Wie sich herausstellte war das Kurbelgehäuseentlüftungsventil mit der Teilenummer 021 129 101 D schuld.
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https://www.youtube.com/watch?v=Ggb0cQYKYnI
Not convinced? This one is also great: http://bit.ly/2DZW7Zo
JBC soldering tools are expensive and rightfully so. The tips (the most important components) are affordable though and can be controlled with a simple DIY station.
If you want something more sophisticated:
sparkybg UniSolder: http://dangerousprototypes.com/forum/viewtopic.php?f=56&t=7218
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https://www.youtube.com/watch?v=GYIiOkr6x9o
Also known as Elektor Profiler this is a nicely made machine but it is not unconditionally recommendable, because of sparse spare parts and closed source software.
Here are the pinouts and a linuxcnc configuration: https://drive.google.com/file/d/0B6S_PcWWM1YlX1VCRFY5WmZhc0U/view?usp=sharing
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https://www.youtube.com/watch?v=RU573U6lsGU
Get yours: http://bit.ly/2DZW7Zo
This miniature soldering 'station' outperforms my daily driver Weller at a fraction of the cost. In this video I disassemble and test both the tip and the handle.
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https://www.youtube.com/watch?v=HgrB5P-rDLw
Keithley 2461 SourceMeter https://www.tek.com/source-measure-units/2461
Keithley Graphical Multimeter https://www.welectron.com/Keithley-DMM6500-Benchtop-Multimeter
Patreon: https://www.patreon.com/marcoreps
Almost but not quite as good: http://bit.ly/2kM1uCv
CERN straws and more: https://twitter.com/GigaBecquerel
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https://www.youtube.com/watch?v=Mk9NQ0XbZUE