RFI Symptoms & Cures
If you ever really use a 1278, eventually you will see the following:
A) Heavy RF noise coming from the TNC, especially as the DCD LED flickers on and off.
B) Tones on TX shift around mysteriously, especially at high power or SWR levels. Sometimes, by moving objects around or placing you hand on the TNC, the tones will shift around even more.
The problem is grounding. That's it--nothing more. And it is easily cured with a pocketknife and a screwdriver.
You see, MFJ likes to put a nice coat of paint on the chassis and cover of their TNCs. A nice, thick coat on the outside and inside. Meaning that the cover doesn't make a good electrical connection to the chassis (which is ground). And the chassis doesn't make a good electrical connection to the front face, either. So basically, as delivered from the factory the TNC is about as electrically shielded as, well, you get the picture.
Take off the cover. Every place that the cover meets the chassis, scrape off the paint down to the shiny aluminum. Remove the threshold control knob and the two screws holding on the front bezel. Scrape the chassis clean of paint, especially around the two screw holes. While you have the covers off, look around on the board for the screws used as grounds. Break them free and re-tighten them--be careful, they have a varnish on them which can make it tough to loosen.
Now reassemble. Hopefully, that's helped quite a bit. If not, then start pursuing the standards: chokes, etc.
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