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: [email protected] -----Original Message----- From: [email protected] <[email protected]> On Behalf Of Andy Durbin Sent: Saturday, June 5, 2021 8:20 AM To: [email protected] 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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected] -- This email has been checked for viruses by AVG. https://www.avg.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
