Flex 6400 - 35 dB change in noise floor like something broke - intermittent at first

Michael Wolff

While operating FT8 on 160 meters, the noise floor on receive while connected to my inverted L antenna on antenna jack 1 started to flake out and shoot from approximately -105 dB up to -70 dB like a switch was thrown, then back down to -105 dB. This happened intermittently, and eventually stuck where the noise floor was up to -70 dB. Initially, I thought it was an antenna problem. So disconnecting all antennas, and changing bands, this seems to be the case through all amateur bands now, where the noise floor is stuck around -70 dB (almost S9 +20 dB), making the receiver effectively deaf to all but the strongest signals with no antennas connected. 

So, I started chasing other cables and power as the cause - I've got it down to where the only two connections the radio has are the Ethernet cable, and a battery that's not connected to anything else to power the rig, and it's still doing the same thing with no antennas connected.

I noticed that when I cycle through the non-transmitting bands (i.e. 2200 m, 630 m, WWV, and GEN), that the noise floor falls back to the normal levels, less than -100 dB. So, I almost wonder if something has gone awry in the tuner (I have the ATU option in this 6400), or if one of the other recalls for which I haven't sent the rig in yet might be the issue (i.e. heat sinks coming undone or something to that effect if memory serves me correctly).

When I engage the wideband noise blanker in the 'ant' menu, results aren't consistent from band to band, but this brings things back down to the 'norm' that I was experiencing on the lower bands. What I don't understand is the sheer intermittence of the problem I had, when I wasn't tasking any action to change anything in the radio, and letting WSJT-X just do it's thing and work stations.

I know there's a pending mod that I haven't scheduled to send the radio in for yet, but I'm not sure that problem is what is going on here.

Sorry if this should have been obviously covered in another post, but when I searched, I didn't see what I thought matched the conditions I'm experiencing.

Best 73,

Mike - KG4Y

Mike-VA3MW