I was using my KPA100/KAT100 combination today to check out some changes to my OCFD antenna. Previously I have had problems with the balun heating up and the SWR increasing over an extended transmit on 7MHz (eg 50w on an FT8 over). I have made an improvement to my OCFD antenna by adding another choke to reduce the common mode current. So in order to test it out, I planned to run 100w CW for 15-30seconds watching the SWR meter on the KAT100 to see if I was going to get the SWR increase. This was done with a “cold” PA.
So I started the test and unlikely previously, the SWR remained good. However at about the 30s mark, I saw some smoke coming from the KPA100 and then it went dead. It also melted my fuseholder in the PA supply (although didn’t actually blow the fuse). I have opened up the KPA100 only to find that the two 22R power resistors around the two final transistors (R38 and R39) have both unsoldered themselves from the board and were lying in the bottom of the case! Clearly it is an easy job just to put them in again but before doing so I want to try and understand what is happening So a few questions: a) I am hoping that my two finals are still OK. Is there a simple test I can do perform applying power? Using a DMM in resistance mode, I measure approx 2M between collector and emitter and 11.2R between base and emitter (other than the missing 22R resistors I haven’t disconnected anything else). b) Any idea what could have caused that much heat that the two resistors unsoldered themselves? It can’t be a DC failure since there is a capacitor blocking the path in the event of a PA transistor shorting to ground. I am concerned that there is some sort of oscillation going on ie the PA is starting to hoot. c) Any advice on how to proceed? 73 Paul M1CNK ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
