开云体育

Good Morning,

I just received/built a "Run 4" box. It seems to work fine until I transmit local SSB audio at any power level over 2W. From 2W up, what appears to be RF feedback into the mic line causes the transmitter to generate ugly noise at full amplitude. Classic RFI into the mic line, which I've struggled with in decades past.

Is there an easy cure? I'm thinking of adding several pF on both ends of the mic audio line on the PC board (each DIN as well as the 3.5mm mic input). I'd appreciate anyone providing a startinig point.

Notes:
1. Ferrites (lots of 'em) on the DIN cable have little or no effect. Maybe lets out another half watt.
2. The RX audio was oscillating upon first power up due to the floating ground on this Run 4 board. The ND2T jumper was added; problem sollved. (TU Tom). I don't think this has any influence on the mic line.
3. Mic is a Heil HC4. Rig is a K4. The 14 dB mic preamp is required for full output. RFI occurs without the preamp, but at a much higher power level, as you would expect. DIN cable to rig is CAT5, using a twisted pair for mic and mic ground.
4. I scraped the paint from the chassis inside at the ground lug mounting screw position.
5. In TX TEST mode (zero RF out), the mic line works fine. Mic directly into the K4 works fine at 1500W.

Thanks for any assistance. CU on the air (?)!

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73 de Bob, K6XX