier. Tuning of the 70E-15oscillator is done by the 'kilocycle' tuning controlin step with all other circuits.

Stability of the 70E-15 oscillator is assured bytemperature-compensated components operating ina sealed and moisture-proof housing.

Separate rectifiers are used to produce automaticvolume control and audio voltages. D-c amplificationof the automatic volume control voltage is providedto obtain essentially uniform input to the detector.Audio power output is held within 3. 5 db over signalinput voltage ranges of five to 125,000 microvoltsat the antenna terminals. A series type noiselimiter clips modulation at 50-85 percent. Thisallows good reception in the presence of strong noisepulses.

51J-4 SPECIFICATIONS

Under the hood...

51J-4 CIRCUITRY

MECHANICAL DESCRIPTION

BAND CHANGE - Collins 51J-4 Receiver covers the frequencyrange of 0.5 to 30.5 mc in 30 bands: 0.5 to 1.5, 1.5 to 2.5,and so on up to 30.5 mc. Each band is one megacycle wide.Circuits affected by band changes are the r-f amplifier grid,first, second, and third mixer grids, crystal selector, andcrystal harmonic tuning circuits. The third mixer isswitched in only on band 1 (.5 to 1.5 mc).

Operations involved in the changing of bands consist ofselecting the proper coils in these circuits by means of tapswitches and changing the position of the r-f amplifier andfirst mixer slug tables. All stages are permeability tunedby powdered iron slugs. The r-f amplifier and first mixerslug tables change position a full megacycle in tuning eachtime a band is changed. This is true of all three slugtables, which tune L104 through L113. However, the tapswitches select the proper set of coils for the frequencydesired.

Slug tables are driven from two sources: the main tuningknob and the BAND CHANGE knob. These two driving sources areconnected to the slug tables through a differential gearmechanism. This is necessary since the coils for bands 4 to7, 8 to 15, and 16 to 30 cover these tuning ranges with onecomplete excursion of the tuning slugs. For instance, theband 4 to 7 slug table tunes its associated coils throughfour megacycles; in one megacycle jumps when operated by theBAND CHANGE knob, and in complete coverage in between whenoperated by the tuning knob. An interesting feature of thedifferential gearing is its ability to combine the movementsof the two driving sources so that the slug table is movedexactly one megacycle in each band change. The other slugtables operate similarly to the 4 to 7 table, except thatthe band 8 to 15 table tunes its associated coils through 8mc, and the band 16 to 30 table tunes its associated coilsthrough 15 mc. These three slug tables are movedsimultaneously by means of separate cams.

Switch sections of the band switch are ganged with the BANDCHANGE knob through an over-travel coupler. This over-travelcoupler drops the band switch at band 16 while the r-f slugtables continue to operate one position for each band asusual. Refer to figure 4-2. This mechanical diagram showsthe gears and connecting shafts associated with band changeand tuning. Shafts associated with changing bands are C, D,G, H, I, K, and the over-travel shaft. On band 1 radiofrequency coils L101 and L110 are switched by means of theBAND CHANGE knob through the over-travel shaft and shaft G.On bands 2 and 3, the r-f coils are selected by the BANDCHANGE knob through the over-travel shaft and shafts G and K,variable i-f section coils, L116 through L119, being used asadditional r-f coils on these bands. On bands 4 to 7, thecoils are selected by the BAND CHANGE knob through the over-travel shaft and shaft G, and the position of the slug tableis changed through shafts C and D. On these bands the samecoils are used for each band. Band change is accomplished bymoving the tuning slug in the coil an amount equal to onemegacycle in frequency. The slug moves in the coil 0. 250inches for a one megacycle change. On bands 8 to 15, the r-fcoils are changed by the over-travel shaft and shaft G, andthe position of the slug table is changed one megacycle perband through shafts C and D. The movement of the slug tablefor a one megacycle change is 0.125 inches. On bands lo to30, the r-f coils are switches through the over-travel shaftand shaft G to position 16 where the band switch remains forbands 16 to 30 while the over-travel coupler allows shaft Gto rotate through to the thirtieth band. The slugs in the r-f coils are driven through shafts C and D. The slugs travel0. 0625 inches during band change. During operation on anyband between 4 and 30 the variable i-f channel is alternatedfrom one variable i-f to the other by shafts G and K.Crystals are selected by operation of the BAND CHANGE knobthrough the 15-position Geneva system and shafts G. H, and I.

TUNING - All r-f, mixer and variable i-f coils, as well asthe variable frequency oscillator coil, are permeability-tuned by powdered iron cores. While tuning, these slugs movein and out of the coils at a rate determined by a cam or bya lead screw. Four slug racks or tables are used in the 51J-3 receiver to perform the function of tuning the r-f, mixerand variable i-f stages. The group of three slug tables inthe rear portion of the chassis tunes the r-f and firstmixer stages when the receiver is operating in the 3.5 to30.5 mc frequency range (bands 4 to 30). The fourth slugtable, located at the right hand edge of the receiver, tunesthe r-f stage, the first mixer grid, the third mixer gridand the variable i-f coils when receiving in the range 0.5to 1.5 mc. It tunes the r-f stage and variable i-f coilsL116 and L118 when receiving in the range 1.5 to 2.5 and 2.5to 3.5 mc. When receiving in the range 3.5 to 30.5 mc, thisslug table tunes only the variable i-f coils L116 and L118.During tuning, positions of the slug tables are varied by asystem of gears and cams; see figure 4-2. On band 1 (0.5 to1. 5 mc) coils L101 and L110 are tuned through thisfrequency range by the main tuning knob through shafts A, B,C and E. On bands 2 and 3 (2.5 to 1.5 and 3.5 to 2.5),tuning is done by the main tuning knob through the sameshafts - A, B, C and E. On band 4 to 7, the main tuning knobtunes coils L104, L107 and L111 over one-fourth of theirtuning range through shafts A, B, C and D and thedifferential shafts.

The BAND CHANGE knob moves this samerack through shafts G, C, D, and the differential in foursteps. Each step is equal to one-fourth of the coils' tuningrange and the shafts are positioned by means of the detent.Thus L104, L107, and L111 are tuned in one megacycle stepsby the BAND CHANGE knob, and between these steps are tunedby the main tuning knob. On bands 8 to 15, coils L105, L108,and L112 are tuned through shafts A, B, C, D and thedifferential. Bands 16 to 30 are also tuned through shafts A,B, C, D and the differential. Each of the two variablefrequency i-f channels covers one megacycle range and istuned by means of the main tuning knob through shafts A, Band E. The proper channel is selected by the BAND CHANGEknob through shafts G and K.

FREQUENCY INDICATION - The band on which the receiver isoperating is indicated on the drum dial that is rotated bythe BAND CHANGE knob through shaft G. The 100 kc divisionsare indicated by a pointer on the slide rule dial. Thispointer is driven from the main tuning knob through shaft A.Kilocycle divisions are indicated by the plastic dialmounted on shaft A. Two scales are necessary on this dialbecause bands 2 and 3 run in opposite directions. Mechanicalstops are mounted on the control shafts to preventover-travel.

ELECTRICAL DESCRIPTION

GENERAL - Collins 51J-4 Receiver is a complete coveragesuperheterodyne receiver capable of AM and CW reception inthe frequency range of 0.5 to 30.5 megacycles. The setcovers the tuning range in 30 bands, each band one megacyclewide. Various portions of the tuning spectrum use single,dual and triple conversion. Three stages of intermediate-frequency amplification and a crystal filter produce thedesired degree of selectivity. The receiver also features alow impedance AVC, a good noise limiter, two stages of audioamplification and a 100 kc frequency spotter or calibrator.

The receiver employs dual conversion on most bands andsingle or triple on others in order to obtain full coverageeconomically with a minimum of image and other spuriousresponses on all bands. Band 1, 0.5 to 1.5 mc uses tripleconversion, bands 2 and 3, 1.5 to 3.5 mc, use singleconversion, and bands 4 to 30, 3.5 to 30.5 mc, use dualconversion. Each band is numbered on the band's centerfrequency. For instance, band 1 covers 0.5 to 1.5 mc, band 2covers 1.5 to 2.5 mc, and so on.

On band 1, where triple conversion is necessary, anintermediate mixer is employed between the first and secondmixers used in the regular dual conversion scheme. The 0.5to 1.5 mc carrier on band 1 is fed to the first mixer whereit is beat against a 12 mc signal from the h-f crystaloscillator to produce an 11.5 to 10.5 mc signal. This signalis beat against an 8 mc signal in the intermediate mixer toproduce the variable i-f or 3.5 to 2.5 mc. The variable i-fis then combined with 3 to 2 mc variable oscillator outputto produce the fixed 500-kc if. On bands 2 and 3, the 1.5 to3.5 mc carrier is fed directly to the second mixer where itis combined with the same variable oscillator output toproduce the 500-kc fixed i-f. On bands 4 to 30 the regulardual conversion scheme is employed. On the even numberedbands the signal frequency is beat against the highfrequency oscillator output to produce a variable i-f of 2.5to 1.5 mc. On the odd numbered bands a variable i-f of 3.5to 2.5 mc is produced. The variable i-f is then combined inthe second mixer with the vfo output to produce the 500-kcfixed i-f. The detailed operation of the various receivercircuits is outlined in the following paragraphs.

RADIO FREQUENCY AMPLIFICATION - One stage of radio frequencyamplification is used on all bands. See block diagram,figure 4-1. The circuit is a conventional r-f amplifiercircuit employing a type 6AK5 miniature r-f pentode, V101.This tube is used because of its low noise and goodsensitivity characteristics at high frequencies.

The control grid of this stage is tuned on all bands, thetuned circuit being selected by r-f switch, S103. Theantenna is capacitively coupled to the tuned circuits in thecontrol grid through r-f switches, S101, and S102.

When operating in the American broadcast band (band 1), theplate circuit of the r-f amplifier is impedance-coupled tothe grid circuit of the first mixer by resistor R105 andcapacitor C117. On bands 2 and 3 the plate of the r-famplifier tube is switched directly to the primary coils ofthe variable i-f tuner, where additional selectivity isobtained. Single conversion is used on these bands. Whenoperated on bands 4 to 30, the plate circuit is tuned andcapacitively coupled to a corresponding tuned circuit in thegrid of the first mixer stage.

The r-f coils and associated trimmers in the plate circuitare selected by the BAND CHANGE knob and tuned through thevarious band ranges via the slug table arrangements. The r-fcoils for bands 1, 2, and 3 are mounted on the variable i-fslug table which is at the extreme right hand edge of thereceiver as viewed from the front. See figure 5-1. The coilsfor bands 4 to 30 are clustered at the rear of the chassisand are tuned by slugs mounted on the three r-f and mixerslug tables.

MIXER STAGES

(a) FIRST MIXER - The first mixer stage uses a type 6BE6miniature pentagrid converter tube, V102. This stage is usedon all bands except bands 2 and 3, where only one conversionstage is necessary.

The grid 1 circuit (pin 1) receives the r-f signal from ther-f amplifier stage. On band 1, this grid circuit is tunedby L110, C118, and C119, and impedance coupled to the plateof the r-f amplifier through C117 and R105. On bands 4through 30, the circuit is tuned by the proper coil andtrimmer groups selected by the r-f switch S104, andcapacitively coupled to corresponding tuned circuits in theplate of the r-f amplifier stage.

The grid 3 (pin 7) input is obtained from the plate of thehfo (V105). On bands 4 through 30, the frequency of theheterodyning signal applied to this grid is such as toproduce an output frequency which falls in one of the twovariable i-f ranges, (2.5 to 1.5 mc or 3.5 to 2.5 mc),depending on which of the bands between 4 and 30 is beingoperated. On band 1, a 12-mc heterodyning signal is appliedto this grid, the output of the stage then being in therange of 11.5 to 10.5 mc, which is again heterodyned in theBand 1 Mixer.

The plate output frequency of this stage is then shown to bein the variable i-f spectrum on bands 4 through 30, and theoutput applied directly to the tuned variable i-f coils. Onband 1, the plate circuit is tuned to the range of 11.5 to10.5 mc by components L114, L115, C139, and C140, and theoutput applied for further conversion to the Band 1 Mixer,V103.

(b) SECOND MIXER STAGE - The second mixer stage, V106, alsoemploys a 6BE6 miniature converter tube. Input to this stageis always either 3.5 to 2.5 mc or 2.5 to 1.5 mc from thevariable i-f coils L116/L118 and L117/L119. The 3 to 2 mcoutput of the permeability tuned oscillator is fed into thesecond mixer tube at grid number one to heterodyne againstthe input signal to produce a 500 kc intermediate frequency.This mixer stage is always used for all bands.

(c) THIRD MIXER STAGE - The third, or band 1, mixer stage isused only when receiving on band 1. A type 6BE6 miniatureconverter tube is used in this application also. Grid number3 of this tube is excited by a 11.5 to 10.5 mc signal fromthe plate circuit of first mixer tube V102, and grid numberone is excited by a heterodyning 8 mc signal from thecrystal oscillator. The output of the third mixer is then3.5 to 2.5 mc, which is then fed to the grid of the secondmixer through the variable i-f coils. This, of course, takesplace only when receiving on band 1 as this stage is notused on the other bands.

Collins Filter Company

HIGH FREQUENCY OSCILLATOR - The high frequency oscillatoruses a 6AK5 miniature pentode tube in a piezoelectricoscillator circuit. No tuned coils are needed to make thecircuit oscillate because in phase feedback voltage isproduced across r-f choke L120. Ten quartz crystals are usedto control the frequency of the oscillator output for thevarious bands. At the minimum, each crystal is used for twoadjacent bands, i.e. 1-2, 3-4, 5-6 and so on, since thecrystal switch S109 changes position only on odd-numberedbands. For instance, the 8 mc crystal used for bands 5 and 6is also used for bands 13 and 14 by utilizing its secondharmonic at 16 mc. In those instances where harmonicoperation is used, a tuned circuit picks off the correctharmonic. This tuned circuit is in the plate circuit of thehfo, V105, and consists of the section of coil L121 in thehfo plate circuit and a number of tuning capacitors. Thelatter are selected by switch pie S108. The circuitconsisting of the section of L121 in the grid circuit of theBand 1 Mixer and capacitors C144 and C145, is tuned to 8 mcand is used when operating on band 1 to furnish the Band 1Mixer with an 8 mc heterodyning signal (second harmonic ofthe 4- mc crystal). At the same time, the other section ofL121 and associated trimmers is tuned to 12 mc (thirdharmonic of the 4-mc crystal) to furnish the first mixerwith the required 12-mc heterodyning signal.

VARIABLE INTERMEDIATE FREQUENCY - The variable intermediatefrequency section consists of two channels, one for afrequency 2.5 to 1.5 mc and the other for 3.5 to 2.5 mc. The2.5 to 1.5 mc i-f is used on the even numbered bands whichemploy double conversion, and the 3.5 to 2.5 mc i-f is usedon the odd numbered bands which employ double conversion.The 2.5 to 1.5 mc i-f is also used on band 2 as anadditional tuned r-f circuit. The 3.5 to 2.5 variable i-f isused on band 3 as an additional tuned r-f circuit and onband 1, in the usual application, as a variable i-f for theodd numbered bands. Using two variable i-f channels in thismanner cuts in half the number of crystals needed by thehigh frequency oscillator, since each crystal's fundamentalfrequency or useful harmonic is used for two bands.Inductors L116 and L118 form the lower frequency i-f coils(2.5 to 1.5) and are the coils in which the tuning slugtravels. The 3.5 to 2.5 mc i-f is obtained by shunting L117across L116, and L119 across L118 to lower the inductancesof L116 and L118. Switch sections S110 and S111 alternatelyswitch the shunting coils in and out as the BAND CHANGE knobis rotated. The variable i-f coils are in the grid of thesecond mixer stage.

Collins Mechanical

VARIABLE FREQUENCY OSCILLATOR - The receiver circuitsdescribed so far have the function of receiving the spectrumin 1 megacycle bands that are presented to the grid of thesecond mixer. The scheme for obtaining high stability iscompleted by a method of heterodyning the signals to a lower,fixed intermediate frequency. In this application, a highlystabilized 3 to 2 mc permeability tuned oscillator, Model70E-15, is employed to heterodyne against the 2.5 to 1.5 andthe 3.5 to 2.5 mc output of the variable frequency i-f. Theresulting 500 kc signal is amplified by the 500 kc i-famplifier.

The coil in the oscillator is cam wound to produce extremelylinear frequency change with linear movement of the tuningslug. The circuit is temperature-compensated and thecomponents are sealed against changes in humidity. Ten turnsof the oscillator lead screw produce a linear frequencychange of one megacycle. The inductance of the oscillatorcoil is trimmed by an iron core series inductor, the valueof which is adjusted at the factory and sealed. A type 6BA6tube, V002, is used for isolation purposes following theoscillator tube and is an integral part of the oscillator.For stabilization purposes, the supply voltages for theoscillator unit are regulated by V116, a type OA2.

CRYSTAL FILTER - Selectivity of the 51J-4 Receiver isimproved greatly by use of a crystal filter in the 500-kc i-f channel. The crystal filter circuit consists primarily of500-kc i-f input transformer T101, a 500-kc crystal, and ahigh impedance tuned circuit T102, connected as shown infigure 4-4. When SELECTIVITY switch S114 is in position 0the crystal is shorted and T101 is connected directly toT102. Thus there is no crystal filter action when S114 is inposition 0; selectivity is determined by the receiver'stuned circuits alone. When S114 is in any other position,crystal filter action takes place-position 4 giving thegreatest selectivity.

To analyze the operation of this circuit consider only theloop containing T101 secondary, crystal Y112, and tunedcircuits T102. Assume that S114 is in position 1. See figure4-5. The secondary of T101 is a low impedance coil with agrounded center tap. The primary of T101 is tuned to 500 kc.Consider crystal Y112 in series with T102 as a voltagedivider, grid voltage to V301 being taken from the pointbetween Y112 and T102. For an i-f of exactly 500 kc,impedance of the crystal is very low-of the order of 2000 to4000 ohms, and the impedance of T102 is of the order of 100,000 ohms. Thus, at 500 kc practically all the voltageappearing across T101 secondary is fed to the grid of V301.

For frequencies a few kilocycles further away from 500 theimpedance of the crystal increases greatly. When the crystalimpedance equals that of T102, only one-half the voltage onT101 secondary appears on the grid of V301. As the crystalimpedance becomes still greater, the voltage appearing onV301 grid decreases. This results in a narrower i-f responsecurve, or in greater selectivity, than that obtained withoutcrystal filtering. Switching S114 to positions 2, 3, or 4merely shunts T102 with resistance, which effectively lowersthe impedance of T102 for those positions. This results in amore rapid decrease in V301 grid voltage as the i-f deviatesto either side of 500 kc. Hence, as the effective impedanceof T102 lowers, selectivity increases. In the sharpestposition the bandwidth at 6 db down is from 200 to 300 cps.

The primary purpose of PHASING capacitor C188 is to producea controllable rejection notch in the i-f response curve sothat unwanted heterodynes may be tuned out. The section ofC188 connected to the bottom end of T101 secondary providesa capacitive path around the crystal that balances out theshunt capacitance of the crystal in its holder and externalcapacitor C187. Varying C188 either side of the balancepoint varies the antiresonant frequency of the crystalcircuit within 3 kc either side of 500. Since the impedanceof the crystal circuit at antiresonance is extremely high,the crystal filter rejects signals at the antiresonantfrequency. Thus at antiresonant frequency points, thephasing action gives a sharp dip in response and theselectivity curve takes on a notch as illustrated in figure4-6.

In order to avoid detuning tuned circuit T102 when varyingC188, a second section of C188 is shunted across T102. SinceC188 has a split stator and a single rotor, the total shuntcapacitance across T102 remains practically constant as thesetting of C188 is varied.

SECOND INTERMEDIATE FREQUENCY - The second intermediatefrequency channel is fixed-tuned to 500 kc. It consists ofthe mechanical filter and four amplifier stages and employs6BA6 tubes in all stages. Input tube V301 is excited by thecrystal filter output coil T102. Permeability-tunedtransformers, with output taps taken off the secondary coilsnear the ground end, are used in the amplifier portion. Bothi-f amplifiers stages and one tube in the mechanical filterare supplied with AVC voltage. Plate and fixed screenvoltages are controlled by the ON-STANDBY-OFF switch and theremote operation relay, K101 on all 500 kc i-f tubes exceptV302. These voltages are removed to render the receiverinoperative during transmission periods.

DETECTOR - The detector in the 51J-4 Receiver consists ofone half of a 12AX7 dual triode tube, V110 (pin numbers 6, 7,and 8). The tube is used as a diode, with rectificationtaking place between the plate and cathode, the grid beingconnected to the plate. R150 and R151 serve as loadresistors for the detector while C202 provides r-f filtering.

NOISE LIMITER - A series type noise limiter is used in the51J-4 Receiver. This limiter employs one-half (pins 1, 2,and 3) of a type 12AX7 dual triode tube, V112. Refer tofigure 4-7. Due to a-c loading of the second detector, heavynoise impulses are automatically clipped from the positiveaudio peaks in the detector. The noise appearing on thenegative side of the audio cycle is clipped by the noiselimiter. In operation, a negative voltage produced byrectification of the carrier is developed across capacitorC205C. This voltage cannot change rapidly due to the valueof C205C and R152. This negative potential is placed uponthe cathode of the noise limiter tube through R153. Thecathode is then negative with respect to the plate of thenoise limiter tube, due to voltage divider action of R150and R151 and current flows. This current is modulated by theaudio which then appears on the noise limiter cathode towhich the grid of the audio amplifier section of V112 isconnected. The noise limiter diode will conduct as long asthe cathode is negative in respect to the plate. However,should a heavy noise impulse be received, the plate would bedriven negative faster than the cathode could follow due tothe time constant of R152 and C205C. If the plate is drivenmore negative than the cathode, the tube will cease toconduct and no audio will reach the grid of the followingaudio tube. The audio cannot reach the cathode of thelimiter tube directly from the diode load because of thefiltering action of R152 and C205C. The value of modulationat which the limiter clips can be adjusted by changing thevalue of some of the components in the circuit. In thisreceiver, limiting starts between 50% and 85% modulation.Switch S116 bypasses the signal around the noise limiterwhen receiving conditions do not require its used.

AUTOMATIC VOLUME CONTROL - The problem of blocking that iscreated by strong signals or heavy static is eliminated byuse of an amplified AVC system and a low impedance AVC line.Refer to figure 4-8. The second triode section of V110 isused as an AVC rectifier to produce control voltage for theAVC amplifier which uses one half of dual triode V111. TheAVC voltage that is applied to grids of the controlled tubesis produced when plate current flowing through one-half ofAVC amplifier tube V111 causes a voltage drop acrossresistor R146. Plate voltage for the amplifier half of V111is obtained from the voltage drop across resistors R165 andR166, which are in series with the center tap of the powertransformer to ground. However, V111 will not draw platecurrent when there is no signal input to the receiverbecause of approximately 11 volts of bias that is placedupon its grid by the voltage drop through R164. This biasvoltage for V111 is taken from the end of R145 through whichthe rectified carrier flows in opposition to the biasvoltage. Thus, when the rectified carrier becomes strongenough to overcome the bias voltage on V111, V111 will drawplate current and produce a voltage drop across R146,thereby producing AVC voltage in proportion to the strengthof the received signal. The bias on the grid of V111 is highenough to produce a delay in the generation of AVC voltageand thus allows the receiver to function at full sensitivityon weak signals. Resistor R144 and capacitor C205B form thetime constant in the AVC circuit. R171, C208, and R167 areused in a degenerative circuit to prevent the AVC amplifiertube from responding to low audio frequencies. AVC is turnedoff by opening the plate circuit of AVC amplifier tube V111.Tubes controlled by AVC bias include the r-f amplifier V101,and the 500 kc i-f amplifier tubes, V301, V108 and V109.

AUDIO AMPLIFIER - Two stages of audio amplification areemployed in the 51J-4 Receiver. The first stage utilizes thesecond triode section of V112 in a resistance-coupledamplifier arrangement. A type 6AQ5 miniature pentode poweramplifier tube is used in the audio output stage. This stagehas fixed bias obtained from the voltage drop producedacross R166 in the center tap lead of the high voltagetransformer secondary. The secondary of the audio outputtransformer has both 600-ohm and 4-ohm outputs. Both theoutputs are terminated on the rear of the chassis atterminal strip E102. Plug-in connections to both outputs arealso made on the front panel.

50 OHM I-F OUTPUT - One-half of dual triode V111 supplies50-ohm 500-kc i-f to coaxial connector J104 on the rear ofthe chassis. This section of V111 is used as a cathodefollower. Excitation is obtained from the voltage dropacross R178, which is connected in a series circuit acrossthe secondary of i-f transformer T105.

100 KC CALIBRATOR - This calibrator is included with thereceiver for use when extreme accuracy of calibration in theorder of 200 cycles is desired. It is coupled to the grid r-f amplifier tube V101, and is made operable when CALIBRATORON-OFF switch S111 is turned on. The calibrator utilizes a6BA6 tube in a piezoelectric circuit, a low drift 100 kccrystal between the control grid and screen, and a 5-25 uufcapacitor C169 between grid and ground. The capacitorpermits the making of small frequency corrections that setthe calibrator to zero beat with a primary frequencystandard. Variable capacitor C224 on the front panelprovides for fine adjustment of frequencies.

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