I have a '73 B that has a problem I can't figure out. When I turn on the headlamps, the tach drops to zero. I've looed at the wiring diagram and can find no relationship between the two. Any help out there?

I have a '73 B that has a problem I can't figure out. When I turn on the headlamps, the tach drops to zero. I've looed at the wiring diagram and can find no relationship between the two. Any help out there?

Possible excess voltage drop. Possible bad instrument voltage stabilizer.

The voltage stabilizer has nothing to do with the tach or headlights.
You are going to need a copy of the wiring diagram for your car and a multimeter.
Start with the battery and clean all the connections you can get to. Measure the voltage at various connections looking for voltage drops.
Check the output of the alternator.
Keep track of your progress on the wiring diagram.
Easy to read wiring diagrams are available at http://www.advanceautowire.com click on stock schematics