My K3S (s/n 10440) developed an intermittent fault a few months ago where it would unexpectedly drop to "0" watts on transmit, and the receiver would drop about 20 dB in sensitivity. The fault initially corrected itself after a few minutes, and could sometimes be resolved by briefly pressing the "Tune" button on the radio. However, eventually it became continuous.
After some experimentation I discovered that turning the power down below 12 watts (8 watts on 6 meters) solved the problem. I now had a perfectly functioning 10 watt version K3S with a non-functioning KPA3A internal amplifier. I was able to bypass the faulty module in the Configuration Menu. I then turned my attention to diagnosing and fixing the problem. First, I tested the components and 12v relay on the KPAIO3A interface board which controls the KPA3A. All tested OK. Next, after reading through similar fault discussions on this reflector, I concentrated on the PIN diode T/R switching circuit on the KPA3A board. Specifically, I checked the PIN diodes D5 and D8, and the red toroid RF chokes L1, L3, L4 and L5. All checked OK. Stumped, I then went back to this reflector's archives and searched for more relevant discussions and ideas. Finally, I ran across two discussions from 2020 entitled "K3S - No high power output - Low power OK" and "[K3S] intermittent RF path interruption." These both described a known issue with the solder connection of a critical resistor, R5. This resistor does not match the footprint mechanically on the PC board, so a solid connection is not possible. As the board went through many heating and cooling cycles over time, this fragile solder connection would eventually break. According to the discussion, Elecraft acknowledged the problem with Rev D of the KPA3A PC board, and corrected it beginning with Rev E. My radio is an early production model and has a Rev A board, and it has the same issue. However, I have never seen a notification or application note from Elecraft concerning this issue. I then examined R5 on my KPA3A board, and it was partially tucked under RF choke L1, with a small amount of silicone obscuring the label. After carefully cutting away the silicone, I gently touched the side of the resistor nearest the edge of the board with a thin dental tool. It immediately lifted up off the board. I had found the problem! In the schematic I saw that the other side of R5 (away from the edge) is electrically connected to C17. As it was also physically close to C17 on the board, I re-soldered that end of the resistor to the same solder pad as C17, which allowed me to re-position the other side of R5 to access the solder pad that was originally underneath the resistor. This fix was successful, and I have now been operating the radio at 100 watts, with no receiver issues, for about 10 hours. I was fortunate enough to have access to the right tools (and eyesight) to work with small surface mounted components like this resistor, but not everyone can. As earlier production K3S radios get older, it is likely that this failure will become much more prevalent. As this was basically the result of a latent design/assembly flaw, I hope that Elecraft will assist those who are unable to make the repairs themselves. 73 de Pat / WA6SCW ______________________________________________________________ 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
