Posted by
P.B. Christensen on
Jan 30, 2018; 2:24am
URL: http://elecraft.85.s1.nabble.com/K3-Power-Reading-SWR-ALC-Problem-tp7637545p7637772.html
Epilog: Today, I replaced both BAS70 Schottky diodes (D36/D37) and now the K3 SWR readings are stable and the internal ALC is working as expected. At this time, it appears diode failure was caused by antenna static.
A suggestion to those who may run into this problem in the future: it turns out that D36 and D37 are easily accessible by removing only the bottom rear plate. Access could not be easier. The diodes are SMT/SMD and so great care must be exercised in the rework process.
What should have taken me less than 30 minutes turned into several hours. I had convinced myself that the diodes were either on the PA module or KAT3 and so I had disassembled much of the K3. The K3 block diagram doesn’t show a module where the SWR bridge resides. The schematic indicates "K3 RF Board," but that's not the PA module. The RF Board is actually the flat motherboard that the PA module and KAT3 plug into. Once that detail was figured out, I reassembled the K3, leaving only the bottom plate open for the repair.
Paul, W9AC
-----Original Message-----
From: Paul Christensen [mailto:
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Sent: Sunday, January 21, 2018 7:12 PM
To: '
[hidden email]' <
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Subject: RE: [Elecraft] K3 Power Reading/SWR/ALC Problem
>"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.
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