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.car...@unh.edu>
To: Elecraft Reflector Reflector<Elecraft@mailman.qth.net>
Subject: Re: [Elecraft] K3S KPA3A No Power over 12W
Message-ID:
<bl3p223mb0385d941e06a62b7319f95bee4...@bl3p223mb0385.namp223.prod.outlook.com>
Content-Type: text/plain; charset="iso-8859-1"
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
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