30 meter mod for the Omni D



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.

Parts list

  • 4x - Type T50-6 powdered iron toroid cores
  • 4x - 56 pF, 50V, C0G/NP0, radial leaded ceramic capacitors
  • 1x - 270 pF, 50V, C0G/NP0, radial leaded ceramic capacitor
  • 1x - 270 pF, 1000V, radial leaded ceramic capacitor
  • 1x - 330 pF, 1000V, radial leaded ceramic capacitor
  • 1x - 560 pF, 1000V, radial leaded ceramic capacitor
  • 1x - 20 to 100 pF trimmer capacitor, mica compression, Arco 406 or equivalent
  • about 45 inches - #18 magnet wire
  • about 50 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. Before proceeding to the low-level driver board and the power amplifier board, the remaining carrier oscillator energy is removed by a 9.00 MHz trap. All other spurs, including the 28 MHz image, are removed by a tuned filter on the bandpass filter board. All amplifiers produce some harmonic distortion, and these harmonics (multiples of the output frequency) are stripped off by a lowpass filter on the lowpass filter board before reaching the antenna output.

Tentec used tuned transformers for the bandpass filters, which featured very light coupling between the two coils. These formed "double tuned, over-coupled bandpass" filters, a wide bandwidth type of filter. This is approximated here by a double-tuned, inductor-coupled bandpass filter formed on three toroid cores. It, like those used by Tentec, has a relatively wide passband, 2 MHz wide. This filter is not selective enough to remove the 9 MHz carrier oscillator, and an LC trap formed on one more toroid core is used to notch it out.

Tentec used standard LC lowpass filters at the power amplifier output, and that filter for 10 MHz is approximately reproduced here.

Performing the mod

(Click any image for an enlarged view)

Building the bandpass filter

Bandpass filter schematic:

The bandpass filter contains three wound toroidal cores. There are numerous instructions on winding available on the web.

Two cores are wound with 23 turns of #24 magnet wire. This number of turns fits easily on a T50 size core, so a larger size wire was used. Any magnet wire from #22 to #26 would probably work ok. If an LCR meter is available, the target inductance is 2.4 uH.

One core is wound with 59 turns of #32 magnet wire. This size wire was necessary in order to fit that amount of wire on the toroid. #30 magnet wire might work, but do not use wire wrap type wire - the thick insulation guarantees that it will not fit! Target inductance is 15 uH.

The T50-6 toroidal cores, with their windings: img0014a.jpg

The cores are stacked on a piece of a zip-tie and hot-melt glued in place. A separation between the cores, about equal to their own width, is provided between them. The 15 uH inductor, with 59 turns, is stacked in the middle.

The cores, stacked and glued to a piece of a zip-tie: img0016a.jpg

The 56 pF capacitors are paralleled to total 112 pF, and the filter is wired as shown in the schematic. One side of the outer inductors is chosen as ground, and the taps are installed at the required number of turns up from this grounded end.

Building the 9 MHz trap

One T50-6 toroid is wound with 15 turns of #24 magnet wire. Target inductance is 1.0 uH for this toroid. This inductor is paralleled with a 270 pF, 50V capacitor and a 20-100 pF trimmer capacitor.

9 MHz trap schematic: 9_mhz_trap_-_schematic.jpg

The 9 MHz trap: 9_mhz_trap_-_photo.jpg

Adding the bandpass filter and 9 MHz trap to the board

The stack is hot-melt glued to the bandpass filter board. It is attached vertically in approximately the same position as the Tentec transformer can for the original mod. There is a resistor which must be moved - see the Tentec instruction sheet. Both of these filters must be kept out of the way of the bandswitch shaft, which is nearby when the boards are re-installed in the radio.

Bandpass board, bottom side, showing pads for 30m mod. At left, output. At the center, ground. At right, input: 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

The 9 MHz trap is inserted in series with the input of the bandpass filter. On the prototype, it was hot-melt glued to an open area of the adjacent RX trimmer board, in such a way that the trimmer capacitor was easily accessible.

Mounting location for the 9 MHz trap on the RX trimmer board:

Adding the lowpass filter to the board

The lowpass filter is built directly on the lowpass filter board. The two inductors are wound from #18 magnet wire - 16 turns, close-wound over a 5/16-inch drill bit. The target inductance is 0.9 uH for both inductors. 1000V capacitors are used in the filter, as it will be handling the full output power of the rig. There is a jumper wire which needs to be removed - see the Tentec instruction sheet.

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

Lowpass board, top side, showing finished 30m filter mod. Right, a 330 pF cap. Center, a 560 pF cap. Left, a 270 pF cap. The 2nd leg of the caps is soldered to the ground plane on the top side of the circuit board, as directed on Diagram 1 of the Tentec instructions. Near the center, the two inductors : img0034a_-_edit.jpg

Alignment and Testing

A general-coverage receiver with a BFO will be needed for alignment of the 9 MHz bandpass filter. Adjust the capacitor with a non-metallic tool only! A metallic tool will introduce a large amount of noise into the circuit, which will pass directly to the power amplifier. Adjust for minimum signal at 9.000 MHz. The rig may or may not need to be connected to an antenna, and may or or may not need to be transmitting (at low power) to hear the signal. This is the only alignment required. Bring up power slowly, with an eye on the SWR meter. High SWR into a resonant antenna or a tuner is an indication that the filters are not working properly and large spurs are being transmitted. With the general coverage receiver, listen to the 2nd harmonic at 2x the transmitted frequency. It will probably be audible, but should be relatively weak.

QR Code
QR Code omni30mmod (generated for current page)