If you use an external mic preamp, significant improvement in Omni 6 tx audio can be achieved by driving the balanced modulator directly and bypassing the Omni 6 mic preamp circuit.
I appropriated the rear panel Audio Input connector for this purpose. In its stock form, the audio input is in parallel with the mic input, so the same functionality can be had by using a "y" on the mic connector. You lose nothing by re-appropriating the connector and putting it to better use.
- Open the Omni 6 top cover
- Remove the speaker panel (two screws on each side and 3 screws on top) after unplugging the speaker connector from the TX Audio board beneath it
- Remove the TX Audio board - remove four screws (including one near the DB25 connector) and unplug myriad of well marked cables. Remove the board by sliding toward front of chassis (allows rear connectors to clear panel holes) and then upward to clear the aluminum sub-enclosure around the hidden Local Oscillator board.
- C52 is a ceramic disc capacitor connecting the Audio Input jack to R53 47K resistor. Find the C52 solder pad connected to the jack end. Heat the pad and lift that leg of C52 to leave a free hole/pad. Don't mistake it for C53, the ceramic disc that connects to ground.
- Insert one lead of a 1 uF capacitor into that free hole and solder. I used Digikey P/N 399-5860-ND
- Locate R72 10K resistor connected to U4-5, the balanced modulator input. Tack solder one lead of another 10K resistor to the end of R72 connected to U4-5.
- Run a wire from the free end of the resistor you just added to the free end of the 1uF capacitor in previous step.