Juno Chorus

[JohnRoberts]

Hi there!
After letting this project on a shelve for a while, i came back to it last night and found out its working somehow!
I have a few issues tho :
- i made the 220pf// to R24 mod but still have an annoying noise that fluctuates with modulation rate (i guess thats what dmp called robot noise?)

- it has high pitched noises increasing when i engage the depth knob - or when this one is fully off…
Could this be due to the fact that, for now, i housed it in a plastic box with metal faceplate?

MN3xxx series BBD Analog Shift Registers generally have two MOS source follower outputs. One is the delayed audio signal, the other a duplicate of the actual signal delayed one half clock tick. Some designs use a trimpot to balance the DC offset caused by different Vgs in the two source followers which generates a clock frequency square wave. If the clock frequency is high enough above audio, a simple LPF can scrape out that clock frequency component. If one of the two outputs is not connected the clock frequency component can be a couple volts p-p.

Unlikely to be related to chassis.

JR

- and lastly, i’d like the rate LED but can’t figure it out… could someone help me with completing this drawing with needed connections?


Thanks very much!!

 

[f.aiseur]

Hi!
So i made all the mods, but still the same "issue"...
Attached is a small recording to showcase what i get :
First is a small dry sequence and then i move the knobs, starting with depth fully CCW and ending with input fully CCW...
Is this normal sounding to you?
Thanks for your time!

edit :
JohnRoberts, dmp, thanks for all your explanations!
Sometimes i'm amazed by my brain... how slow it can be!
It took me a while to understand what you were talking about, even by reading daily your different posts....
TRIMMERS, YES! TRIMMERS!
But after tweaking them all ways around, i couldnt notice any change in the unwanted noises - or any change at all.
Maybe they are bad, so i will change them and update my progresses here
Thanks again!

 

[f.aiseur]

Hi!
So after changing the trimmers, i still have the same behavior…

I noticed that r81 (r81 on gligli pcb = r85 on original schematics) was in fact 22k in original schematics, so i’m wandering why did you change this one to 10k, dmp?

Not sure it can affect my issue buy i’m curious!

Thanks!

 

[dmp]

The original had 10k (R15 I think) in the schematic in the service manual
This sets the bias of the transistor in the buffer of the input signal going into the MN3009. I don't think it would cause the robot noise we've been talking about.

 

[f.aiseur]

Isnt it the green part that we are talking about?
I’m far from secured with my understanding of schematics…

But yeah this is just curiosity..

Do you have any recommendations on what i could try to tweak? Maybe a different cap value in parallel with r24?..

Thanks!

 

[dmp]

Isnt it the green part that we are talking about?

Your screenshot has different part numbers than the schematic I referenced from the service manual.
Do you see how "service manual" is a hyperlink in my posts?
If you want to use your schematic please post a link to it

 

[dmp]

Post the BOM you used with updates to all the parts you changed, showing what all your installed values are? I can have a look at it

 

[f.aiseur]

Well the screenshot i sent this morning comes from this website :

https://www.florian-anwander.de/roland_string_choruses/
He has other schematics there, pretty nice source of infos on Roland’s choruses btw!


I do see the link in your posts yes and i checked it out. i saw some differences and it seems the net is full of various schematics…

For the BOM, i used the one Gligli provided and did the exact same mods as you explained here except for the switch that i didnt use.
I bought almost everything from tayda so the sk30a or maybe sa1015/sc1815 could be somehow bad - but i guess its not related to my robot noise issue

 

[dmp]

I can't help if you don't post what parts you used. Assuming you used all the right parts isn't going to help figure out the problem.

Similarly, if you don't post what schematic you used it's hard to be on the same page. That page you linked to has a juno 6 chorus schematic and it agrees with mine, not yours. I'm not going to hunt through all the other schematics there to try to figure out which one you've been looking at because you can't be bothered to just answer the question.

 

[f.aiseur]

Hi dmp,

Sorry for the schematics link, i thought i posted both links but only one made it thru…
Here is the juno-60 schematics i took the screenshot from :

https://www.florian-anwander.de/roland_string_choruses/JUNO-60_schem_chorus.jpg
I didnt really use it to build it, just after, to check if i could spot something that would fix my issue.

I used gligli’s bom provided here :

https://github.com/gligli/juno-chorus-clone/commit/86dc4d6e32addefe5640765b14b110dfda580ac2
And followed it exactly, then just did the mods you explained here, a few messages before. No more no less.

I’m sorry if i didnt answer your question the way you expected. If there is still some misunderstanding i could post a picture of my pcb?

I was a little confused with all those different revisions schematics, but, as stated on the github, this project is based on juno-60

 

[dmp]

OK, juno 6 and 60 use the same chorus, the 60 does the switching in some different ways to allow recall. But I can refer to 60 part numbers to keep things clear. The R19/R24 and adding 220pf would be the same for the 6 and 60. A way to diagnose this is to see if you have the robot noises in both right and left as the issue only appears in one side (as it is reversing the phase of the chorus modulation) for the R19/R24 phase inverter. The way 'space' is provided to a stereo chorus effect like this is to simply invert the phase of the modulation to one of the L/R chorus chips.

You might also try lowering the level into the chorus and see if you still have the "issue"
Reroute wiring to the In/Out and any pots to see if it has any effect.

Other than that you'll probably have to try to rule out any other way the modulation is leaking into the signal. How did you connect RV1 & RV2? Maybe post a picture. If the ground lug of those pots are connected to a different ground location it could possibly let the modulation leak in. Should be close to p2 p5 of U1

 

[f.aiseur]

Thank you very much for those leads!
I won't be able to experiment on this the next weeks but will definitely do it at some point and hopefully will come back here with good news!