We firstly disconnected the transform primary to measure the SCR gates G1 and G2. The picture of wave forms is shown below. During measurement we found the firing angle was changed from pulse to continued voltage. I thought this might be caused by the feedback.
In order to send a correct control pulse to SCR controller, we sent a pulse generated by the function generator. This time we connected the transform primary, the waveform was shown as left bel. We found only SCR gate 2 pulse was modified, the gate 1 was not changed anymore.