[1000mp] Mark-V fan stuck on

[George Gorsline VE3YV]

After operating in digital mode for maybe 20-30 minutes, the fan comes 
on and then does not go off, even after the rig is turned off and sits 
over night.  This has happened twice - the first time it eventually went 
back to normal cycling on/off after a few days of casual operating.  
Power used on PSK/RTTY is 30-50 watts, controlled by audio input level 
through the back panel jack.  Once the fan is stuck on, mode, band, etc. 
have no effect.  I'm assuming that except for the fan noise (one of the 
few things about ALL Yaesu rigs I've owned that I don't like), this is 
just an irritating "feature."  Any thoughts on whether I should be 
worried and how to find/fix?  tnx

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73,  George VE3YV / K8HI