RPM measure on dash board

[wakespirit]

Dear all,

I have a correct craft Ski NAutique of year 1990.
Since today I do not have any more RPM measure on the dash board..

Where can I check to verify where it can com from.
Whre is the source of RPM measure to identify if it comes from tacho or Measurement device it self on dashboard.

thnaks fro help
regards
serge