Re: uBitX SSB transmit oscillations on 40m only #ubitx

Howard Fidel

I have seen this problem, but only with my  power amplifier. It is caused by RF getting into the audio stages. The stock audio input is unshielded, you can try replacing the cabling to the uBitx for the microphone with a shielded audio or coax cable. Also, check you lead dress, and make sure the output RF cable is routed no where near the input audio cables. You can also fiter the audio input with a PI filter network to remove the RF, but I would try the above steps first.


On 7/26/2018 3:20 PM, Bruce wrote:
uBitX v3 oscillation and audio from speaker when transmitting on 40m. I'm looking for (1) anyone else who has this whether they have fixed it or not and (2) suggestions on how to diagnose it. I have a 200MHz dual channel analog oscilloscope, an RF-output interface to the 'scope, a power-SWR meter, a dummy load, a 5-band horizontal loop antenna, and a digital multimeter.

Tried all the other bands on SSB with no problem. Clean on CW with expected power out from 12w at 40m and below tapering off to 1w at 28MHz. During 40m SSB oscillation, it is heard through the speaker or headset. On the other bands the audio output is silent. Here is what I have tried and still the oscillations occur:

1. Change microphones; tried 3, oscillations continue with slight level differences between mics
2. Disconnect audio output devices; speaker, headsets
3. Disconnect USB cable extension that runs from Arduino to back of enclosure (saw a comment on that on this site.)
4. Change ground placement on SO-239
5. Connect dummy load directly to SO-239 with no transmission lines
6. Test with metal top on and then off enclosure
7. Test using 12.3v battery and using 13.8v power supply (didn't make sense but why not)
8. Wrapped microphone cord in ferrite clamps at plug and at microphone head
9. Use 1.2:1 40m meter antenna instead of dummy load
10. Check microphone jack wiring, continuity to ground, etc.

I have video and audio of 20m vs 40m if that helps.

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