[Elecraft] KAT3 ATU Problem

[Tom Boucher]

Mike (K8CN) and others - thanks for your inputs. I'm talking to Paul at
Elecraft Tech Support who, by the way, is being most helpful.

He reckons I have a problem with the Low Power Amplifier which takes a few
seconds to come up to full output power. I'm not at all sure how that
affects the SWR measurement and ATU setting, but I'm going to follow along
with his advice and see where it gets me!

Then perhaps I'll start looking in detail at the SWR bridge components.

The rig has a 100 W pa and second Rx Mike.

73
Tom G3OLB

*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*