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30 meter mod for the Omni D

References

Introduction

The World Administrative Radio Conference approved the WARC bands at nearly the same time as Tentec developed the Omni 545 and 546 - 1979. The original Omni series came with 10.0-10.5 MHz receive only. A kit was later offered to upgrade the series to 10.1 MHz (30 meters) transmit and 24.89 MHz (12 meters) receive and transmit. As of this writing (2017), the kit has been unavailable for some time. 18.068 MHz (17 meters) was not offered, because it was a multiple of the Omni's 9 MHz carrier oscillator, and would have had unacceptable spurious output.

This mod is a partial duplication of the WARC kit, for 30 meters only. Tentec encouraged amateurs to perform this mod themselves in the kit instructions. However, as parts are no longer available from Tentec, substitutes were developed. Omni owners contemplating this mod should see the instructions at warc_install.pdf to familiarize themselves with the steps. In particular, the steps listed under the heading "SUBJECT: Modification to permit operation on 10 MHz band only" should be read. The disassembly information given there will not be repeated here. The mod is "no tune", so no alignment is necessary.

Parts list

  • 3x - Type T50-6 powdered iron toroid cores
  • 4x - 56 pF, 50V, C0G/NP0, radial leaded ceramic capacitors
  • 1x - 270 pF, 1000V, radial leaded ceramic capacitor
  • 1x - 330 pF, 1000V, radial leaded ceramic capacitor
  • 1x - 560 pF, 1000V, radial leaded ceramic capacitor
  • about 45 inches - #18 magnet wire
  • about 40 inches - #24 magnet wire
  • about 40 inches - #32 magnet wire

Theory of operation

A 9 MHz carrier oscillator is produced on the SSB generator board. It is modulated with signal information - SSB audio or on/off keying for CW. For the 10 MHz band, VFO output varies from 18.99 to 19.49 MHz. This is mixed with the carrier oscillator on the TX mixer board, producing two outputs: the desired signal from 9.99 to 10.49 MHz, and an image at 27.99 to 28.49 MHz. The 28 MHz image is removed by a tuned filter on the bandpass filter board before proceeding.

Performing the mod

Click any image for an enlarged view.

Bandpass filter schematic:

Bandpass board, bottom side, showing pads for 30m mod: img0025a_-_edit.jpg

Bandpass board, top side, showing finished 30m filter mod (between bandswitch and the shield can at the right): img0032a.jpg img0033a.jpg

Lowpass board, bottom side, showing pads for 30m mod: img0019a_-_edit.jpg

Lowpass board, top side, showing finished 30m filter mod: img0034a_-_edit.jpg


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