Yeah it works, it just sounds...off. When I hit the sustain footswitch it just turns into a single ascending high pitch sound. Very strange.
That sounds like an issue with the CD4046 lowpass filter or the CD4066.
Check the values of R9 (22K), R12 (330K) and C6 (220nF).
You can try reflowing the solder on those parts, and if it doesn't help, try replacing C6.
Does the Sustain mode LED work/toggle when pressing the sustain footswitch?
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Thanks, those parts and values check out. Haven’t got another 220n ceramic at the moment. Would a film cap be okay there? I know in the instructions it specified ceramic (I bought 2 nice C0G caps for this).