I purchase a K4 synth in bad condition. The light of the display is ok. There is a line of characters, no letters, no numbers. I have changed the volt regulator IC of the main supply, because there was not volts (nine) within the circuit. It was a success. There is nine volts in the circuit now. But there was not success for to make sounds. I am searching for someone who had been trying to repair this synth. Someone who manages K4 circuit diagrams or information about the electronics of the keyboard, or anyone who has K4 circuit boards in good condition. I purchased too, The K4R module. This device, it is the same K4, but without keys and without FX, the circuit must be the same. My K4R is playing well. Anyone who knows about it, please, help me. Thanks for all.