The 640 is in Net mode and can only be zeroed in Gross mode.
Press G/N to return to Gross mode, then press Zero.
Trade up to a POINT and get $500 value for your old indicator!
Qualification:
Any scale brand and scale model qualifies.
Any condition qualifies. (Yes even broken scale indicators)
Any age of scale indicator qualifies (Yes even 40 year old scale indicators)