[Elecraft] K3 RF Feedback Problem

[Joe Subich, W4TV]

Jamie, 

We had to modify all microHAM interface cables for the K3 
to move the PTT ground from Pin 8 to the shell of the Foster 
(mic) jack to avoid the "Pin 1 problem" caused by the common 
RF choke (L7) on the front panel board.  

I suspect your multi-switcher also connects PTT to pin 2/8, 
mic to 1/7 and a shield between the chassis of both units. 
Moving the PTT ground from pin 8 to the shell of the Foster 
plug (or connecting PTT ground to the chassis in the 
multi-switcher and leaving pin 8 open) should eliminate 
the problem completely.  

As W9AC reports, replacing L7 with a jumper seems to be the 
best solution but it requires disassembling the front panel 
of the radio.  A good "work around" is to install a jumper 
between pin 8 and the shell of your mic connector. 

73, 

   ... Joe, W4TV 
 


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of James 
> C. Hall, MD
> Sent: Saturday, November 01, 2008 1:13 PM
> To: 'Dr. James C. Garland'; elecraft at mailman.qth.net
> Cc: 'W. Douglas McDowell'
> Subject: RE: [Elecraft] K3 RF Feedback Problem
> 
> 
> Hi Jim:
>  
> I'm wondering what external station controller you are using. 
> I'm using a NCS Multi-Switcher and I ran into very similar 
> issues. I certainly felt (as did Doug at NCS) that it was a 
> ground loop problem. I used transformer isolated mike cables 
> and found that it was NOT a ground problem. I can give you my 
> long details, but the bottom line is that the mike gain in 
> the Multi-Switcher (internal) needed to be driven a bit 
> higher than normal, and the mike gain on the K3 need to be 
> quite low (5 or 6) to avoid the 'growl' effect. I'm 
> forwarding this to Doug at NCS as he is quite interested in 
> what's going on that might be causing this.
>  
> 73, Jamie
> WB4YDL
>  
> 
> 
> 
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Dr. 
> James C. Garland
> Sent: Saturday, November 01, 2008 8:57 AM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] K3 RF Feedback Problem
>  
> Gang,
>  
> I'm pulling my hair out over an RF feedback problem with my 
> K3/100, and wonder if anybody has any ideas? I use an 
> external station controller that switches key, microphone, 
> CAT, line in/out, etc. between different rigs.  The 
> microphone audio connects to the K3's rear panel microphone 
> jack and when the K3's power exceeds about 15W (in SSB), the 
> audio becomes greatly istorted. It does this when all the 
> other cables to the K3 (except the microphone and power cord) 
> are unplugged. It also does it when the audio is fed to the 
> K3 via the rear Line In jack. Everything in my station is 
> well-grounded. Here are some things I've noticed:
>  
> 1.  The feedback still occurs when a military-type completely 
> shielded 100W dummy load is screwed directly to the K3's 
> antenna port. This suggests the RF from the K3 is getting 
> into the audio line from some other source than radiation 
> from the coax cables in the station, most likely the 12V 
> power cord. To support this conjecture, I also notice that 
> the feedback threshold (15W) is not changed when the K3 
> drives a linear amplifier. 
>  
> 2.  I normally use a 30A Kepco 12V power supply to run the K3 
> and my other station accessories. The RF feedback problem is 
> improved but not entirely eliminated if I use a separate 12V 
> supply for the K3. A common mode choke on the 12V line 
> doesn't appear to make any difference. 
>  
> 3. I can see some modulation-induced fluctuationss on the 12V 
> line with an oscilloscope that increase with the K3's power 
> setting. I haven't looked at these fluctuations closely 
> enough yet to know whether it's RF noise or just 
> audio-frequency  fluctuations caused by the K3's modulating 
> current draw from the power supply.
>  
> To summarize, my conclusions so far are that (1) the 12V 
> power cord is a source of RF leakage from the K3,  even 
> though the K3 has a filter at its 12V power connector to 
> minimize this leakage; and (2) The audio/DSP circuitry in the 
> K3 is very sensitive to RF - much more so than other 
> transceivers in my station.  (I've not had this problem with 
> other rigs.) 
>  
> Because of this RF sensitivity, one evidently has to be very 
> careful  when hooking accessories to the K3. The front panel 
> microphone jack is (to me) wired in a curious way, with the 
> Mic ground and PTT ground (shield) connections floating above 
> the K3's chassis with a 100uH rf choke. Similarly, both the 
> "hot" and "shield" side of the rear panel mic input jack has 
> series 100uH rf chokes, which isolate the input from chassis ground. 
>  
> The problem with this arrangement is that most accessories 
> that would connect to the rear panel audio input jacks have 
> single-ended outputs, with a shielded cable that is directly 
> tied to chassis ground at the accessory end of the cable. 
> Thus, unless one is very careful, it is easy to have a 
> situtation where the signal grounds in the audio circuitry of 
> the K3 can fluctuate with respect to the K3's chassis ground, 
> and this may be the source of the RF feedback sensitivity.
>  
> Unfortunately, I can't figure out the detailed mechanism for 
> the feedback closely enough to figure out a solution. Foir 
> example, I don't know whether it's better to leave the minus 
> side of my 12V power supply floating, or to tie it to the 
> chassis gound..  And even if I left it floating, it would be 
> tied to the chassis anyway by the other accessories hooked to 
> it, and this might cause more problems than grounding it at 
> the power supply terminal. Somehow, it seems like a wiring 
> change in the ground configuration of the K3's audio circutis 
> to improve RF isolation may be needed to lick the problem completely.
>  
> 73,
>  
> Jim Garland W8ZR
>  
>  
>