Usually the problem doesn't start there, though. Electricity is like water: it will always take the easiest path -- i.e. shortest path back to ground. Usually a bad ground forces (or a short allows) the electricity to flow where it isn't supposed to, in this case through the fuel gauge sender ground when you operate the turn signals. So I would also look for a bad ground in the turn signal circuit, or you can probably expect to be revisiting this again when it burns out your next printed circuit. It's certainly possible it's all the printed circuit's fault after 40 years, but bad grounds elsewhere happen a LOT more frequently.