Total controlled SCR tripase bridge problem

Hi,

i've a system that a total controlled thipase bridge wich control the primary of an step-up transformer. The secondary has a traditional greatz bridge. With a pasing control of the scr i can regulate the dc output from

Some day ago this "machine" had this problem:

withauot load i set a DC output ant the the set point is maintened but there is an oscillation with a very low freq. around 600mHz.

When i connect the load the dc tension start to oscillate and the scr bridge give strange noise.

After these oscillation sometimes the dc stabilize and bridge "sound" good.

So.........is it possible that a scr foult?

How can i check if a scr is dead?

Thanks for any help