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
Sent from my home-brew I5 Core PC
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Jack WA9FVP
Sent from my home-brew I5 Core PC
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