just to paste here what I wrote already in e-maldidn't want to wait for all 3 gray cells ..................... just measure voltage across R28 - to determine which current is going through optocoupler LED it seems that optocopupler's transistor is eating much of Q7+Q8 CCS current , and that's reason why you must increase so much R26 I resume ( but first - measure and write to me that voltage across R28 ) that optocoupler gone south from some reason ; what's value of R26 on other channel ? ......................