[Elecraft] KPA500 T/R Switch Board – D8 Identification

[Alan Ibbetson]

Andy K3WYC commented that there have been reported failures of R7, D8 and
D9, and maybe other components. Does anyone have a root cause analysis for
these failures?

I can see why Q5 was changed from a FET to a Darlington NPN: the 5T voltage
is marginal for turning on the FET. But why was R3 changed to a zener
diode? Are there huge voltage spikes on the 5R line?

Thanks, Alan G3XAQ

Andy Durbin via Elecraft
<https://www.mail-archive.com/search?l=elecraft@mailman.qth.net&q=from:%22Andy+Durbin+via+Elecraft%22>
 Sat, 06 Jun 2026 11:33:46 -0700
<https://www.mail-archive.com/search?l=elecraft@mailman.qth.net&q=date:20260606>
wrote
:-

My notes on TR switch R7 include - "Several reports of R7 failure some also
with failure of D8 and D9. Results in approx 20 dB RX signal attenuation when
BYP compared to OPER."

The voltage of almost every component junction in the TR switch were researched
and documented here - https://www.qsl.net/wb4kdi/Elecraft/KPA_TR.html

Click on the schematic links for the two TR switch states.

More info on KPA500 at the parent level - https://www.qsl.net/wb4kdi/Elecraft/


This is a great KPA500 resource and is being maintained.  Please contribute if
you learn something new.

73,
Andy, k3wyc

-- 
Alan Ibbetson
alan at g3xaq.net