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www.americanradiohistory.com 448 the difference between the two frequencies present in the grid circuit. By tuning the oscillator circuit this third frequency may he wade equal to that, say 535 kilocycles (560 meters), to which the receiver proper has been tuned preciously and the signal from the broadcast station will be amplified in the receiver and heard in the loud speaker. From the foregoing explanation it is apparent that an installation, which includes a timed -R.F. receiver and the Pre -Selector, really amounts to a superheterodyne receiver; in which the Pre -Selector functions as the so- called "first detector" and oscillator, while the R.F. amplifier of the receiver proper serves as the intermediate amplifier of the combination. FEATURES OF THE CIRCUIT The tremendous selectivity afforded by the Pre- Selector is due largely to the frequency- changing process involved. In addition, the Pre- Selector combination provides better selectivity than the average superheterodyne receiver; because the R.F. amplifier of even a very broad -tuning broadcast receiver is tuned much more sharply than are the coupling transformers ordinarily employed in the intermediate stages of a regular superheterodyne receiver. Regeneration in the detector circuit of the Pre- Selector is not required for the sake of selectivity, but is used solely for r 30 TURNS TAPPED 15?" TURN. #30 D.S.C. PRIMARY 116= DIAMETER. -Id- N IÌI 114 TURNS SPACED %28 D.S.C. SECONDARY` 2 "- DIAMETER ) 64 TURNS *36DSC. TICKLER 11/2."-DIA. LI ri Radio News for November, 1928 47 TURNS Z8 D.S.C. 2 "- DIAMETER 12 -TURN# 36 D.S.0 PICK -UP 14.-DIAMETER 47 TURNS 4128 D.S.G. 2 "- DIAMETER Fig. 4 Details of the coils; the primary of L1 is mounted on a slide, which is moved out of the secondary by a cam on the tuning condenser Cl as the rotor plates rise. the amplification it provides. Ordinarily the regeneration control, C4, is left with its knob set at zero. It is only in the case of reception from very distant stations that regeneration is required and, for such reception, the knob is turned up to a point just below that at which the detector goes into oscillation. The regeneration is ob- R1- SW1 IIIIIIIIIIIIIIIIIII C4, TO FRAME OF CONDENSER Illllllllllllllllr + V2 J' i- IlPnnnnuq m I :,, n'lll ... . 1 1 =°ï' ° Illla IIc : M ' Imïlüüiüiüilddlau nu 1 IIII L2 MOUNTED ON FRAME OF CONDENSER C2 I1 r si aÌÍÍÌÍIIII l I JIM PIPIT !I ailíi.!IiI ii: 11 1 r- !lgtIIIIII SOLDER TO BRACKET A- OUTPUT B+ B- A+ A- SW2 AER. RECEIVER 45V Fig. 5 <strong>Com</strong>plete wiring layout of the "Pre- Selector;" the apparatus is *lightly spread apart in the picture to show the connections. COIL LI MOUNTED ON FRAME OF CONDENSER Cl GND 1
www.americanradiohistory.com Radio News for November, 1928 449 T-1 2 3' N 14' 3/g DRILL 5 HOLES } 5/32 DRILL AND COUNTERSINK, 2 HOLES 3' 1 /g DRILL 6 HOLES 5/32 DRILL 3 HOLES 10' 542 DRILL, B HOLES 3/g DRILL Fig. 6 Drilling details of the "Pre- Selector" panel and binding -post strip. waveband. This a decided convenience, particularly when tuning for weak, distant stations. CONSTRUCTION AND WIRING The diagrams and photographic reproductions provide practically all the data required for the construction of the Pre - Selector; the only point that needs be touched upon is the mounting of the coils. Both LI and 12 are mounted directly on the frames of their respective tuning condensers. L2 should be mounted in a vertical position with the small pick -up coil at the top. A tapped hole will be found in the rear of the frame of condenser C2, and a 6/32 screw passed through the hole in the brass mounting bracket of the coil and screwed into that in the condenser provides a convenient and substantial means for mounting. The curved brass bracket which comes with LI should be mounted on the frame of the coil by means of the two small screws provided. Its position should be that, when the coil is mounted in the position shown in the top view (Fig. A), the slotted lug on the bracket should be toward the panel and pointing down. Next, mount this coupler on the rear of condenser Cl by means of the screw provided. When in proper position, the notch in the lug of the mounting bracket should be astride the rib on the back of the condenser frame. tamed through the use of a feed -back coil mounted in a fixed position inside of the secondary of the input coupler, I.I. It will be noted from the diagram,Fig. 1, that a switch, SW2, has been included in the ground side of the antenna circuit. This switch is required, not for its effect on selectivity but rather to permit the smooth and consistent control of regeneration. Where a very large antenna is employed, with the full primary winding in the circuit, the absorption effect is so great as to make regeneration ineffective. In such a case the switch is set so that only half of the winding is in the antenna circuit, thus reducing the absorption effect to overcome this difficulty. The by -pass condenser C7 has been included in the ground circuit of the Pre - Selector, simply as a safety measure, to prevent any possibility of short- circuiting the "A" current in cases where the same device is used to provide the filament current for both the Pre -Selector and the receiver. The only other circuit feature not covered is that which employs the small adjustable condenser, C3. The purpose of this condenser is to permit enough capacity to be RADIO MEWS FREE BLUE PRINT ARTICLE' No. 68 d set of large blueprints and a list of parts used in the construction of the "Pre- Selector" shown here will be sent postpaid to any applicant. Write to the Blueprint Department; write your name and address and "No. 68" legibly on a sheet of paper separate from letters to other departments. We regret to inform our correspondents, especially abroad, that U. S. postal regulations do not permit sending blueprints with magazines; nor can we undertake to accept blanket requests for "all blueprints." added to the tuned oscillator circuit to cause the two tuning -condenser dials to read alike. When it has been adjusted so that the two tuning controls read alike for a given wavelength, their readings will remain alike throughout almost the entire SUPPLY AND INSTALLATION If it is used with a battery -operated receiver the same set of batteries may be used for the Pre -Selector. Or, if the receiver uses a `B" socket -power unit, that may also be used to provide the plate current for the Pre-Selector. If the receiver employs A.C. tubes, it will then be necessary to provide a set of three standard dry cells to supply the Pre -Selector's filament; one set of batteries will provide approximately 200 hours of actual service. The rheostat (R1) included in the filament circuit of the Pre -Selector is of resistance sufficiently high to permit these tubes to be operated safely from a six -volt source. With the supply voltage the rheostat should be turned on half -way to provide proper operating voltage for these tubes; if the supply source is 4/ volts the rheostat should be turned on three -quarters. When dry cells are used the three -quarters position is correct while the cells are new; but the rheostat must be advanced as the cell voltage drops. In some cases as, for instance, where the "B" socket -power unit is built permanently (Continued on page 483) -BATTERY HOOK -UP WHERE BOTH RECEIVER AND PRE - SELECTOR USE SAME 'A' AND 'B" BATTERIES- BATT 0 0 0 -B- BATT. BATT - BATTERY HOOK -UP WHERE PRE - SELECTOR IS EQUIPPED WITH SEPARATE BATTERIES - TO - (NEG.) TERMINAL 0 0 0 'B BATTERY 45%, - OF RECEIVER 'A' BATTERY. TO RECEIVER 'AER' TERMINAL TO RECEIVER 'GRD' TERMINAL 'B'- TERMINAL MuJSI BE LEFT UNCONNECTED. PRE - SELECTOR A- AER. A+ B+ B+ B+ B- ONO 45V. 90V. 135. OR /BO V. NORMAL CONNECTIONS TO RECEIVER TERMINALS. PRE - SELECTOR TO 'A- TERMINAL # OF RECEIVER NOTE ; THIS CONNECTION IS MADE FOR USE ONLY WHEN RECEIVER FILAMENTS ARE TO BE TURNED OFF AND ON BY SWITCH SI OF PRE - SELECTOR. IN MAKING THESE CONNECTIONS REMOVE NORMAL CONNECTION BETWEEN RECEIVER -A-' AND BATTERY 2. Here the "B -" connection is left unused, to avoid a short across the filaments of tic 199 -type tubes. Fig. 3. Even though separate batteries are used, the connection shown makes remote control of the receiver easy.
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