original Conquistador

I recently purchased a Conquistador with the round holes in the speaker. The board does not say el dorado on it, it says Umax. There is only one pot on the board that I could find, it was in the bottom right had corner. Is this the ground balance adjustment?
I really like the slow retune, just hover over the target and it gets smaller and smaller but I cannot hold it still enough for it to completely disappear. Thanks for your help.

I am pretty sure that it is the ground balance pot. because all the other adjustments are on the faceplate. But to be on the safe side mark it before you adjust it in case you need to put it back where it was.