Juno 106 PWM affected by LFO, but only from the panel


#1

I’ll elaborate on the terse subject line:

The issue with this Juno 106 is that when I move the LFO slider, it changes the pulse width of the square wave. That happens with the PWM switch being set to manual. To isolate the problem, I controlled the LFO speed from the computer via MIDI, and I was please to see that the problem does not persist. So the issue is somewhere on the panel or on the way from the panel to the CPU.

The way I understand the schematic is that voltage “from” the sliders is passed to ICs 1 and 4, which are dual multiplexers. The MUXs’ outputs are buffered and passed on directly to the CPU. So the problem is somewhere leading to the CPU because we know the CPU works fine. My assumption is that the issue isn’t between the outputs of the MUXs to the CPU because then I wouldn’t have seen it affecting more than just the PWM control.

So the problem is either with the MUX or the sliders/panel. I tried to measure for shorts between the PWM and LFO sliders paths, but nothing unusual there. Could it be a bad MUX? Is it possible that there’s a leakage only between two inputs of the MUX? I’m not familiar with multiplexer failures, so not sure where to check next. Would love some feedback before I pull out IC1 and swap in another multiplexer.

Thanks!


#2

I wanted to update on this. I know my original post (at least the title) made no sense, so I’ll try and clarify. If the PWM was set to manual (so the LFO shouldn’t affect it), moving the LFO slider would cause the PWM to change.

Anyway, like I suspected it was an issue with the multiplexer and replacing it solved the issue. I still have a couple more issues left with this Juno and then, hopefully it’ll be done.