Getting the Utmost From the AR-88.pdf

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82 THE SHORT WAVE MAGAZINE April. 1951and. working with the receiver turned on itsleft side, to tune the top and bottom trimmersat the same time. With care and attention tothe curve centreing and amplitude, it is quitea straightforward job, but one which does takesome time. Unless the receiver has been badlymis-used, no trouble should arise from the IFcomponents, as the writer has yet to find thefirst fault in this respect. Do not be misled;nto thinking the IF transformer is a "dud."It is almost always your tuning which is atfault!Testing IF Sensitivity. Remove oscilloscopeand wobbulator leads from receiver. Connectmatched output meter to output terminals atback of receiver. Set signal generator at 455kc and feed through .01 flF condenser to pin 5of mixer as before, and-with selectivity switchin pos. 3-rock the signal generator tuning togive maximum output on meter, then switchselectivity switch to pos. 2 for future readings.Set modulation on signal generator at 30% andfeed in sufficient signal to bring output meterreading up to one watt. Read off the inputon the scale of signal generator. This maybe anything from 600 to 1000 microvolts onthe first test. A note should be made of thisfigure.Now take out the first and second RF valves(6SGTs) and removing each of the 6SG7 IFvalves in turn, substitute one or other in turnof the RF valves. This should show on theoutout meter if the valves are all of a standardefficiency. It a substitution results in an increasein output, leaving the valve which causedthe impravement in circuit. Should sparevalves be available they should be tested inturn, to obtain the best team in the IF stages.It better valves have been found for the IF'sthe input required from the signal generatorwill, of course, be smaller. to give one wattoutout. The IF's should now be trimmed upwith the substituted valves in use. using theoscilloscope as before, and the input requiredfor one watt output again measured. Thisshould be lower than the previous figure. butmay still be around 600 to 700 microvolts. Toimprove this figure still further remove the6SA 7 melal mixer valve and substitute a 6SA 7glass type. If one or two of the glass ones areavailable. try them in turn. not forgetting tore-align with each change.The result in almost every case is that therequired IF input for one watt of output willdrop frolll aboul 600 microvolls 10 belween 80and lOO-which is a considerable gain .The IF efficiency measurement is alwaysmade with selectivity switch in pos. 2. butconstant checks of signal frequency are required,to be made by switching to pos. 3and rocking generator frequency.Once this figure is obtained with the curvescentral on all positions of selectivity switch.the IF's can be sealed by painting the trimmerspindles with a spot of shellac to preventaccidental rotation.If any difficulty is found in adjusting a particularIF transformer, the signal generatorshould be fed through the grid of the preceding.IF valve and the stage roughly aligned: thesignal generator is then returned to pin 5 of the6SA 7 for final alignment.Crystal Phasing adjustmelll. Connect a highresistance sensitive DC voltmeter to point Con last IF transformer. Feed signal generatortuned to 7 kc off IF resonance to pin 5 of6SA 7 mixer, and adjust C75 for minimumresponse.Adjustment Crystal Load Circuit(I) With set-up as for IF alignment andselectivity switch on pos. 3, rock signal generatorthrough IF frequency and adjust L34 forsymmetrical round-topped curve. (2) Withselectivity switch in pos. 4 adjust C81 forsymmetrical curve. (3) With selectivity switchin pos. 5 adjust C80 for symmetrical curve.rocking signal generator as for I and 2.Wave Trap Adjustmetll. With range switchon Band I, feed a modulated signal of 455 kcinto aerial and ground and adjust L57 on backof chassis for minimum output on meter orspeaker.This adjustment should be made before finalRF alignment.I 2 3 45 ' 54 3 2 "'
J·olulIle XI THE SHORT WAVE MAGAZINE 83The.AR88 with tbe lid open, showing chassis layour and pla cement of main parts. The accompanying article s uggests how therecenercan be checked and adjusted for nlaximunl pcrfonnance on the amateur bands. This should not he undertaken withoutp roper test equipment and experience in its use.RF Alignment (Stage .t)Connect matched output meter to outputtermmals of receiver, then connect signalgenerator with 30% modulation \·ia a 200-ohmresistor to aerial and ground. (Shorting linkfrom other aerial terminal to ground is kft illposition.) The manufacturers ~recornmend thatthe meter be connected across the voice coilof the speaker. but the figures quoted later inthiS article refer to readings on J. i\1arconimatched output meter without speaker Ic'ad .Control ~t.,~ett i" gsTOlle COll1ra/AIIICfIf//l TrimlllerOn-nf] .\'11,//('11Range S'II"itchRF Caill:1udio Caincl VC5iefe("lil'i/\' SI\'ifchFu ll y clockwisL'Adjustt:d for rH:!X . outputat Hr t'nJ 01 ,,',1.: 11h; .lIld ill tum.Rl"c. ~'Lh" !.To hand hr.: ln ~ ~t! i~nl"dMax.M::tx.Pusitilln ..J..Pu"iti(l[iBegin alignment on Band 6 by feeding 30.5me from signal generator. T une receiver formaximum output and adjust aerial tuning onfront panel for maximum output. Adjust 1stand 2nd RF trimmers C45 and eG8 carefullyfor maximunl output. Then se t signal generatorto 22.5 me and retune receiver for maximumoutput, but do not alter the previous settlllgof aerial tuning on front panel. (This is tunedfor maximum at the HF end only of each bandin turn, and is left so tuned wh~n the remainingadjustments arc made at the LF end of eachband.) Adjust aerial inductance trimmer Ll2(on back of chassis) and 1st and 2nd RFtrimmers L21 and L31 fo r maximum output.Repeat these adjustments at the HF and LFends several times to obtain peak output.The writer obtained 5 watts output for onemicrovolt input consistently at both ends of thishand on a properly aligned receiver.Switch off modulator fr011l signal generatorand read off the noise-Ie\el on meter. The
Page 1 and 2: Volume XI THE SHORT WAVE MAGAZINE 7
Page 3: Volume Xl THE SHORT WAVE MAGAZINE 8

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