Diode Trio
With diode trio removed from end frame, connect an Ohmmeter to the single contact and one of the three connectors. Observe reading, then reverse leads on the same connectors. A good diode trio will give one high and one low reading. If both readings are the same, replace diode trio. Repeat test between single connector and each of the two other connections. Do not use high voltage such as a 110 volt test lamp to check diode trio.