Juno 106 chorus distortion

I’m slowly going mad trying to figure this out and could use some guidance!

Fully repaired Juno 106 (new voice chips, recapped PSU, muting transistors, recapped chorus section with new 3009 chips) and I’m experiencing distortion WITH the chorus on, VCA at +5, on a couple of patches, especially Bank 5 Patch 7. The voice chips are sitting in sockets but even having 3 chips in place it’s still present.
I haven’t checked the resistors surrounding the chorus section yet but I’ve whittled my options down and the only question now is “is this just how it’s supposed to be?”
I’ve tuned and retuned the chorus many times, without luck.

Please help!

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Just to be clear, have you completed Step 11 (the Chorus Bias step) of the adjustments in the Juno-106 Service Notes? That’s the one where you need both an audio signal generator and an oscilloscope to do properly.

I’ve done the chorus calibration section several times and followed the entire calibration step by step.

You might be able to track down where the distortion is being introduced to the signal using an oscilloscope and probing along different stages in the BBD audio signal path. The nature of the distortion, whether it’s hard clipping or nonlinearity, will be visible on the oscilloscope and is a clue to diagnose the problem. I’d start probing at TP1 on the jackboard then go backward to the input of each active component (transistors and ICs) and look closely for when the distortion begins. Then do the same with TP2. Is it both or just one of the channels showing distortion?

And to be clear, you used a 10V p-p 1kHz sine wave injected into TP2 for the chorus calibration?

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This is a known issue and occurs with the 6/60/106. In most cases the fix is the replacement of the muting transistors which you need to replace with the same 2SC2878B also the FETs which are 2SK30A, bewqeHowever sometimes this doesn’t fix the problem either and some techs have had luck in removing certain resistors but I don’t have the details of that. On machines I’ve done replacing the FETs and muting transistors did the trick. Some techs don’t bother and just tell the users just keep the VCA level at 0


I have done the 10VPP 1K sine wave to TP2. The next step is to do what you’ve suggested and check the chorus path. I’ll post back with my findings. Appreciate the advice!

So I’ve swapped out the muting transistors but not the FET’s so I’ll give that a try! I also read on another forum that swapping R36 for a higher resistance would help attenuate (I guess) the output. What do you think?

Slight mistake in my post was the muting transistors are 2878A. Not sure about R36, reducing R29 or R33 in value slightly will definitely attenuate the signal though.

I’ll try that out. I really appreciate the help!

Would a 40k resistor be good enough?

Try and see, in particular if we’re talking about R29 it’s original value is 47k. Making R29 smaller will reduced the gain, increasing it will increase it probably giving you more distortion.

I’ll try that out. Cheers!

So it helped a bit but I now noticed that the distortion is much more pronounced when the square wave is activated. Could the wave chips be faulty?

I used 22K resistors at R29 and R33 and the distortion seems to be gone. I’ll confirm after having the 106 on for a few hours.