Juno 106 - wierd distortion fault

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Mooger5
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Re: Juno 106 - wierd distortion fault

Post by Mooger5 » Fri Nov 02, 2018 8:33 pm

Any news?? It´d be good to know if at least one worked :thumbright:
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polysixer
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Re: Juno 106 - wierd distortion fault

Post by polysixer » Mon Nov 12, 2018 12:35 pm

Mooger5 wrote:Any news?? It´d be good to know if at least one worked :thumbright:
Sorry, been out of the loop for a week. Logarithmic slider did mostly fix the issue, though still present on extremes and I'm not really that fond of the resulting curve. Increasing resistance at R36 seems the most obvious better sounding solution. I'm also waiting on some TR7 and TR8 vintage replacements which could make a difference, will post more updates this week.

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Re: Juno 106 - wierd distortion fault

Post by Mooger5 » Mon Nov 12, 2018 9:51 pm

All waves on, vcf fully open etc, at +5 will distort...
The 10k resistance can be tweaked, such as 5k for a less pronounced curve. It can even be across the wiper and the other terminal of the pot for an anti log curve.
R165 can be temporarily bypassed with a pot wired as rheostat to vary the resistance to a suitable value after which it could be replaced by a resistor.
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polysixer
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Re: Juno 106 - wierd distortion fault

Post by polysixer » Tue Nov 13, 2018 1:58 am

Ok I fixed this. I can confirm it was the muting transistors that were degraded, TR7 and TR8. You have to replace them with the exact part numbers 2SC2878A, no equivalents or others will do (I had put equivalents with the same specs when I began investigating the issue. Both the originals and replacements distorted). As a nice side effect the chorus is not nearly as noisy. I can now increase the VCA as much as I want, it will never clip, even with all waveforms.

Thanks a lot Mooger5 for your patience and kind explanations and let's not forget this will also help Juno-6/Juno-60 and even Jupiter-6 owners who have this issue, which now seems pretty frequent.

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Re: Juno 106 - wierd distortion fault

Post by Mooger5 » Tue Nov 13, 2018 9:34 am

Wow I would never have thought of the mutes! Great job :)
My local supplier has the parts in stock. I've got to try this. Maybe the Juno has more useful headroom than we all have thought.
Congrats for the fix. Well done!
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Re: Juno 106 - wierd distortion fault

Post by Mooger5 » Wed Nov 14, 2018 9:08 pm

Nope. Mine sounds the same, normal as usual...
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Re: Juno 106 - wierd distortion fault

Post by polysixer » Wed Nov 14, 2018 9:50 pm

Mooger5 wrote:Nope. Mine sounds the same, normal as usual...
Yes, you get a very distinctive distortion in some settings when these degrade. You can hear it in most of Synthmania’s Juno-60 videos and others.

I wonder how many confuse this issue with faults on the MN3009 BDDs. I’ve seen this as a suggestion in most of the older threads about it. The chorus does make things worse when active but now with the new transistors it's definitely not as loud and the typical swooshing is lower too.

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Re: Juno 106 - wierd distortion fault

Post by Mooger5 » Thu Nov 15, 2018 7:16 pm

No, that was not what I meant to say.

I´ve replaced the muting transistors and I don´t hear any difference.

My 106 remains normal, i.e. the final VCA must be set to zero or below with full waves on, otherwise it will clip just like any working 106 out there.

Any value higher than zero is for quieter sounds, low VCF cutoff, etc. It´s how it works.

I was hoping the new parts would bring an improvement still, but they didn´t. Sorry.

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Re: Juno 106 - wierd distortion fault

Post by polysixer » Fri Nov 16, 2018 11:39 am

For reference, this is where I got my original NOS 2SC2878A pair:

https://www.soundtoparts.com/en/compone ... c2878.html

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Re: Juno 106 - wierd distortion fault

Post by Mooger5 » Fri Nov 16, 2018 11:43 pm

Mine cost 0.50 each and they are the same as yours.

Look, it´s the reference or part number that matters, not how it´s written.

The schematic calls for a 2SC2878a and that´s what it is. A difference in performance between batches if any won´t be perceptible and certainly not as audible as the fault you have described.

It would have to be a real sloppy design if it only worked with a very, very specific part from some magic batch, don´t you think?
Afterall it´s only a muting circuit where the transistors temporarily shunt the signal to ground everytime the unit gets powered on or a memory button is pressed, etc. After that, and most of the time, they act as open circuit.

So you could have tried just by removing them from the circuit.

Heck, the old transistors in the photo are originals and one ends with an "a" while the other ends with a "b", and they are OK.
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polysixer
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Re: Juno 106 - wierd distortion fault

Post by polysixer » Sat Nov 17, 2018 1:22 am

Oh yeah I know, nothing special about those, just pointing out for future reference as source, I couldn’t find them locally.

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Re: Juno 106 - wierd distortion fault

Post by Jakobz » Thu Nov 29, 2018 9:25 pm

Lots of great info here :) im going to try changing Tr7 and Tr8, to see if it helps with my noise issues.

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Re: Juno 106 - wierd distortion fault

Post by 1way2life » Thu Dec 13, 2018 4:27 pm

Hi,

I have an early 106 with a similar problem. Hissy chorus as well as some output crackling only when the chorus is engaged. I solved the hiss issue by replacing the BBD 3009. But, I still get the crackling.

I scoped the output by tracing along the patch. I get a clean output through to the outs of IC6 (TA75558S). However, I saw the crackling on TR7 and TR8. I decided to replace these with some exact NOS A versions. This is to no avail, the crackling continues.

This is not an overdrive distortion sound, but a literal crackling... like when you twist an old cable on an amp.

Any ideas?

Thanks.

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Re: Juno 106 - wierd distortion fault

Post by Mooger5 » Fri Dec 14, 2018 12:11 am

Was the crackling still present with tr7 and 8 removed?

If not, tr9 could be a source of noise. Also c18 and c19 are good candidates. Redo the joints first.
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Re: Juno 106 - wierd distortion fault

Post by 1way2life » Fri Dec 14, 2018 12:44 am

Thanks Mooger5,

I will try all those suggestions and post my result.
Thank you!

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