Getting the Utmost From the AR-88.pdf

84 THE SHORT WAVE MAGAZINE April. 1953reading should be below one watt. but if itis higher remove the 1st RF valve and substitutea 6AC7. or 717A. or 6SH7 for the 6SG7and re-align_ This should reduce the noise considerablywithout alIecting the output withmodulation on generator signal.Band 5. Tune 22.4 mc generator signal onreceiver. adjust aerial tuning and C43 and C66.all for maximum output. Tune 16.4 mcgenerator signal on receiver and adjust LlO(on back of chassis) and L20 and L30. all formaximum output. Repeat several times. Outputshould be between 4 and 5 watts for onemicrovolt input.Band 4. Tune 16.4 mc generator signal onreceiver and adjust aerial tuning and C41 andC64. all for maximum output. Tune 12.2 megenerator signal on receiver and adjust L8(on back of chassis) and Ll9 and 29 for fulloutput. Repeat several times. Output shouldbe 3.5 to 4.5 watts for one microvolt input.Band 3_ Tune 11.5 me generator signal onreceiver. adjust aerial tuning and C39 and C62.all for full output. Tune 4.6 me generator.signal on receiver and adjust L6 (on back ofchassis) and Ll8 and L28 for maximum output.Repeat several times. Output should be3_0 to 3.5 watts for 1 microvolt input.Band 2. Tune 4.4 . mc generator ' signal onreceiver. adjust aerial tuning and C38 and C60,all for maximum output Tune 1.7 me generatorsignal on receiver. adjust L4 (on back ofchassis) and Ll6 and L26, all for full output.Repeat several times. Output should be 2.510 3.5 watts for one microvolt input.Band 1. Tune 1500 kc generator signal onreceiver. adjust aerial tuning and C37 and C59.all for full output. Tune 550 kc generatorsignal on receiver and ad just L2 (on back ofchassis) and L14 and L24 for maximum output.Repeat several times. Output should be I to1.5 watts for input of one microvolt.When the receiver ' has been tuned up topeak performance with the modifications mentioned.it may be found that IF oscillationappears. as indicated by unwanted whistles atodd places on one band, or perhaps several.To eliminate this with certainty, the writerspent many hours before finding that thetrouble was caused by the close proximity toeach other of the anode, screen and cathodeleads of the last 6SG7 IF valve. These leadseach go to .05 ... F fixed condensers on thechassis front. and should be cut out andrenlaced by screened leads. as short as possible.with the screening braid well grounded.Since the discovery of this cure, the manufacturershave included secened leads at thispoint . as a standard modification. This willtherefore only apply to earlier models. Yourscan be identified by the main tuning dial.which is of translucent material with figuresin black. Later models (which include themodification mentioned) are provided with dialsof the same material. but with alternate bandsmarked all black with translucent figures.Readers wishing to fit an S-meter (5 mAf.s.d. right hand zero) will find the adjustingpotentiometer in the centre back of chassis.provided with screwdriver slot.The "0" in the AR880 indicates that provisionis made in the receiver design fordiversity reception. R.C.A. manufacture astandard rack to accommodate two AR88D's.and a diversity switching unit which automaticallyselects the most useful signal fromseparate aerials, and feeds this signal to one orother of the receivers. The output is thereforekept as near as possible at a constant level,irrespective of fading in one or other of thesignal paths.So, if you want to keep track of that fadingOX, this is the set-up you require. and thewriter does not know of a better. Beforeorderin,g. however, make sure you have animport licence, a dollar bank balance. and anaerial farm !It is realised. of course. that much informationcan be obtained from the manufacturers'handbook, but the writer has tried to set outhis own results and conclusions, arrived at aftermonths of concentrated study and experiment.together with modifications which the readermay care to try out.MORE ABOUT THE TIFD AERIALT}le article by G2NS in our issue for January, 1953.describing an unusually effective multi-band aerial forconfined spaces developed by W3HH. has aroused agood deal of interest. Many versions of the design arenow being operated by G stations, and following aresome further notes on the TIFD offered by G2NSafter correspondence with the States: The best angle oftilt is 30° and the optimum value of the terminatingresistance is fairly critical for best results, being 390ohms for a 300-0hm feed line. and 650 ohms for a 600-ohm line. These resistors should be non-inductiveand capable of dissipating the power involved. Wirewoundresistors will do, but if they are used the systemas a whole tends to become non-aperiodic and calJsfor changes in the method of coupling when goingfrom band to band. High-wattage wire-wound resistorsare still readily available, but carbon types are preferableif they can be obtained. It has been found in practicethat a three-band version of the T2FD can be loadedup and operated efficiently on five bands.