I design and test high power equipment that uses large SCRs (silicon controlled rectifiers). Sometimes one will go out. I use 6 in a three phase bridge configuration and if one goes out, its difficult to find the bad one without taking them all out. You can make a "in circuit" tester out of a flashlight.
Step 1: What is an SCR?
An SCR is a Silicon Controlled Rectifier. They are used in power supplies, welders, inverters and other equipment for controlling power. The one below can carry 350 amps of DC current with a rating of 2400 volts. Think of them as a diode that can be turned on with a small current to the gate. Once turned on, they stay turned on till the current flow is interrupted or below the minimum current. Large SCRs like this one requires 150 milliamps at 3 volts to turn on. The SCR behaves like a solid state latching relay. The large base at the left is the Anode and attaches to a heat sink. The "pigtail" end at the right is the Cathode and the white wire is the Gate. The additional skinny red wire is connected to the cathode and is used with the gate when connected to a triggering circuit.