[Elecraft] K3 RF Feedback Problem

[4CX250B]

Very interesting, Paul. I was aware of the isolation chokes, but hadn't known anybody to try jumpering them. I'd be interesting in hearing one of the K3 designers explain what they had in mind by wiring the audio inputs this way. Perhaps it has something to do with using balanced line audio sources. Minimizing ground loops can be a headache when one has multiple audio devices, powered by different power supplies.

73,
Jim W8ZR
________________________________________
From: Paul Christensen [w9ac at arrl.net]
Sent: Saturday, November 01, 2008 10:03 AM
To: 4CX250B; elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3 RF Feedback Problem

Jim,

This may, or may not be the root cause of your RFI problem, but it is important to note that the K3 elevates the ground potential on both the front and rear panel MIC connectors.  This is not a trivial issue.  L7 is a 100 uH choke that isolates the ring of the rear panel TRS connector (J20) from chassis ground.

In the same manner, L4 (another 100 uH) RFC) isolates ground pins 7 & 8 on the front panel MIC jack from chassis ground.  The net effect of these chokes results in an "ungrounding" of the MIC inputs.. The addition of the chokes is something completely opposite of what is required in this part of the circuit.

When conducting other K3 audio mods, I went in and removed both L4 and L7, and jumpered the solder pads with 24 AWG buss wire.

In my case, RFI problems 100% solved.

Paul, W9AC

----- Original Message -----
From: Dr. James C. Garland<mailto:4cx250b at muohio.edu>
To: elecraft at mailman.qth.net<mailto:elecraft at mailman.qth.net>
Sent: Saturday, November 01, 2008 9:57 AM
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



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