If by "signal lamps" you mean turn signals/hazards, then you've got a bum headlamp switch or positive feed to it. Very simply put, there are four separate circuits for your lighting: headlamp, running/courtesy lamp, turn signal, and hazard/brake. Only two of them meet inside the headlamp switch: headlamp and running. If you blow the running lamp fuse, the headlamps will work as will the brake & turn lamps, but not the rest of the running lamps, dome light, etc. The headlamps run off a separate circuit which is not fused; it has a self-resetting circuit breaker as a safety feature.
When I first got my '72 Charger SE, while barreling down a back road at dusk at 60 per, everything went dark and I heard the headlamp doors clunk shut. I stopped the car, reached up behind the cluster and fiddled around with the switch and connector, and lo, the Lord said "Let there be light!" I popped the cluster out and the connector itself was badly melted around the main power feed. The road I was on was quite rough and the vibration allowed that one wire to back off the switch. A connector scavenged from my '73 fixed the immediate issue, but what kept it from returning was my going around the car to literally every lighting ground, cleaning it up with emery cloth, and using dielectric grease on the surfaces to prevent further corrosion. Obviously, you want to start with the headlamp grounds, but the taillamps draw more than you might think when both filaments are lit (braking at night) which can seriously tax the ground connection.