MXR - Micro Chorus  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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Fender3D
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Post by Fender3D »

Sorry for double post,

did you try with more than 9V?
Does it behave the same?

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Post by sabelya »

Hi fender3D,
Thank you very much for your time, your advices are very helpful.

First i checked everything again and i found an error in R6 value (22k instead of 100k, strange..), but this didn't really change the noise level.
I tried an TL072 IC, no difference.
Battery is not an option but i did the test, and wouah.. this reduce the noise in a low level even if i still hear it.
So i added a big capacitor (2200uF) in the box to use the 9v DC transformer, well the result is good enough.
After, I lowered R27 and R36 to 33k each, no really difference.

Should i remove the IC sockets and soldering directly the ICs on the board ? (to reduce some parasitic effects)

It seems that it's a shielding problem or something like that, no ?

Thanks

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Post by Fender3D »

Indeed C20 and C18 are slightly lower to me... and there's no V+ filter capacitor... (hmm no protection diode either)
but I got values from MXR stock micro chorus, and I was aiming at a 1590B boxing...

The bias divider R27,TR1,R36 could be lowered, I think that 10K, 22K, 10K will work the same and bias network won't be affected by ICs current draw.

You may add a small cap (47-330p) at BBD's pin 3 and see if it helps

I usually don't use sockets... BBD aside

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Post by sabelya »

Sorry but BBD means the MN3007 chip I suppose..
So, as you suggested me, i added 250pF at the input (pin3) of the Chorus chip and the result is surprising.. no effect on the "sinusoidal noise" but it seems to have slightly modified the depth of the Chorus, slightly more, interesting like that :)
This one is for my brother and i'm planning to do another one suitable to the bass guitar, i will use every tips discussed, even for the bias divider values.
Do you think the one side PCB could be better (ground rail around) ?

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Post by Fender3D »

sabelya wrote:Sorry but BBD means the MN3007 chip I suppose..
yep :thumbsup

If the cap from pin3 to GND doesn't work for your issue, it may be a problem with the clock going too much "down"...
Try altering R28/R34 ratio and check... (put 2 ~470K resistors at "sides" going to a 1M trimmer, wiper to R28/R34 junction, find the best behaviour then measure resistance and sub with 2 fixed resistors)
sabelya wrote:Do you think the one side PCB could be better (ground rail around) ?
I didn't try it, I can't tell...

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Post by sabelya »

Sorry but some words are difficult to understand for me..
Do you mean like that ?

http://postimg.org/image/g3drn90eb/

Thanks

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Post by Fender3D »

yes, :thumbsup
the same as bias control...

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Post by sabelya »

Ok thanks.
Just figured out now that I should remove the 2x 1M resistors in place.. :slap:

I need to order the componants, keep you informed, thanks

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Post by sabelya »

Hi Fender3D,
Back to this small box :D

As decided, I built a new 'micro chorus', same PCB, fit to the bass guitar (for me). The previous one works very well in a guitar pedalboard.

So I changed,
- C1, C9 : 220nF, C12 : 1uF (more bass frequency)
- C19, C20 : 1uf (should be no effect for C20, but C19 I ask myself.. ?)
- R27, R36 : 10K, TR1 : 25K (bias bridge as discussed previously)
- TL062 to TL072 (upgrade to low noise amp, should be ok)

:hmmm: Result : no noise, good clean sound but no 'chorus' effect... ! Even with RV1 and TR1 on left or right position :scratch:
I don't understand. I checked everything again and again. I missed something again.
Any idea ?
Thank you very much for your help.

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Post by Fender3D »

What's bias voltage?

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Post by Fender3D »

Now that I re-read your post (damn edit button....):

C19 should smooth LFO waveform in a different way according to LFO speed (R34 + C19 is a time constant),
maybe too much capacitance prevents VCO from functioning...

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Post by sabelya »

Okay, this makes sense..
I don't know if I have a 0.33uF cap, but maybe disconnect C19 will show if this is the right way ?
I will measure the bias voltage as soon as I get home.
I really appreciate your support, thank you so much.

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Post by sabelya »

Is there any 'edit' button ? Sorry but didn't find it..
Bias voltage (TR1 center) is about 4.7v in the middle position.
I removed C19 but no change at all.
I redid all welds..
A defected component ? The TL072 works apparently, the MN3007 was tested with the previous build, maybe the TL061 ?
I'm lost..
Thanks

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Post by Fender3D »

Does it sound the same with and without MN3007?
Whether it does then is an issue with BBD path, BBD itself, bias not ok or VCO not working.
If you hear any difference, then we may assume BBD and VCO are working ok, then it might be an LFO issue.

Please check bias voltage at BBD's pin 3... D1 is mounted correctly?

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Post by sabelya »

:slap: of course, bias voltage at BBD's pin 3 !!

TR1 in the middle (4.7v)

MN3007
pin2&6 : 3.7v
pin3 : 3.6v
pin4 : 0.6v
pin7&8 : 2.8v

Q1E : 3.6v
Q2E : 2.2v (?)

I thought the orientation of D1, but yes it's correct.
Unfortunately, I didn't use IC sockets this time.., more difficult to take off the chips..
I should do it of course.

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Post by Fender3D »

I have no 3007 at hand, but if I recall, bias at BBD should be slightly more than 1/2Vcc (ie ~5V).
sabelya wrote: pin7&8 : 2.8v
I guess they should br around 1/2Vcc, but I could be wrong...
sabelya wrote: Unfortunately, I didn't use IC sockets this time.., more difficult to take off the chips..
BBDs are extremely sensitive to static... a socket here is not a waste of money or time....

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Post by sabelya »

Hi Fender3D,
Just to let you know that the MicroChorus Bass Mod is working now.. ! :applause:
The problem was a mistake, again :roll: , sorry for that, wrong value for R18.
Thanks a lot for your support.

I tried to increase the Chorus effect, I know that there is no simply way to change the depth, so I played with R20 and R17 values, changing the wet / dry signals ratio if I'm not wrong. Result is not interesting enough..
Any suggestion ?

Thank you.

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Post by Fender3D »

Great sabelya...

I'm happy you can enjoy your MC :horsey:

If "increas(ing) the Chorus effect" means altering the wet/dry ratio, you may sub R17 for a 180K-200K, lift C9, sub R20 with a 47KA pot and connect wiper to C9. It will act as a wet level control.

If you want more depth, as I said before, you may try altering R34, and/or R29-R30 values.

Did you try it?

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Post by sabelya »

No I didn't try altering R28/R34 ratio, I thought this was to help the noise I had..
Decreasing R34 does not change the sound for me.
I found something effective, a 50k pot instead of R18 (in addition with 30k for the min value), sounds like an intensity pot :applause: . Is there any problem doing that ?

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Post by Fender3D »

R18 sets, together with R39, Q2's bias point.

It's better a pot instead of R20

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