When transistors fail the fault is either a SHORT CIRCUIT or an OPEN CIRCUIT in one or more junctions.
An OPEN CIRCUIT BASE/EMITTER junction makes the transistor DEAD. With no current flowing in either the base or collector circuits.
A SHORT CIRCUIT BASE EMITTER junction will allow current to flow freely between these terminals with no voltage drop but no current flow in the collector circuit.
The aforementioned faults are by far the most common but occasionally a BASE/COLLECTOR junction goes short circuit allowing current to flow uncontrollably, sometimes damaging other circuits. This failure is common when the collector/base junction voltage rating has been exceeded, a common reason why a transmitter output transistor could fail when there
All of these faults can be found by measuring voltages or by testing the transistor out of circuit with an ohm meter or transistor tester as previously mentioned. (Some types of transistor testers can check the device in circuit.)
The origin of some of the text on this page is from the RSGB with additions by the web master