The SCRs can be burnt out in several ways.
Over current which causes over heating.
Overheating because of poor cooling through mounts and fan.
A type of over voltage called dv/dt caused by bad snubber networks.
And an under drive condition on the gates, that caused spot heating and burnthrough on the SCR die.
The gate drive voltage could be hard to read with a regular multimeter.
Depending on the SCR drive circuit it could be an optocoupler type or the more common pulse transformer type,or something else altogether.
One thing that could be checked with a multimeter , would be the SCR drive board voltage.
But I don't know how much of the circuit board specs you have at hand.
My personal favorite for killing them is to drive an inductor at too high of a frequency, and dv/dt them to death.
Robert
Over current which causes over heating.
Overheating because of poor cooling through mounts and fan.
A type of over voltage called dv/dt caused by bad snubber networks.
And an under drive condition on the gates, that caused spot heating and burnthrough on the SCR die.
The gate drive voltage could be hard to read with a regular multimeter.
Depending on the SCR drive circuit it could be an optocoupler type or the more common pulse transformer type,or something else altogether.
One thing that could be checked with a multimeter , would be the SCR drive board voltage.
But I don't know how much of the circuit board specs you have at hand.
My personal favorite for killing them is to drive an inductor at too high of a frequency, and dv/dt them to death.
Robert