I recently posted about a K3 40 meter SWR anomaly, where the K3 SWR reading on the front panel meter tended to increase as I tuned up higher in the band. I first observed the problem while using my Steppir yagi, which retunes itself so that I should have seen a low SWR across the entire 40 meter band. I saw a 3:1 SWR on the K3 meter at the high end of 40 meters, while at the same time I was reading a much lower SWR on my inline LP-100A. Further investigation showed that this situation also occurred when transmitting into a known-good dummy load. I was seeing about 1.1:1 at 7.001 MHz, and this rose to 3:1 at 7.299 MHz. When the high SWR was being sensed by the K3, output power was also being folded back. For example, I would have about 36 watts output when the K3 was set to 50 watts at 7.299 MHz. This anomaly also affected 40 meter operation with the KPA1500, as I observed what appeared to be power overshoot and experienced the need to frequently readjust drive power on 40 meters. A fair amount of digging in the email archives found a couple posts which touched on an issue with the 8.215 MHz trap (C10 and L10) on the KAT3 tuner board. Keith WE6R at Elecraft then suggested removing C10 and L10, and adding a shorting wire in place of L10. This mod does indeed appear to fix the false SWR issue – at least it did in my K3. I now have flat 1.1:1 SWR readings into a dummy load on 40 meters at all power levels. The trap is not included in the design of the KAT3A, so K3S rigs with the KAT3A should not see this SWR problem. Any K3 users who have retrofitted the KAT3A tuner into their K3 should not see this anomaly. The problematic trap, from comments by Keith, may be due to some lots of capacitors which for reasons unknown to me have a high failure rate. It may be prudent for K3 owners with the KAT3 tuner board (S/Ns under 10,000 or vicinity) to check SWR readings on 40 meters using a dummy load to ascertain whether they need to apply the KAT3 trap fix. How to check your K3: Although I do not know how many KAT3 tuners are affected by the trap issue, I do feel it is the kind of problem that some users may have but just have not noticed. It is fairly easy to check for; just connect the K3 to a dummy load and observe the SWR as shown on the K3 meter at several random frequencies across the 40 meter band. Of course (assuming a stable 50 ohm dummy load) you should see a fairly flat 1:1 or near to it. But if the SWR rises as you approach 7.3 MHz, and you are using the older KAT3 tuner board, you most likely have the trap problem. Upon visual inspection, you may see evidence of burning on the trap components. Although my trap components looked fine, I apparently had some internal failure in C10. The faulty trap may also exist in the older KANT3 module (the trap parts would have different part designations) but I don’t have a KANT3 and have not researched this enough to comment further. Trap fix: As Keith suggested, remove C10 and L10 on the KAT3 board, and install a jumper where L10 was. I cut out C10, and also cut the leads on L10 while leaving enough length to strip some enamel coating and solder the leads together, thereby creating the jumper. To remove the KAT3 board, use proper wrist strap procedures. First remove the K3 top cover and one rear panel corner screw near the ANT connectors on the rear panel, and unplug the coax from ANT1 and ANT 2 connectors going to the KAT3 board. The KAT3 board can then be carefully pulled out for the trap mod. I did not have to remove the subreceiver or the K3 side panel. Results: For me, I now have a flat SWR reading across the 40 meter band when using the K3 internal SWR meter. Power output is now stable and fixed with no foldback, and I am not experiencing any KPA1500 drive faults. Thanks to Keith at Elecraft for his assistance on this issue. Please correct me Keith if I have not documented this issue accurately. --Ed, N3CW--
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