According to the yesterday's current and voltage measurement, we concluded that there must be a position broken to block the current flow through transform primary. The most possible is the relay K4 on R03 board. So I removed the relay to check.
The relay K4 module is P40C42A12D1-120. I found the pins were burnt due to the high current. One pole was almost melted together, and could not be detached. This broken relay effected the SCR normal operation, might cause SCR breakdown. I have not found the relay from Newark, they suggested to replace it with TYCO 1423254-6. However there was no in stock, we have to order at least 9 pieces if we really want to buy. So I ordered the similar module -- P40P42A12P1-120. Although it's not the same relay, we can replace the old pins with new ones. I think it's the fast way to fix the problem.