Common causes and solutions for bulb failure in old LeFeng instruments are:
1. The light fuse is broken. The solution is to replace the fuse with the same resistance.
2. A fault in the vehicle voltage regulator causes the vehicle circuit voltage to be too high, causing the lamp to burn out. The solution is to replace the lamp with a new one and replace the voltage regulator.
3. The battery wires are in poor contact, causing the generator's no-load voltage to be too high, causing the lamps to burn out. The solution is to correctly fix the battery wires and replace the lamps.
4. There is a short circuit between the generator armature and the magnetic field coil, causing the voltage to be too high, causing the lamp to burn out. The solution is to repair the generator coil to normal and replace the bulb.