Bastl Instruments POPCORN v1.1 Assembly Manual page 20

CLEANING (OPTIONAL)
After the soldering is done you might want to ​ c lean your PCB. You can use e.g. isopropyl alcohol. Put
some of the liquid all over the PCB using the brush (be aware to not let it flow into the pots), let it act
for a while and sweep it off. Then just let it dry. You can repeat these steps until you are satisfied with
the result.
SCREW + NUT
You can also mount the screw and nut to the voltage regulator now.
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