Tom, I'm going to go out on a limb here, but hey, it's only a virtual limb!
I am going to withdraw my previous statement that the K3 SWR bridge is operating correctly. Given what you've said, the ATU control algorithm is receiving bad information from the SWR bridge. Here's how to test this hypothesis: -- use a different resistive load than 50 ohms, e.g. two 50 ohm loads in parallel using a tee-junction, or build a higher resistance load using several non-inductive power resistors, or use a 4:1 balun reversed to present a 200 ohm load to the ATU when balun terminated in 50 ohms. -- put the KAT3 ATU in 'bypass' mode -- measure SWR at low TX power (whatever power the ATU uses automatically is fine as long as your loads can tolerate the dissipation briefly). Schottky diodes D36 and D37 in the K3 SWR bridge detector circuit are known to be sensitive to high voltage transients, e.g. ESD or lightning-induced voltages. Failure of either or both diodes produces bad SWR estimates, which may not be observed when using a good 50 ohm termination. If you have a good high impedance DC voltmeter (DMM), you might check the voltages VFWD and VREFL at the outputs of the LM358 op amp unity gain buffers (U16 on the K3 RF board) to observe how each behaves with different load resistances at the ANT port(s). If the VREFL voltage doesn't change predictably with mismatched loads, it could be that diode D37 has failed. A search of the old Elecraft reflector and the separate K3 groups.io forum reveals lots of discussion about anomalous behavior of the ATU, frequently associated with SWR bridge component failure. 73, Mike, K8CN ______________________________________________________________ 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
