i just usually have limited spares onboard and don't want to take a chance
of blowing one till i find the actual fault. with access to shoreside spares,
sure, replace. if it pops again, then keep looking.
you are right, it is puzzling that it would behave until activated once.
especially if the rest of the autopilot
is still functioning (i.e. it is
not clamping the processor due to overcurrent. if the processor
also quit functioning then it would make more sense).
the only explanation that comes to mind immediately is so
far fetched i hesitate to mention it.
could be that
the transistor is damaged and the inrush current
when the clutch
is activated is punching through the base of the PNP
or zenering the junctions effectively shorting
it when it is under load. makes it act like an SCR.
the diode across the
clutch coil used to disssipate the transient when the transistor turns off
would be in the wrong direction or i might suspect it of zenering.
i would look at the driver circuitry first, that explanation is just too wild.
capn dan has it right.
replace the transistor, see if the problem goes away.
if not, start checking drivers back to the processor.
should be simple as i can't imagine a very complicated path between
clutch and processor output pin. not much else dependent there.
come to think of it, put a voltmeter across the clutch and see if the
voltage is switching on and off or not.
If its a sticky clutch, you got the wrong boy here.
I rustle electrons, if its big enough to hit with a hammer you are out of
my job description.