Hach DR300 Service Manual page 42

Repairs
Step Description
6 Turn the optical unit and insert an aperture.
7 Insert the interference filter.
Note: Make sure that the wavelength is correct.
Note: Make sure that the wavelength label is readable.
8 Push the LED head in the housing. Bend the pins 90°. Note
the polarity and direction.
Push the housing in the optical unit.
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Picture