Tach not working but blipping at start
 

Looking for some advice or suggestions please.
1970 911T with a tach where the needle blips up as you turn on the ignition key. It blips up and falls back down immediately and only does this when you turn ignition on before cranking the car to start.
once started, the tach needle doesn't work and doesn't move at all.

Where should I be looking please?


Thanks in advance.