[TenTec] OMNI-C BFO alternative alignment

[Marinos Markomanolakis M.D.]

Some thoughts on aligning the Omni-C BFO without an oscilloscope or RF mV meter.

Due to aging, after some years, the BFO crystal will drift some in frequency and realignment is in order. Of course you may use the instructions of the manual, but an alternative method can be as follows:


  1.  Connect the receiver to the antenna and listen to the white noise pitch at a clear frequency spot. Put the MODE to SB-R and slightly move C3 either way (Generator board). You will notice a change in the white noise pitch, one way will increase low audio frequencies and sound "duller" while moving C3 in the opposite direction will make the sound crisper. Find the best position to equalize this Hi-Low  audio response. This can be done by ear or by connecting the audio output to a computer while running an audio spectrograph program.

  2.  Tune at the frequency of a broadcast station eg BBC, while still on SB-R position. The digital readout of the rig should read exactly 3Khz higher than the actual transmitting frequency of the station. If not, adjust the counter time base trimmer so that the readout is reading exactly 3Khz UP. (even with the 100Hz resolution, you can have good accuracy when the last 2 digits just stabilize from 9.9 to 0.0).

  3.  Switch Rx to MODE SB-N while still listening to the broacast station. The readout of the radio should change to indicate the exact transmitting frequency of the station. If not, adjust trimmer C4 to achieve this with an identical process as above. (The time base of the counter is NOT readjusted).

  4.  While listening on another radio, transmit on a dummy load with the OMNI and zero beat your transmission by the audio feedback from the mic of the radio picking up its transmission from the monitoring receiver.

  5.  Put MODE to LOCK position while still on a dummy load, advance drive level to achieve comfortable listening volume from the monitoring RX. A tone approximately 750Hz should be heard. The exact frequency of this audo tone can be checked by heterodyning it with the tone produced by a computer running this program. https://www.szynalski.com/tone-generator/ (no connections are required between the monitoring RX and the OMNI, just listen to both tones from the computer and the RX at approximally equal volume.)

  6.  If the audio tone heard in step (5) is not exactly 750Hz, adjust C5 to make it so.

  7.  In case the C5 needed to be adjusted in the previous step, repeat step (3) touching up C4.

  8.  Finally verify that conditions (2), (3) and (4) do ALL apply. If you find slight differences, touch up the respective trimmers to achieve it.


The above process may appear too compicated, but once you understand how its different steps work, it is extremely easy to reproduce. Please refer to the circuit description of the SSB generator from the OMNI manual.


The final result is a balanced (almost* symmetric) audio response between LSB – USB in RX and TX, perfect CW TX tone and last but not least, being able to be certain with a 10Hz accuracy of your operating frequency.

Since the PTO of the 546 is extremely stable after 30 min warm up, you can even detect slight errors in the transmitting frequency of a station while on QSO, even if he is using a modern synthesized radio.


*Absolutely symmetric audio response is almost certainly not obtainable due to the different slopes of the crystal filter response at their hi and low skirts.

Thoughts and suggestions are welcome.

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