[Elecraft] K3 RF Feedback Problem

[4CX250B]

Thanks very much to Joe W4TV and  Ian  GM3SEK  for their helpful comments.Here's what I've learned.

1. RF leakage from the K3 into the 12V power cord does not appear to be a problem, as Ian surmised. Furthermore, as Ian also noted "If the PS is allowed to float and the common negative return for all the accessories is moved very close to the K3, that might solve the problems."  This was largely correct. When I floated the negative return of the 12V supply, the distortion in the audio disappeared, so long as no other accessories were powered by the same 12V power supply. I measured about 200 mV p-p voltage fluctuations between the negative supply voltage and the chassis of the K3, when I talked into the K3 at 100W output.

2. 12V accessories ordinarily have their negative power terminal tied to their chassis ground. This is true of all the accessories in my station, including a homebrew keyer, an Alpha 4510 wattmeter, an EQ Plus audio compander, a 60 stereo amp used to drive bookshelf speakers,and my homebrew station controller. When I connected ANY of these to the same 12V line powering the K3, the distortion returned.  Evidently, the K3's internal circuitry really like to have its local circuit ground isolated from its chassis.

3. A complicating factor is that I use a RIGrunner 12V distribution panel (model 4010S) to distribute 12V around to all the items in the station using standard Powerpole connectors.  I discovered that the  frame of the RIGrunner is internally connected to all its  negative (black) Powerpole connectors, making it difficult to run an isolated negative return on my 12V line.

My fix for all of these is to use two 12V power supplies: a 30A Kepco supply with floating outputs for the K3, and a 6 Amp Lambda supply to power the accessories, via the RIGrunner distribution panel. No other devices in my station, other than the K3, have any problem with the negative 12V line tied to their chassis.

I know it is good engineering practice often to isolate internal circuitry from chassis ground. Lab quality signal generators, for example, usually tie the shell of their RF output connector to the chassis via a (typically) 100 ohm resistor. This isolation minimizes ground loop problems and current imbalance in the coax cable. So I don't fault Elecraft for doing this with the K3. The problem is that doing can introduce compatibility problems with otherstation  instruments, as in my case.

Thanks again to all for your suggestions and commens.

73,

Jim W8ZR