[Elecraft] K3S KPA3A No Power over 12W

[Mike Walkington]

Hi Mike

Thanks for the feedback.

I think D2 and D3 are OK. ie Both are 0.5V forward biased and open when 
reversed.


D5 is 0.7V and 2.3V. Am I correct in believing this is a problem.

D6 is a really tiny diode. Did you mean D8? Which is in comparable 
position to D3 on circuit diagram.

D8 is 0.5V and 2.3V. Am I correct in believing this is a problem.

Cheers

Mike, VK1OO



Message: 16
Date: Sat, 3 May 2025 12:28:13 +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
Message-ID:
<BL3P223MB0385160B8F084445682B3620E48C2 at BL3P223MB0385.NAMP223.PROD.OUTLOOK.COM> 


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G'day, Mike,

You mention checking diodes D7 and D10.  On the KPA3A module, the 
relevant diodes in the T/R switch are D2 and D3 in the receive path, D5 
and D6 in the transmit path.

The loss of receiver sensitivity when the POWER control is advanced 
above 12 W indicates that D2 or D3, or both, are damaged OR their 
switching control voltage on their cathodes via Q4 and the 7R control 
line is not pulling the cathodes low on receive.

The inability to develop transmit power above 12 W indicates a problem 
with the diodes D5 or D6, or both, OR their cathodes are not being 
pulled low by Q6.

In the past others have found the T/R diodes closest to the antenna, 
i.e. D2 and D5, are more susceptible to transient overvoltage on the 
antenna port due to nearby lightning strokes and static build-up and 
discharge on the antenna due to wind and precipitation.

I suggest you check D2 and D5 on the KPA3A module first, then check the 
other two diodes of the T/R switch.

73,
Mike, K8CN