John and all, Sorry for the previous blank email - slip of the mouse!.
There are several possible explanations for repeated K2 PA transistor failure - the most common of which is that the PA bias is not correct. The K2PAKIT comes with 2N2222A transistors that are to be replacements for Q11 and Q13. If one or both of those transistors are damaged, the bias to the PA transistors will not be correct and the result is their early demise. Q11 and Q13 often fail as a secondary fault when the PA transistors fail. There was also a short "glitch" in the PA transistors being used by Elecraft. The original PA transistors did not need to be matched because they came in consistently with an HFE in the 50 to 60 range. Then the original supplier of the PA transistors no longer produced these good transistors, and it was discovered that the HFE of the transistors being supplied varied substantially. If one were unfortunate enough to be supplied with transistors having substantially different HFE, one would hog the current and eventually fail. Once that condition was discovered, Elecraft began supplying matched pair transistors for the K2 PA. No problems have been observed since the transistors have been matched. Perhaps your friend was trapped in that interim period between the mismatched transistors and the time when Elecraft began matching them (or he had a Q11/Q13 problem) - without more details about the time frame this occurred, it is difficult to make any assessment - what I can say is that this condition has been corrected. 73, Don W3FPR On 11/12/2011 6:39 PM, John wrote: > Thanks to all for the answers. A friend of mine had a K2 (QRP) that > kept blowing finals no matter what he tried so I was curious if there > was some known stress point in the design. He has since moved to a K3 > and no longer has the K2 so a resolution to the blowing finals is not known. > ______________________________________________________________ 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
