[Elecraft] K3S KPA3A No Power over 12W

[Mike Walkington]

Hi Mike

I appreciate the ongoing support.

I measured resistance across both D5 and D8. Both measured 10K with both probe polarities. I suppose that is the effect of R11. Does that mean these diodes are probably OK?
I will need the weekend to de-solder a leg of D5.

I can see from an email in the reflector last September that D5 is probably an MA4P4006B-402 not an MA4P7470, and looking at data sheets would seem to confirm that. Of course the corollary of Murphy's Law for electronic failure is nigh, something about taking out the more expensive component. D5 at $55 versus D8 at 19 cents

Cheers
Mike, VK1OO








Message: 4
Date: Sun, 4 May 2025 11:46:35 +0000
From: Michael Carter<Mike.Carter at unh.edu>
To: Elecraft Reflector Reflector<Elecraft at mailman.qth.net>
Subject: Re: [Elecraft] K3S KPA3A No Power over 12W
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Hi Mike,

You are correct about the diode mis-naming of course. D8, not D6.
(Note to self: clean eyeglasses and laptop screen, and use better lighting when reading schematics!)

The reverse polarity tests on D5 and D8 are concerning.  Since there are alternate parallel paths for current to flow other than directly through D5 and D8 when those are each back-biased and remain in-circuit, I suggest measuring resistance of each diode in circuit with reversed DMM probe polarity.  The alternate path for a D5 test is via L5, R11, R5, and L1.  I would expect to measure slightly more than 10k_ohms on that path if D5 is intact.  If you can temporarily lift one leg of D5 and perform the same reverse polarity resistance test, that would confirm if D5 is OK.

D5 and D8 are PIN diodes, both rated for sufficiently high reverse voltage withstand not to have failed simultaneously.  If D5 was damaged by an extrinsic event, you would experience signal 'suck out' on receive by a low resistance path through D5.  A failure of D8 may indicate a problem in the PA circuit itself, but let's not get ahead of things quite yet.

Let us know what you find from additional tests of resistance measurements, especially when the DMM probes are reversed across D5 and D8.

Cheers,
Mike, K8CN