I built one of the last few PAiA Proteus I keyboards ages ago and was disappointed in one of the oscillators - I believe it was the LFO.
I could not tell much difference between the square wave output and the triangular wave. On my oscilloscope, I discovered that regardless which waveform I selected both were present. A triangular waveform showed many spikes leading to the rails to form a square, and vice versa. While one waveform was prominent, it would look like the area between was shaded in.
At the time, I concluded that it was a defect in the design of one of the ICs used. I’d noticed the same behaviour with the XR2206 function generator and somehow got it in my mind that that is what the Proteus used for the LFO. Looking at the schematics now, I’m suspecting something is wrong with C33 and that first op-amp.
Has anyone encountered something like this?