[Elecraft] KPA500 Relay Failure

[Wes]

Good work.

On 5/1/2021 9:45 AM, wa9fvp wrote:
> 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
> --
> Sent from: http://elecraft.365791.n2.nabble.com/