(c) 1999,2022 Peter McCollum
The RT/A-3 Transmitter, a part of the BN-2 (aka HRT-4)
[See the “Beacons” section for more info on this RT-3 variant]
The RT/A-3 is based on a 'standard' RT-3 transmitter, but with the following modifications:
· The tuning chart plate has been replaced by a plate that reads "RT/A-3 PART OF BN-2 TUNING RANGE 1500-1800 KC". The original RT-3 ID label is still there.
· The band switch has been removed, and replaced with a screw that plugs the hole (to keep the unit watertight). Inside, all of the bandswitch components are gone.
· A fixture has been added to the FT-243 crystal socket: it allows a crystal to be installed in the horizontal position, laying across the BN-2 label plate. The original crystal socket is still usable, and the second socket (for crystals with wider-spaced pins) is still there, also.
· The antenna tuning cap has been replaced by a dual-section 365 pf unit, with both sections wired in parallel for a total of 730 pf. The cap looks like a commercial broadcast radio type, with a compression trimmer on each section. To mount the cap, three holes were drilled and countersunk in the front panel. The original watertight shaft bushing has been mounted upside-down on the top of the panel, and there is a rubber washer under it. This allows everything to remain watertight, but allows the new capacitor to mount close to the panel. However, since the shaft bushing is mounted on the "wrong" side of the panel, the antenna tuning knob is now higher above the panel than the original.
· There is a schematic (marked "RT/A-3 TRANSMITTER") glued to the inside of the case. On one RT/A-3 unit, many points in the schematic have a small hand-drawn check-mark next to them - it's as if the technician was checking off the modifications as he did them, then he glued the schematic in the box when he was finished.
· The ant. current indicator is a #43 lamp, instead of a #47 (this is indicated on the schematic, also). The lamp's parallel resistors are a much smaller value than the original. Most of the rest of the circuit is the same - although the plate-tank is a toroid transformer (two separate windings), and the oscillator tank is also a toroid (single winding). These new toroids are about 1" diameter, and are mounted on plastic standoffs with nylon screws.
· Because of the change in the antenna current lamp circuit, it only glows when the antenna impedance is low (around 20 ohms or less). This probably indicates something about the intended type of antenna for the RT/A-3 (perhaps a bottom-loaded vertical whip?).
· The outside of the case and lid has a 2"-wide yellow stripe painted on it.
· The original code-key is there, and works normally, although the units were adjusted so that the contacts were closed all the time. So, the unit would transmit a carrier as soon as it was powered up.
RT/A-3 transmitter. Note the missing bandswitch control, the modified crystal socket, and the yellow stripe on the case.
The RT/D-3 is a variation of the RT-3 transmitter which includes the addition of a burst-coder connector, similar to the T-784/GRC-109. On the ID label, the "RT-3" has been covered by a small foil sticker that reads "RT/D-3". On the lid, the letters "RT-D-3" have been stenciled in yellow, about 1/2" high. There is an "MWO 39" marking on the panel, just like many RT-3 units. This modification adds an RF choke between the final tank circuit and ground, thus providing a DC path to ground for the antenna. Two examples of this model have been seen so far - both are late-production RT-3 units.
The panel of the RT/D-3 transmitter. Note burst-coder connector in the upper-right, and the "MWO-39" marking in the lower-left.
This is another variation of the RT-3 transmitter. The difference is in the frequency coverage - the "E" model covers 3-30 MC, instead of just 3-22 MC. The bands are 3-5.5, 6-9, 10-16, and 17-30 MC. The original tuning chart has been covered with a new chart (black on white, instead of white on black) to indicate settings up to 30 MC. On the panel, the original "RT-3" marking has been painted over in black, and "RT/E-3" has been painted over it. The "E" model does NOT have the burst-coder connector like the "D" model. Three examples have been seen. A CIA memo in January 1963 states that the cost of the RT/E-3 increased from $150.40 to $185.40, due to a “safety modification” done by an outside contractor.
The reason for adding the 22-30 MC coverage was to allow more propagation options when operating at certain times of day, or over certain distances, where the lower frequencies were not always acceptable.
The conversion of a standard RT-3 to an RT/E-3 is perhaps described by "MWO 43", as this marking is seen on one example. The circuit modifications are as follows:
· Remove 3 turns from L3 (the final tank coil for bands 3 and 4), to extend the frequency range of band 4.
· Reverse the end connections on L1 (the oscillator tank coil for bands 3 and 4). This moves tap closer to the end, to extend the frequency range of band 4.
· Change R6 and R7 from 15K to 2.7K each (the oscillator plate load resistor). This will increase the plate current in the oscillator.
· Replace R8 with a wire (the final grid-leak resistor is now only an RF choke).
· Change C8 from 18pF to 15pF.
Bench testing at about 18 Mhz shows that the RT/E-3 has noticeably higher power output compared to a standard RT-3.
RT/E-3 serial #5724 was found to have a 6AG7 oscillator instead of a 6AC7. Are the two types intended to be interchangeable in this transmitter, or is one a mistake? Since the oscillator runs at a higher plate current due to the modifications, it is possible that a 6AG7 was the intended tube.
The ID markings on an RT/E-3.