I'd go to the ignition coil, one terminal should have two green/blue wires, the other terminal should have two black wires.
One of those black wires goes to the connector on the tachometer.
Theat's the power wire
I'd check the solderless connector at the coil for security of wires, corrosion, etc .
All of the circuits in the tachometer share a common ground, it's the solid brown wire on the connector .
According to the wiring diagram, the black wire does not go through any connectors, joints it's one piece of wire .
I'd do a continuity test from the coil end, to the pin on the connector that has the black wire .
After 38 years and who knows how many owners, anything is possible !!!!!!