Vortex Sound Research

I was hired to repair a prophet V, the problem is that the whole PCB controls (filter, mixer and amplifier as well as oscillator control modules) are bypassed, tweaking the knobs doesn't affect the signal at all, we're getting signal out of the presets, but cant tweak the presets, after disassembly our theory suggests that the problem is somewhere in PCB 3, probably in the data bus or DAC, maybe in a IC, but we don't have a clue at all, the guy before me, said the malfunction appeared after repairing the keyboard, There are some modifications to the original circuit, including jumpers, The guy also mentioned something about the mux pot