Strange things these days. My tempeture on the display makes jumps from 200 to 160 and back in secondes… Seems there is some bad connection. Are the white cables to the nozzle the once for the heating?
Hello,
It seems a problem with the NTC temperature sensor itself, or wiring / fasle contact.
With link below, you’ll find the printhead assembly manual. This may help you troubleshoot the problem.
Replacement parts: Heater & Sensor Vertex Delta:
HEATER CARTRIDGE AND THERMISTOR ASSEMBLY SPARE PART SET FOR VERTEX DELTA 3D PRINTER - Whadda
Best regards,
Vellmean Support
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Did some reconnecting yesterday. At first sight i stay stable now. Will do a test when im back home… Thanks for the file!
Hello @Human
@VEL450 give you the right track to solve this kind of issue.
You can also check the “temperature control loop”, no short cut of the cables, and température sensor, testing the temp. sensor with an ohm meter, good contac of the connectors, …
P.S. : This could explaine your previous under extrusion issue … has in the marlin firmware there is a protection to prevent extruding under 170°C ! (#define EXTRUDE_MINTEMP 170)
Have a nice day !
Gonna give it a try today. Also orderd the parts mentioned above.
Always good to have replacement components, as they are rather essential for printing 
Good luck in this adventure of finding the wrong thing. Have a nice day !
Got the parts yesterday. While fixing it i saw one cable was damaged (the one thats close to the nozzle). Could fixed it but replaced all the parts to be sure. Had to stop because the nut to tighten the sensor was missing in the package. Bought one in the hardware shop today. Now testing and seems to be fixed. Tempature stays stable at 200°. Print look good so far…
Thanks for the help!
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You’re welcome,
Well done 
Nice to hear it’s working again.
Best regards,
Velleman Support
HI,
I can’t get access to the printhead assembly PDF. Is there another link?
G
Thanks PPAC,
Much appreciated.
G