+++++++++++++++++++ From: "Stuart Rohre" To: , "Trevor Day" Subject: Re: [Elecraft] RF Feedback Date: Wed, 10 Jul 2002 18:08:52 -0500 I have seen various appearances of RF feedback in SSB radios. Some of the causes I found were: Use of a coiled cord mike cable with a coiled cord paddle/bug cable, and the shields formed inductors, since there was RF on ground. Magnetically coupled RF, because the SSB rig was set on top or too close to the side of a transmatch with large tank coil. The RF makes its way to the audio circuits and can be easily rectified in input preamps of the audio chain. Then it latches up the ALC, and the ALC indicator will be excessively high, or on all the time, if an ALC light. Try beads and pf value caps to decouple the RF, if it is on hot mike leads. You may need a bead over the outer shield, if the inductor effect is a causative path for RF entry to rig. Is the RF amp in steel case or aluminum box? Aluminum does not provide adequate magnetic shielding. Spacing the amplifier away from other rig may help. Bypass everything in and out of amplifier, filaments, B plus, use shielded B plus cables, etc. Make sure no coax jumpers approach a quarter wave at the band in use. Gl and 73, Stuart K5KVH - Original Msg Date: Wed, 10 Jul 2002 23:09:50 +0100 To: elecraft at mailman.qth.net From: Trevor Day Subject: [Elecraft] RF Feedback Hi All, well I have now completed installing my new DEMI 6m transverter into the EC2 case and it works fine. I particularly appreciate the enabling software for transverter use with the K2, it makes band changing and operation so easy :)) Now my problem; I am getting tremendous RF feedback when I key up my single 4CX-250B PA on 6m. It produces a conservative 200 watts output, but at that level the PA Anode current just lifts to max and stays there! By unplugging cables and manually operating the PTT line etc etc I have determined that the problem is on SSB only and appears to be entering the rig via the mic (an EH2). This is the same setup used with my previous transverter plus TS430S which seemed quite happy. I should mention that the DEMI is the low power PCB only option driving a single stage amp to 2 watts prior to the 4CX-250B. Output at the 2 watt level is very clean. I have tried changing cable lengths, re routing cables etc. I have also reduced SSBA and SSBC settings to minimum which improved things very slightly in that the feedback now waits until I speak rather than just operate the PTT:) My next step will be to de-couple the mic lines but I am conscious of not wishing to degrade the fine audio from the Heil mic. Has anyone else had this problem and if so what was the cure? Thanks, 73 Trev K2 #2637 -- Trevor Day +++++++++++++++++++