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(c) 1999,2015 Peter McCollum

RT-3 Variants

 

The RT/A-3 Transmitter, a part of the BN-2

[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:

 

RT/A-3 transmitter. Note the missing bandswitch control, the modified crystal socket, and the yellow stripe on the case. Author's collection.

 

The RT/D-3 Transmitter

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. Author's collection.

 

The RT/E-3 Transmitter

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.

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. Author's collection.

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