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Build directions
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This design is very easy to build. You should start with all low parts. Solder in the relays at last.
Soldering
This pcb has very thick copper traces, 95 um. This means that they cool very good. The result of this is cold joints if you don't observe some advice's. The R1, K1, K2, X1-X4 demand very much heat. You must really burn the solder joint for at least 5-10 seconds. Make sure that the tin really melts and becomes shiny and smooth. K1 and X1 have traces on the component side so these parts must be very carefully soldered because you won't be able to inspect the soldering.