Yes, the "resonating board" may explain the difference between various perceptions of KPA500 owners about the hum level. To check for that, it is a simple matter of lifting the KPA500 up with your hands - if the hum level diminishes, then the surface it is sitting on is acting as an acoustical amplifier.
There have been other items mentioned in this thread - the transformer contacting the Z bracket is one valid problem caused by improper assembly. The transformer itself has isolating foam washers that were to be placed above and below the transformer core for acoustical isolation. I can imagine if the bolt were tightened enough to compress those foam washers completely, then hum might be transmitted to the KPA500 enclosure - try loosening the bolt slightly to see if the hum goes away or diminishes significantly. 73, Don W3FPR On 9/27/2011 5:17 PM, Guy Olinger K2AV wrote: > Another issue is what the amp is resting upon. > > Back when I first got my Alpha 76PA, I had a noticeable hum when > transmitting. When I wised up a little, I tried it from different places > and noted that the volume had a very large variation depending on where > placed on the upper shelf or simply where elsewhere it was placed. I cut up > a little mouse pad that had become ugly into 1.5 inch circles and placed > them under the feet of the amp. Hum out in the room simply went away. You > can barely hear the note from a violin string that is strung between braces > on a workbench, but the same bow on the same string on a violin can be heard > thoughout the concert hall. The shelf was serving as the violin to the > Alpha's hum. > > I still use the mousepad circles under the amp. If you put the the mouse > surface of the pad *on the bottom* then the amp will slide around if pushed > a little and is much easier to position or get into place. > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
