Ruby_tuesday
I had the same thing happen to me.
The problem is the "Instrument voltage regulator" This little gizmo resides on the back of the instruments & controlls the voltage that the instruments see, which is less than battery voltage. I think it is as low as 6 volts.

The only fix is replacement with a servicable item. I do not know if it is interchangeable with the regulator from other Nissan Datsun models, but i bought a new one, so it was a looong time ago.