What's in those attenutators, exactly, besides a potentiometer? I have a similar looking Radio Shack model. Got it at a garage sale for a nickel or something. Maybe it's defective. I tried it a couple of times, and it seemed to have a range between distorting the signal and really really really distorting the signal.
Back to tubejunke's question. I see no clear statement that
any source works well on channels 3 and 4 and I will respectfully renew my advice to clean the blasted tuner already. Quick and easy. I could show a child how to do it. Costs about 3/8 of a cent. Can't hurt, and it just might help
Phil