The waveforms show control circuitry firing the scrs (good oor bad)at
a very early phase angle, suggesting that NOTHING is actually working
as intended. The control circuit might as well be open loop.
I would expect, under light-load test conditions, to see a relatively
late phase angle firing in a functional circuit that simply controls
the gross line frequency amplitude, downstream.
In a PWM circuit with two scrs (the second might be un-noticeably
small) intended to convert at a frequency higher than the line
frequency, you would see alternating gate inpulses to the two devices.
How are you determining that everything is 'normal'? It's possible
that the 'good' scr might meet an early death, if loaded under current
circumstances.
The 'good' waveform might result from bad or open connections in the
output power train.
If the 'bad' latching thyristor results in output overvoltage, check
the feedback path to the control circuit for open contacts etc etc.
RL
