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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Noth
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Post by Noth »

Fender3D wrote: 17 Sep 2011, 12:00
theehman wrote:I don't suppose this new board will fit into an old MXR Micro-Chorus housing? I have 3 w/ bad SAD512Ds.
I reworked on this 'cause some requests...

I redrawn the schematic, some minor changes from mictester's http://www.wizardinside.it/foto/schemi/ ... _schem.pdf

m_chorus_schem.png

Then PCB, it'll fit into MXR's boxes http://www.wizardinside.it/foto/schemi/m_chorus_PCB.pdf.
Hi,

I'm modifying my micro chorus which has a dead SAD512, according to the Fender3D's upgrade.
So basically I just have to:

- replace two resistors (R27 / R36) in order to adjust the bias
- replace two capacitors (c14 / c16) to have a proper clock
- unsolder the old SAD512 and connect to a dedicated board with the MN3007 and a few components

Actually there's one thing that surprises me: the MN3007 is supposed to be powered with a negative voltage, thus -9V on Vcc.
According the the Fender3D's schematic, it seems to be powered with 9V instead.

Did I miss something?
Thanks !

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Noth
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Post by Noth »

Oh my bad!

I just took a look at the MN3007 datasheet: pin 1 is actually GND (labled VCC on Fender3D's schematic) and pin 5 is VDD (labeled GND), so everything is fine!

Now I have to find out why I still have no sound out of this MN3007 :)

When measured stand-alone, the clock signals seems to be fine, but when connected to the MN3007 they magically drop to extremely low levels.
Maybe it has to do with the clocks transistors, I kept the original ZTX109 instead of the 2N5088 on the new version?

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Fender3D
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Post by Fender3D »

On my test circuit, I used the first transistors couple I found in my drawer... BC549 I simply reversed the tranny case...
Any NPN BJT should work, just check out clock frequency, if it gets too low, it will "whistle" or make funny/annoying noises, in case, just lower caps value
I don't know if MN loads clock signal heavily, I think I've never checked with and without chip.

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Noth
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Post by Noth »

Hi,

Thanks for your reply!

I double checked and triple checked everything, thus I'm starting to wonder if all my 4 MN3007 are dead.
I don't remember where I sourced them, so this is an option...


For the record and if it may help others be trying to fix theirs:

The original clock, with Chorus speed to minimum, measured at SAD512 pin 1 (chip removed):
Image
-> Frequency oscillates from 73kHz to 86kHz (13kHz range).

The original clock, with Chorus speed to maximum, measured at SAD512 pin 1 (chip removed):
Image
-> Frequency now oscillates from 77kHz to 83kHz (6 kHz range).


The upgraded clock for MN3007, with Chorus speed to minimum, measured at MN3007 pin 2 and 6 (chip removed):
Image
-> Frequency oscillates from 68kHz to 82kHz (14kHz range).

The upgraded clock for MN3007, with Chorus speed to maximum, measured at MN3007 pin 2 and 6 (chip removed):
Image
-> Frequency now oscillates from 73kHz to 79kHz (6 kHz range).

One thing that surprises me is that, after changing C14 and C16 capacitors from 20pF to 15pF, the clock frequency is slower (while I was expecting a doubled frequency).
As far as I can tell on my board, the CLK1 (on pin 2) signal is still fine when the MN3007 is in place, while the CLK2 seems to be heavily loaded and cannot be measured anymore.

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Post by theehman »

Ron Neely II
Electro-Harmonix info: http://electroharmonix.vintageusaguitars.com
Home of RonSound effects: http://www.ronsound.com
fx schematics and repairs

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Post by Noth »

[Edit: Post moved here: viewtopic.php?t=32615 ]

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Post by theehman »

Fender3D wrote: 17 Sep 2011, 12:00
theehman wrote:I don't suppose this new board will fit into an old MXR Micro-Chorus housing? I have 3 w/ bad SAD512Ds.
I reworked on this 'cause some requests...

I redrawn the schematic, some minor changes from mictester's http://www.wizardinside.it/foto/schemi/ ... _schem.pdf

m_chorus_schem.png

Then PCB, it'll fit into MXR's boxes http://www.wizardinside.it/foto/schemi/m_chorus_PCB.pdf.
Old thread, but I can confirm that this PCB does work and fits perfectly in the MXR chassis.
Ron Neely II
Electro-Harmonix info: http://electroharmonix.vintageusaguitars.com
Home of RonSound effects: http://www.ronsound.com
fx schematics and repairs

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