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Re: RFI Into Mic Line

 

Bob

I think by "Run 4", you mean a YCCC box from the fourth run.

The only thing I see that you didn't comment on was that paint needs to be scraped at two spots- one is the
where the ground lug is, and the other is to make sure the board is grounded.? If? you look at the pc board, you
will see that one of the mounting screw holes is connected to the board ground.? Where that screw and?
spreader meet the chassis also needs the paint scraped.? I don't know that that would solve your problem,
but it was in the construction documents.

Dennis W1UE


On Sun, Jul 17, 2022 at 8:48 PM Paul Young <k1xm@...> wrote:
Hi Bob,

What do you mean by a "Run 4 box"?? I have a K4 but I haven't done any SO2R phone operating with it.

The SO2R Box microphone switching is very simple - there is a relay which connects both the mic hot and ground to the appropriate radio.? The mic ground is not grounded to the box or to the power supply minus.? There isn't a jumper but it would be pretty easy to add a wire which grounds it.

I would try a wire or maybe a capacitor between the mic ground lead and the box ground.? If you do this you'll have to open the box - remember to remove the nut on the speed pot before you open it, otherwise you will break the pot.

??? Paul

On Sat, Jul 16, 2022 at 2:22 PM Bob Wolbert, K6XX <k6xx@...> wrote:
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 (?)!

--
73 de Bob, K6XX
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