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

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.


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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.


Hi. Eliya
My Juno-106 has exactly same problem. PWM affected by LFO, but only from the panel
Replacing MUX4052, TL022 OPAMPs didn’t help…nothing changed.
So would you please give some hints for me? Where I have to replace.

Good luck to you and your Juno-106


Which 4052 did you replace? The one that multiplexes the LFO and PWM CV is IC1. You could also try replacing IC2 which is the buffer, but I doubt it contributes to the issue.

Other than that I would suspect a short between the two sliders.

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IC 1(left 4052), IC2 (left TL022 buffer) was replaced. I checked short circuit between two sliders but nothing was unusual. Do you have any good idea to check short circuit between those? I just put multimeter’s probe on every single pin. It bleeped only when ground pins were matched.

I must admit that I don’t know very much about circuit theory, so I’m not even sure if a short between the two sliders is what’s doing it. Measure the resistances between two different sliders that you know are working well. Then compare these measurements to those between the PWM and LFO slider.

It is also possible that your replacement Mux is bad. If it’s new old stock or from an old batch maybe it has gone bad.

Lastly, maybe most importantly, how did you verify that the LFO and PWM otherwise behave correctly? I used the Juno Editor software. Have you used that software or another to make sure the problem isn’t at the CPU?

Thank you for your help. Eliya.
Measurement finished.
There weren’t any strange short circuits between two sliders.

Yesterday although I switched IC1 to IC4, right part of panel works very well. but left’s PWM, LFO still has same problem.
Maybe it’s not the MUX problem. Tonight I’ll replace buffer once again.

CPU shouldn’t be the problem. I checked with software called j106(http://www.jarvik7.net/juno-106/)

If replacing TL022 doesn’t work, could it be a PCB failure? I hope it isn’t…

I think any continuity issue with that could be caused by the PCB would’ve shown up when you were testing between the pins of the sliders. I still think it’s something with the MUX, it’s a pretty simple path between the sliders and the inputs to the chip.

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Had a closer look at the JUNO 106 schematics - I don’t think you have proven anything yet by using programmed LFO vs Slider LFO except that programming works and physical does not.

If you have a closer look - the 4052 is nothing more that an Analogue MUX - switching and selecting 2 sets of 4 analogue signals and routing them out to the opamp IC2 and IC2 is no more than a buffer. These signals and then routed directly to AN inputs of uPD7810. The AN inputs are A-D conversion inputs - so the uPD7811 has 8-channel AD converter built in.

The digital selection does not get selected from the same points. Therefor I would assume this to be possibly a firmware bug.

First - we need another 3rd JUNO 106 user to do the same and also verify from each user what version firmware each is using. If all 3 have the same problem then we do have a FW issue.

I like your reasoning, but the issue did go away once I replaced the MUX on my Juno. It’s also possible that rladusrn and I are experiencing slightly different issues.

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I too had the exact same issue and it was 90% fixed by replacing the 4052. I still have some weird “leaking” from the LFO Rate-slider to PWM; when PWM is set to manual the rate-slider still affects the PWM slightly.