ramps fails to heat hotend higher than 140 celcius

i build a SmartRap with an Arduino Mega and a Ramps 1.4 board
it suddenly stopped heating the hot end up to 200 degrees, only at 140 or lower

i reloaded the Marlin firmware
i mounted a new hotend
i replaced the arduino
i replaced the ramps 1.4 board
no effect

when i use a separate power supply for the hot end it still does not reach the needed 190 degrees
i measured the resistance of the thermistor in the hot end , it is 103.4 Kohm.
the blinking of the led might indicate that the machine thinks the hot end is hot enough, but it isn't
when i change the PID values the blinking stops but i don't get 200 degrees

the strange thing is that the problem suddenly started to occur without changing anything.
the same Repetier software that drives my delta printer also drives the SmartRap on the same computer.

does anyone know what is wrong
i am out of options here

thanks in advance,
Doede.

Its not a "Smartrap/Smartrapcore" specific problem, you will have better luck in the ramps/electronics sections.

The heater cartridge might be the issue.

i replaced the hotend again with a new one and it works ok now
so the problem is solved but i still don't know what's wrong with the other ones
thanks