[Elecraft] K3 Power Reading/SWR/ALC Problem

[Paul Christensen]

>"From experience, I'd say nearby lightning strikes would be the 
culprit."

Highly unlikely.  Clear skies for days.  No lightning activity here in a long time here.  Two years ago, I had a similar problem with static-damaged Schottky diodes in an Alpha 9500.  Those diodes are used to sample RF for the amp's EBS circuit.  

>"I strongly urge everyone to use protection devices on all 
coax and balanced lines at the point where the feed lines and rotor 
lines enter the house.  I prefer Polyphaser devices."

Already installed and we never disconnect at the remote site.  The site conforms to Motorola R56.  No lightning-induced damage in almost 3 years of operation in a very lightning-intense location near the Okefenokee Swamp.   High power Alpha-Delta arc-plug devices are located at the master ground bus where all coaxial and control lines enter the station.  Low power (200W) Alpha-Delta devices are installed between the K3 and amplifiers.  The Alpha-Delta devices use field-replaceable arc-plugs.  Like Polyphaser, these devices are intended to mitigate lightning-induced charge, not necessarily antenna static charge.  Still, it's possible that one or more of the Alpha-Delta arc plugs has failed, leaving the K3 vulnerable to static damage. 

In looking at the K3 SWR bridge circuit, both sides of the Schottky diodes have a reasonably low impedance to chassis ground.  One side is slightly less than 22K-ohm.  However, it may not be low enough. To supplement protection, I may add a pi-section 1 mH choke between T3 and the chassis.  It's the same choke used as a static drain in the Alpha 89 and 87A amplifiers to prevent static-induced PIN diode damage, and the type used at the output of many vacuum tube amps to function as a safety device in the event a plate blocking capacitor fails in a shorted condition that would otherwise allow high-voltage on the output network, transmission line, and antenna.

Although fun to discuss and contemplate fixes, it's conjecture until validated and the root-cause may not be related to any of this.

Paul, W9AC

ly failed.