hi ,need help with a carlsbro flanger , alu series late 70s..cant find schem

[bobster]

hi all,
im hoping for some advice to help me improve a flanger i just got from e bay
it is same as the one in this link [ third one down ]    http://www.pedalarea.com/flangers.htm
it works but im finding when i switch the toggle to 'invert' setting , the volume takes a serious dip
if set to a slow sweep it comes up a bit in volume at some points but mainly stays very quiet in comparison to the normal flange setting.
i can find a schemo for this but it has five 8 pin 741's , one LDR , one 4009 , one 4049 , one TDA1022 , two small transistor and three trim pots. a few tantalum caps here and there and what looks like a tiny rectifier bridge [ its probably something else ]
its a bitch to take apart but i had a look and all seems ok visually. i marked where the trim pots were and tried them in different positions but the original setting were by far the best.
anyone familiar with this model and have any ideas?
thanks
bob