[Elecraft] K3 Power Reading/SWR/ALC Problem

[Paul Christensen]

Dick & Don,

Thanks for confirming.  Although unlikely to be the source of trouble, It
also appears U6 and U8 are in the VFWD and VREF circuit paths.  I'll order
up everything and have it on hand when I replace this week.  It could also
be a passive problem like a marginally-corroded connector, or failed
resistor.  

By the way, the problem manifests with high SWR and power set readings that
no longer match anything close to  actual output power into known good
loads.  The SWR reading fluctuates excessively with change in the PWR
control and slight changes in the control quickly lead to 120W power spikes
when the K3 display indicates 15-20W.  This tells me the ALC circuit is not
getting a good forward power sample.  So, it appears the SWR bridge or
surrounding circuitry has partially failed.  

Paul

-----Original Message-----
From: dick at elecraft.com [mailto:dick at elecraft.com] 
Sent: Sunday, January 21, 2018 1:18 PM
To: 'Paul Christensen' <w9ac at arrl.net>
Subject: RE: [Elecraft] K3 Power Reading/SWR/ALC Problem

I don't know where purchasing actually buys the parts, but...

D36 and D37 are BAS70INCT Diode, 70V, SOT23, like Digi-Key BAS70INTR-ND
U16 is LM358, DUAL OP AMP, SOIC8, SMD, like Digi-Key LM358DR2GOSTR-ND

73 de Dick, K6KR


-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Paul Christensen
Sent: Sunday, January 21, 2018 08:02
To: elecraft at mailman.qth.net
Subject: [Elecraft] K3 Power Reading/SWR/ALC Problem

I have a K3 at a remote site about an hour north of my home so
unfortunately, I cannot quickly investigate an issue that's recently
developed.  After reviewing the K3 schematics, the root cause is likely
isolated to the following parts on the PA board: D36, D37 (BAS70 - Schottky
directional coupler sampling diodes), and/or U16 (LM358 dual op-amp used as
a voltage-follower).  

My question:  are these three parts leaded or SMT/SMD?  Perhaps I'm not
looking hard enough on the Elecraft website but I don't see a detailed
B.O.M. for the K3/K3s.  I want to be ready for replacement without wasting a
two-hour trip to/from the site.  Thanks.

Paul, W9AC

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