The driving voltage is tapped down on the driver tank coil to provide a proper match. It was found, experimentally, that a point % turn from the ground end of L3 provided the proper amount of drive.
The audio section uses a 12A X 7A and offers a good match to most of the handheld crystal or ceramic microphones. I used choke modulation because 1 later plan to build a small companion receiver. By using an audio output transformer for the modulation choke, the audio section of the transmitter will also serve as the audio amplifier and output section of the receiver For the present, the 8 ohm winding of the transformer is left open. At 85% modulation the voltage drop across R1 just exceeds the firing point of an NE-2, so the bulb was panel mounted for a visual check of modulation. During operation the microphone gain is advanced until the NE-2 just flickers on voice peaks foi a safe modulation level. Construction
The main chassis is a x 5" x 2" U shaped piece of aluminum. The front panel is the cover from a 5" x 4" Mini-box cut to shape. A 3" x 4" aluminum shield is mounted between the i 2AV7 and the 2E26, The top, back and sides were covered with perforated cane-metal aluminum (not shown).
Under the chassis, the three coils are mounted at right angles to each other and the three tuning capacitors are mounted in line across the front edge- The microphone jack, microphone gain pot, and power plug are mounted on the rear. The receiver antenna connection is also rear mounted. The push-to-talk relay was scrounged from an old surplus computer chassis, along with the small 24 volt transformer to power it. Any 4-poIe double-throw relay would have served just as well and could be powered from a divider across B+. (This, however, would result in B+ appearing on the microphone switch when the transmitter is un-keyed,) A rigidly mounted, preferably silver-plated coil should be used for the output tank coil — the other coils were wound from No. 16 AWG nyclad copper wire.
Tune-up and operation
After the wiring has been completed and checked* plug in only the 2E26 and apply power. The first step will be to neutralize the 2E26. With the meter switch in position 2 (P. A. grid), set the doubler tuning capacitor (c3) to about half range. Turn the P, A. screen switch on. With the microphone in one hand and the other hand on the P. A, tuning capacitor, key the microphone and quickly rotate the P. A. plate tuning capacitor through its entire tuning range. Do not keep the mike keyed for longer than 2 or 3 seconds or the 2E26 may be damaged. If there is a flicker of grid current while swinging the capacitor (it is almost certain there will be on the first try), change the setting of the neutralizing capacitor slightly and try again. When properly neutralized, the 2E26 will show no grid current at any setting of C3 or C4,
After the 2E26 has been neutralized, remove power, turn the P. A. screen switch off and plug in the other tubes. With the meter switch in position 1 (doubler grid) peak CI and C2. The meter in this position has a full scale deflection of approximately 200 volts, so you should get at least half scale deflection. CI and C2 should also be peaked as quickly as possible, since both halves of the 12AV7 depend largely on grid leak bias for protection. Both plates will show color rapidly with the transmitter keyed and the two capacitors not peaked.
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Next, place the meter switch in position 2 and peak C3. (After the set is loaded you will have to come back and re-peak C3 to a slightly lower capacity setting.) After the P. A. grid has been peaked, turn the P. A. screen switch on and put the meter switch in position 3 (relative power). Connect a G. E. No. 46 (blue bead) light bulb to the antenna jack for a dummy load, key the mike and peak C4 and C5. As you peak C5 the light bulb should glow with a'brilliant white light. Turn the meter switch back to position 2 and re-peak C3, The rf section is now tuned and loaded.
White talking in normal tones into the microphone, advance the microphone gain control until the audio light on the front
panel just flickers on voice peaks. The dummy bulb should show an appreciable increase in brilliance when you talk into the microphone. To load into an antenna, simply peak C4 and C5 with the meter switch in position 3,
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