A look at an old AVR (Automatic Voltage Regulator) PCB that I was asked if I could fix. these devices are used in generators to regulate the output voltage, which they do by varying the DC excitation of the rotating (rotor) central core of a generator. A generator works by spinning a polarised magnetic field inside an external (stator) group of coils to induce current in them as an alternating AC waveform. By varying the magnetic field of the internal rotor by applying DC current via slip rings, you can adjust the voltage generated.
In this case the AVR (Automatic Voltage Regulator) is powered by the output voltage and also uses it to monitor voltage and frequency. It doesn't regulate the frequency, since that is done by the device rotating the generator, but it can detect if it's dropping too low and stop applying excitation for protection against high current flow. It monitors the voltage and compares it to a set value to determine if DC excitation of the rotor is required, and if it is then this unit uses a thyristor to switch the power to the rotors excitation coil on one half of the output waveform.
The operation of these generators generally requires a residual magnetic field in the rotor to even start producing electricity, so if a generator has not been used in a long time and that field has diminished too low to induce enough current in the output (stator) windings to provide its own excitation current, then it is sometimes necessary to disconnect the excitation windings and apply a DC current briefly (often from a starter battery) with the correct polarity to put in a base magnetic field to get the system running again. This is called flashing.

This video also shows how some fairly innocent passive components can fail as opposed to the more common active components. In this case it was a resistor that was being used well within its rating, but failed in a manner that left no obvious sign of the damage you normally see with less passive resistor failure.

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