I fitted a commercial EMI filter (intended for 240V AC operation) inside my P3. It is a fully sealed unit with solder tags. This unit was connected in the + and - DC supply line. This filter completely removed any trace of noise from the power cord to the P3. I used a short coax with a one inch extension of the center wire to act as a "sniffer" to determine where the noise was coming from. This coax was connected to the K144XV input. Did this solve the problem? No! not entirely. I discovered there was now a small amount of noise being radiated from the RS232 lead and the IF input coax. These cables had been free of noise before. It seems to me that it is acting a bit like a boiling kettle - Block the spout and the steam comes out the holes in the lid! Why was there so much noise generation in the first place? I used my sniffer inside the P3 box to try and track down the precise source of the noise. But no luck. The noise was overwhelming everywhere at S9 + 20dB rising to S9 + 40dB near any of the ICs. This was on any frequency in the 2 meter band and beyond. At this point I re-visited the earthing between the panels that make up the P3 box. If you have been following this thread, you will know that I had previously cleaned all the mating surfaces. I looked again at the front panel. It is connected by four screws to the corner blocks. You can see where rectangles of masking tape have been placed to prevent the black paint from reaching the area which will mate with the corner blocks. The masking tape has not quite reached the edge of the board, leaving a one mm wide strip of paint right at both corner edges. It seemed very unlikely that this was preventing a good connection between the panels. Nevertheless I removed all trace of the paint in all four corners of both the front and back panels using a file and sharp knife. Did this have any effect? Well Yes! It reduced the noise by about 6dB. The noise which had started at S9 + 5dB was now down to about S3 which was the target value. The carriers at 19KHz spacing were just, but only just, perceptible with the 2 meter antenna connected. It would have been interesting to know what the noise would now be like without the EMI filter. Sorry guys (and gals), I did not do that experiment. When you have got something that works - leave it alone! I was also able to connect my other accessories to the switched 12V line from the K3 without affecting the noise. Another effect of my paint scraping exercise was that the position of the P3 was no longer critical. Before, if I pulled the P3 forward, the noise went down, but returned with the P3 in its correct position aligned with the K3.
Roger Crofts, VK4YB ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
