[Elecraft] KPA500 receive loss

Sat Jun 5 11:20:34 EDT 2021

This happened with my backup amp at the contest station on J6.  I left it
connected by accident overnight (too much rum) and we had some static
storms.  Took out the pin diodes (well, more accurately the diodes that act
like PIN diodes).  When put in operate, the receive drops by 30+ dB but
still transmits fine.  A simple replacement of the four diodes and three (?)
transistors solved the problem.  Dirt cheap.  You can make these
measurements at the test-points to confirm:

TP2= RX 13.1, TX 0.8
TP4= RX 3.9, TX 13

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ 

email:  bill at wjschmidt.com

-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On
Behalf Of Andy Durbin
Sent: Saturday, June 5, 2021 8:20 AM
To: elecraft at mailman.qth.net
Subject: [Elecraft] KPA500 receive loss

There was a similar report fairly recently.  I copied this to my KPA500
notes:

"Reported problem(s):
Receive signal is attenuated when in operate. Receive signal is normal when
in standby.

Problem(s) found:
Confirmed problem. Found that D8, D9 and R7 in the T/R circuit had failed.

Repair(s) performed:
Replaced D8, D9, and R7 on the LPF board."

I didn't record who posted this but I expect you will find this and similar
reports in the archive.

Here is another that may be related:

"Elecraft list May 1 2021:

I'm repairing a KPA500 for a friend that has the RX loss in standby mode.
After a time, he also lost receive in operate mode.  After doing a teardown,
I found the problem.

With the PA board removed, I checked K23A relay.  The normally closed
contact on the antenna side of the relay was open.  K23B was OK.  With relay
on the board, there was no safe way to energize the relay so I didn't check
the contacts on the normally open side. I also noticed that resistor R7 was
un-populate and pieces of it was stuck to bottom cover.

Looking at the PA schematic I came to the conclusion that the relay K23A was
also having problems on the operate position. With no or a poor connection
to the antenna, the 500-watt output voltage can go sky high. It can get
rectified by D9 and feed a high DC voltage through L4 and damage resistor R7
or even un-solder itself.

I replaced the relay and R7.  I remove the cover on the old relay and using
a 12v power supply, I set up bench test. With the relay energized, I
connected a Fluke meter in diode mode and it proved that the antenna
connection in operate mode was intermittent.

-----
Jack WA9FVP"

73,
Andy, k3wyc
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