[Elecraft] KPA500 failure

[Wes]

I've been incorporating a new transceiver into my station and was going through 
setting the per band power limits for driving my KPA500.  When I got to 50 MHz, 
a band I seldom use, I was dismayed to note that there is no amplification, it 
operates as if in Standby.  The RF frequency sensing fails to switch bands and 
even manually switching to 50 MHz makes no difference.

I have the schematics, but it's unclear to me where to start. Any ideas welcome.

Wes  N7WS