Noise in transmitted audio. Noise in microphone.

Rick WN2C · May 30

I get noise in my transmit audio when running 100 watts and higher power. I usually run VOX and it will keep the transmitter keyed until I shut off the VOX. I purchased on 5/19/2026 and have yet to use the radio on any band except 10 meters. I replaced a 6400M with this radio and the only change that was made was the radio. All peripherals mic, tuner, amp, are the same. I believe there is a problem with the radio. I have used a different XLR cable to no avail. I tried the FHM3 hand mic and the audio is weak. I use a Heil PR 781 and have heard noise coming the Mic even though the Bias is turned off, which is why the VOX is held open.

Any thoughts or suggestions?

Rick WN2C · 3:23AM

Well, I think I have solved my problem with one toroid on the mic line. It is a jumper with an 8-pin male foster and 2 lines that come out on the other end. One for the mic and the other for PTT. I got this from FLex when I had my 6300 years ago. I still had the problem on the dummy load even at 100 watts although it seemed intermittent but not when I turned the amp on. The issue is gone on 40 meters where it was more prevalent, so I still have to check other bands.

Arnold Goldman · May 30

i had a problem with the 8600M had to put RF chokes on the mic, tgxl and pgxl cables; look at palomar enfineers they sell RF packages for them;

Rick N0VT · May 31

I've made it a rule to first suspect and rule out RFI anytime something goes wonky when transmitting.

Mike-VA3MW · June 1

I think we need some more details.

can you describe the noise?
Does the problem go away at lower power? If so, it is an RFI issue.
What are you using for an antenna? Is it an EFHW? Is it near the radio? If EFHW, you will need chokes (Mix 43) on both ends of the feedline to keep the RF off the feedline and radio. If an EFHW, I would very highly recommend that you add a couterpoise to the to the antenna to move the RF off the feedline
Do you have Noise on your transmit audio if you do not use a MIC but use PC audio?

Take a moment to answer these and it will help to narrow things down.

John KB4DU · June 3

What Rick said.

Rick WN2C · June 12

These are the results I got during testing. I have short video but can't upload it to here. I have taken out the amp and external tuner. The noise sounds like digital noise. The Bias is turned off, however I get sound that I hear coming from the microphone. Like there is a voltage applied to a dynamic mike when there should be no voltage. The antenna is a wire dipole, Alpha Delta LB Plus. I have been using it for years. I have changed XLR cables. I had a help desk ticket in, but it got closed after it was suggested that i replace the XLR cable. It just seems really strange that just unhooking one radio and putting a newer version of said radio in its place would cause RFI issues when I had non before. I can't use pc audio as I do not have SDR on a computer nor do I use one with the 8400M.

Mike, VA3MW if you want I can send you the video by email if you like.

Mike-VA3MW · June 12

You can always upload the video to YouTube and share the link.

Your next test is to test using a Dummy Load as that will eliminate any antenna system concerns. They are not expensive from DXEngineering if you don't have one. (under $50 if I remember correctly).

KD0RC · 3:54AM

Hi Rick, one other possibility is a ground loop. A 1:1 audio transformer in the mic line (only for dynamic mics) might just make a difference. Amazon sells them as "Ground Loop Isolator" fairly inexpensively. Another thing to try is to unplug the PTT line and just use the MOX button to see if that helps. If it does, then you most likely have a ground loop.

Noise in transmitted audio. Noise in microphone.

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