Test Lamp Method

Use No. 57 bulb connected in series with 12 volt battery and test probes. Touch one test probe to heat sink (positive diodes) or to rear end housing (negative diodes) and other test probe to individual diode lead, then reverse test probes. Lamp should light in one direction and should not light in opposite direction. If lamp lights in both directions, diode is shorted. If lamp does not light in one direction only, diode is open.