Flex 6400 - TX Audio on 15 and 20m Severely Distorted.

Hi Len. I ask about the mic settings as I am trying to solve a problem with phone that appears to only affect 15, 20, and 160m. This Flex 6400 is new to me as of yesterday and I have been using the IC-7300 and FTDX-10 as my primary radios. Digital modes are my current interest.

The antenna setup is a My Antenna's 80-10m fed @ the top (25 AGL) sloping downhill to a 25 foot post. RG8X coax from the antenna to an Alpha Delta 4 position switch. The second antenna is a MyAntenna 9:1 balun w/220 feet of wire. Both antennas perform as expected with reasonable SWR throughout the amateur bands. Both the IC-7300 and FTDX-10 are grounded to a large copper rod hammered into the ground. No hot RF on/near the radios or equipment that I am aware of. Occasionally, my computer monitor will have wavy lines on-screen - very slight and you have to really pay attention to see it. Nothing glaring at all. I can touch coax shield connectors when transmitting without a RFI shock.

Now onto the Flex 6400. It is connected to my laptop docking station through a 6 foot Ethernet cable. The ground cable moved from the IC-7300 to the Flex. Everything else is the same - except for the Flex. Removed the coax from the 7300 & attached it to the Flex ...

I ran through the bands doing FT8/FT4 - no issues. Made a few phone contacts on 10m - audio reports great. 15m is where the problem manifested. My calls went unanswered - many attempts to multiple very strong stations. On 20m, one operator was replying with 'I can hear a signal that is heavily distorted'; he could hear the W in my callsign, then bad hash for the rest of it. My phone signal was unintelligible. When I say bad, I mean real-bad. From the recording, you could not tell a human was speaking - only loud white noise.

My mic setting was FHM-3 default and I found the record feature. Sure enough, my voice was entirely wiped out - replaced by massive hash. Through trial and error, I found that when I turned the power down from 100w to 10w, my voice was clearer though clipping and static were present.

Changing the mic settings seemed to make a difference - slight differences. But anything over 50w on 15 & 20, my transmissions are severely degraded. DAX on or off - no difference. I played with the MIC sliders and only slight changes were heard in my recording. FHM-3 DX appears to be the least-worst of the selections. Again, really only manifesting on 15 & 20m. SWR is clean on these bands, with and without the tuner enabled. If anything, a slight touch-up when enabling the internal tuner.

Moving on to 40m and 100w - perfect. Clean as it should be - 80m too. 20m is more affected than 15m and the other bands are seemingly ok.

A friend has a 6600 and we went through his TX and microphone settings - mine match his exactly. I changed the settings, disabling BIAS and other microphone settings. This had an impact, but did not solve.

So here we are ... as a new Flex owner, I am trying to solve this problem.

My guess is RFI, but from what and where? The IC-7300, FTDX-10, and even my old TS-440 do not exhibit any has on 15 & 20m, only the Flex. Perhaps it is not the MIC settings but something else.

I reset the radio using the published process. Power off, disconnect from power, wait, power on, observe white button, continue ...

Looking for the community to help solve this.