also tested with a normal resistor of 100kohm. But the values stays the same.
4.95V (therm 1) and 4.65V (therm 2).
What does it read if you connect the thermistors back and enable heating?
Does the voltage change?
therm1: 4.85V ; therm 2: 4.65V
in repetier the temperature stays constant.
enabled heating of the extruder and bed, but repetier doesn’t show a output-curve. Measuring the output of heaters = 0V.
Is there a way of checking if the internal AD-converter is still working?
Just to be clear motors and fan are still working.
The marlin firmware disables heating if the temp is below 5°C to prevent overheating the hotend with a broken thermistor.
What i tried is just what you meant. Measure the voltages to see if it changes and compare to the adc’s.
Did you change anything in the firmware?
Perhaps try reflashing the board. (don’t forget to load firmware EEProm defaults in repetier after flashing)
reflashed it yesterday, doesn’t fix the problems.
did you reload the firmware defaults to the eeprom?
yes
Does the reading change if you short the THERM1 pins?
yes: 738°C
Then the ADC on THERM 1 works correctly.
Does it do so on THERM2 too?
also 738°
So the ports are ok.
Then there is only a wiring fault left over as a cause.
Have you got a spare hotend thermistor in your delivery?
no
Hmm, then try to unsolder it and connect it directly to the board pins. (perhaps by jumper wire)
Or perhaps try the same first with a 100KOhm or (something near) resistor if you have one handy.
Also check the plug itself.
Someone here on the forum had a bad connector plug, leading to similar issues.
I’ll check/try it tomorrow.
thanks for the help
No, problem 
checked and measured again.
made a mistake: the resistance of the smd-resistor on the board at the therm2-side is 0.7 khom not 7kohm (should be 4.7kohm).
Also the (+)pins of therm1 and therm 2 are connected with 5.7kohm. I assume this is not normal and that there is a short somewhere.
Open circuit: 4.95V on therm 1 ; 4.65V on therm 2. Not normal, even when the resistors are not equal this should be the same. I think there is a leak-current somewhere.
also connected some normal resistors to the pins instead of thermistors. The therm1 (extruder) side gives different temperatures. But the heated bed (therm 2) stays at 25°C.
If therm2 resisor is 700 ohm it is defective.
The resistance you measure between therm1 + and therm 2+
Is because both pins are connected to +5V ( pullup for the ADC line) over the resistors.
You can find the schematics of the controller if you google reprap 3drag controller.