Check Filament

When measuring voltage, wrap tin foil around the top of the negative probe. Then press the foil against the wire with your finger.

Fig 1: Identifying Tin Foil
Attach probe circuit tester (in Volt range) to middle portion of each filament.

Fig 2: Attaching Probe Circuit Tester (In Volt Range) To Middle Portion Of Each Filament
If a filament is burned out, circuit tester registers 0 or battery voltage.
To locate burned out point, move probe to left and right along filament. Test needle will swing abruptly when probe passes the point.

Fig 3: Attach Probe Circuit Tester To Burned Out Point