Have you tried just running the mic into your K3 without the external station controller ? What is that controller ?
73 Stewart G3RXQ On Sat, 1 Nov 2008 07:57:10 -0600, Dr. James C. Garland wrote: > Gang, > > > Im 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 K3s rear panel microphone jack and when the K3s 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 Ive noticed: > > > 1. The feedback still occurs when a military-type completely shielded 100W dummy load > is screwed directly to the K3s 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 doesnt appear to > make any difference. > > > 3. I can see some modulation-induced fluctuationss on the 12V line with an oscilloscope > that increase with the K3s power setting. I havent looked at these fluctuations > closely enough yet to know whether its RF noise or just audiofrequency fluctuations > caused by the K3s 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. (Ive 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 K3s > 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 K3s chassis ground, and this may > be the source of the RF feedback sensitivity. > > > Unfortunately, I cant figure out the detailed mechanism for the feedback closely > enough to figure out a solution. Foir example, I dont know whether its 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 K3s audio circutis to improve RF isolation may be needed to lick > the problem completely. > > > 73, > > > Jim Garland W8ZR _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
