[Elecraft] KPA500 receive loss

[Dave]

Does that confirm correctness, or brokenness?

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 6/5/21 8:20 AM, Dr. William J. Schmidt wrote:
> 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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