One guy from the Greek A/T club had a problem with the tachometer as well. He found that the problem was the R5 39Ω blue resistor.
He replace the resistor with a new one and the tacho is ok again.
Αν θες τα όνειρά σου να βγουν αληθινά, μην κοιμάσαι.
If you want to make your dreams true, do not sleep.