[Elecraft] K3 RF Feedback Problem

[Joe Subich, W4TV]

Jim, 

> 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.

When initially trying to resolve the issues with microHAM 
interfaces and the K3, I also thought this was the problem. 
Several users went to the extent of powering their radios 
from batteries in an attempt to prevent return currents from 
finding their way back to the power supply via the common 
chassis grounds on the accessory (microHAM) equipment. 

While is was successful in some cases, the only universal cure 
was fixing the "pin 1 problem" with the front panel microphone 
connector. 

> 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 case of the K3 is just the opposite of what you describe. 
The internal circuitry is tied to the chassis but the external 
circuits are "floated" on the RF chokes in the return leads.  
Specifically, the rear panel PTT/Foot switch return, front and 
rear panel mic returns, the common on the ACC connector and the 
RS-232 return are all go through RF chokes before reaching the 
circuit common AND chassis ground. 

The cause of the problem is the use of a common choke for the 
front panel PTT and mic returns.  When external interfaces 
connect the PTT return to the chassis that 70 mV of ripple 
appears at the top of the common RF choke (L7) - here's the 
problem - effectively in series with the microphone signal!  
Anything that eliminates the common impedance (think 
"modulation choke") - including moving the PTT return 
connection from pin 8 to the shell (chassis) of the of the 
front panel mic jack or replacing L7 with a jumper - resolves 
the problem by eliminating the place the RF gets into the 
system.   

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of 4CX250B
> Sent: Saturday, November 01, 2008 5:02 PM
> To: elecraft at mailman.qth.net
> Subject: RE: [Elecraft] K3 RF Feedback Problem
> 
> 
> 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