S.E.P. joule/energy meters are failing. Any ideas

SEP joule/energy meters are failing at our school. They pass current for a few seconds to a minute and then drop to a very low current (mAs). Left for a few minutes it will repeat the fault.
The spares list shows that a 7A fuse is available. I suspect its a semiconductor/PTC type of fuse but I cant see one in the "guts" of the unit. I also suspect the kids either from ignorance or trying to make smoke may have overloaded the units.

Any thoughts or experiences of fixing these? I'd almost fixed one and then I ran out of nails.
Thanks Tech Q.

Found the little blighter. It was surface mount, being dead modern, so looked like a fat sm diode. It had indeed overheated but needs the unit pulled apart to get to the *@$£& thing. Looking to add a thermal fuse as in your illustration if I can find the right places on the PCB.

I'll post a fix if it all works.
We restrict ours to 10V on the source if they're the old style ones (with power jack rather than USB type connection)
The older one we have has been shelved for this reason.
We have a simpler one now with the fuse (13A) in the plug now. Much easier to replace when its predictably overloaded.