[Sequencial] [Prophet 600] – [No sound on Osc B via Mix pot]
Hi, I’ve been working on a friends Prophet 600. When manual programming is selected, there is no sound from Osc B, Osc A works fine. However, if I select a preset sound Osc B plays and I can switch the waveforms off on B and the ‘Mix’ pot works as I can change the amount of each osc in the mix. At first I though it was the 4051 that controlled the CV Vol of Osc A and Osc B. I swapped it out for a new one. Still, In manual programming mode, I can see the effect using the scope on pin 14 (osc A vol CV) of moving the Mix pot, but I see no such change on pin 5 (Osc B vol cv). It’s all very strange and perplexing any thoughts? Many thanks, Craig
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That is a very peculiar issue! The nature of it makes me think it could be a corrupted OS EPROM. Minor corruptions in an OS can lead to all sorts of weird bugs so I always like to rule that out. I had a look at the schematic and I can’t really think of anything else that would cause that behavior.
Hi, I’ve solved this! It’s partly my ignorance of the Gligli mod. I’d scoped all B oscillators and they were fine on all wave forms…what I didn’t know was the Gligli reconfigures the ‘Glide’ pot to be Osc B volume!!! So when I turned the ‘mix’ pot all it did was control Osc A volume…I had glide set at zero while working on the synth. Anyway, note to self, read all manuals before working on something.
This synth has another issue which is it glitches and sometimes plays the same note as the last one played or plays notes very out of sequence and out of tune. It’s a random event so not ascribed to any pericular voice. I think it’s to do with the S&H circuit. I have read that some of the old CD4051’s can’t keep up with the speed of the Teensy ++. I intend to swap out the old CD4051’s for something faster. If that doesn’t cure it then next I’ll replace the caps on this circuit. There are three tantalums on it which I’ve already replaced. Thnaks for your thoughts and feedback. Craig