The board should be silk screened on the back side and you’ll see the + and – markings by the power terminals. Note which side is closest to the edge of the board per the board markings(+ or -). Now flip the board over and see if the red + lead on the jst connector is actually on the correct side. I’ve seen some boards where the connector leads are backwards and in every case the markings on the board indicated the correct polarity, the problem was always the connector.
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