Onkyo TX-SV636 only working in Bypass mode. All prologic features have failed. Replace DSP chip or...?

Answer 1

It sounds like the DSP chip has "bought the farm".But, there could be other chips involved also which makes finding the problem even tougher. Finding the exact replacement could be very difficult. Installing it (if you find one) could be agony. As a former electronic assembler (victimized by the crash and burn of the high-tech industry) I know that replacing chips is NO picnic.
The TX-SV636 was a great receiver in its day. But, with the advances that receivers have made over the years, I don't think that trying to repair (or have repaired) your receiver would be worth it. Buying a new Onkyo receiver somewhere in the price range that your existing one costed back then could be an awesomely HUGE upgrade in MANY ways. I'd say, go check out some of the new Onkyos at Circuit City or some other dealer who carries Onkyo.