Hi Elecrafters, Hoping that some of you may be able to point me in the right direction. I'm not having much luck with power supplies!
Although a suitable level of audio volume is a somewhat personal and subjective assessment I have spent some time now reading the list archives re 'low audio' and I'm pretty certain that I have a problem. My volume control needs to be set at 3:00pm or higher in the quite environ of my shack with the RF gain at max to provide a listening level that for me is just not quite loud enough. And no, I'm not hard of hearing. Now I 'think' that I might know why I may have an audio problem. Which brings us to the saga of the Power Supply Unit. Having completed the K2 I moved it from the work-bench to the operating table next to the computer. Ready to connect the K2 audio out to the computer sound card for doing the filter alignments (Baudline and Linux). In doing so I connected the K2 to a different 13.8vdc power supply from the one that had been used during the build process. This power supply is quite new, about 6 months old and sees daily service running my 2m rig. I then plugged a 3.5m stereo extension lead into the front panel headphone jack with the K2 happily running and then clambered under the desk to get to the back of the computer tower. My trouble started when I went to plug the other end of the stereo extension lead into the 'line-in' connector on the sound card (can anyone see where this is going?). There was a brief blue 'spark' upon plugging the audio lead in. That for the split second in time that it takes for the mind to go "oh help that was an arc!" and then "but that's impossible it's a audio line level cord" followed by "oh s@&%! The K2 has powered itself off!" before I yanked the cord back out. Seemed like an eternity but it probably all happened in less than 2 seconds. Power and audio were restored to the K2 as if nothing had happened. At this point instead of getting scientific about my approach to fault finding I went off in random directions and probably made things much worse. I triple checked the wiring of the speaker, external speaker connector and plug. Having satisfied myself that all was perfectly correct I then connected the stereo lead to the computer first and then plugged the other end into the K2 external speaker connector. Please don't ask me why I did this, just put it down to pure stupidity!! This time I'm watching the K2. Sure enough big blue spark, K2 powers off and I yank the audio lead back out again. K2 powered back up again. What the?? At this point in the sad and sorry tale my immediate impression was "oh help, I've blown the speaker". As the speaker audio level seemed to have dropped by at least half and at that time I thought there was noticeably more crackling on audio peaks. At least that was my "impression". Only at this point did I put my brain into gear and start to do some logical fault finding (too little, too late) and this is what I found: The power supply in question has a correctly regulated 13.8vdc across its output. But between the power supply case which is grounded to mains supply earth and the negative terminal of the same supply there was a voltage potential of nearly -25 volts (-24.7 something or other). This is to say that the PSU negative terminal appears to be 25vdc below ground potential. When the K2 was interconnected with the computer via the audio lead there would have been a 25vdc potential difference between the ground levels of both machines. Shorting the negative terminal of the PSU (now my least liked shack accessory!) to its own case would cause the power supply output to fall to zero (hence the K2 powering on and off). Holding that 'short' in place for an extended period blew the mains fuse inside PSU. So my questions are (and sorry this has been so long): Can anyone think of a good reason as to why this behaviour from my PSU would be normal or exceptable and why I shouldn't take it back to the place of purchase and accidentally drop it on the foot of the first salesman I find there (it's linear and quite heavy)? Can anyone with a better understanding of the K2 circuit than I, predict the result of placing +25vdc across the external speaker jack if only for a micro-second or two? Is this likely to be the cause of what I perceive as a low audio problem or am I worried about a red-herring and my low audio problem, if I even have one is likely to be unrelated? Thanks again in advance! Cheers, Steve VK2SJA K2 S/N 5556 _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
