[Elecraft] KPA500 fan won't shut off

[Andy Durbin]

"the KPA500 fan was still running at a high speed even though the amp had been turned off all night by using the front "on" button.  The only way the fan stops running is if I turn the amp on with the front switch or if I shut the amp off by using the rear on/off switch. Menu setting for Fan is set to normal."

Yes, as Jack suggested, a stuck 60 V enable relay in the PSU fits the symptoms.

That leads to a question though - what does the firmware do if commanded to shutdown but 60 V never bleeds down?   I'm guessing it eventually times out or else the fans should have stayed at low speed trying to bleed down the 60 v supply.   I also assuming that, when the firmware shuts down, the fan speed control lines default to high speed setting.

When the KPA500 is next powered, but still off, the stuck 60 v enable provides power to the fan and the defaulted control lines make it run at high speed.  Once the firmware starts running it detects low PA temperature and sets all fan control lines to off.

Interesting puzzle.  Was there a prize?

73,
Andy, k3wyc