MXR MicroChorus BBD Mod

[Noth]

Hello everyone,

I'm moving my post which was initially on the MXR MicroChorus dedicated thread (viewtopic.php?t=6946) here, since it seems to be the proper location for that!

So, long story short:

• the MXR MicroChorus uses a SAD512 BBD chip which is hardly sourceable.
• Fender3D provided on page 5 a complete rebuild schematic and a PCB in order to use a MN3007 chip instead
• Since the original schematic and the updated one are quite close, I just made a small additional board

And now I'm a bit confused!

So I've built a small dedicated board with the MN3007, based on the Fender3D upgrade, as follow:



Basically I did:

• change the clocks capacitors from 20pF to 15pF
• change the two resistors for the bias adjustment
• unsolder the SAD512 and connect the IN, OUT, VCC, CLK1, GND signals
• pick the CLK2 signal

In the following tests, the MicroChorus is fed with the following test signal:



When testing without the MN3007 chip I can measure the following:

- GND (5) to VCC (1): 9V
- GND (5) to VGG (4): 0.6V

I can measure the following clocks signals:



The input signal at pin 3:



And the output signal at pins 7-8 (basically coming from VCC, since the chip is not there):



Note: my son played with my probes, thus there's a x10 issue with the scope captures here

So the clocks looks good, I have a 4V bias as expected on the input signal.


Now if I connect the MN3007 chip:

- GND (5) to VCC (1): 9V
- GND (5) to VGG (4): 1.9V

I can measure the following clocks signals:



The input signal at pin 3:



And the output signal at pins 7-8:



There is many odd things here to me:

• What happend to the clock signals?
• My input signal is now a 8Hz signal with a 2V bias
• The output is flat

Note: my son played with my probes, thus there's a x10 issue with the scope captures here

I did try with 7 different MN3007 chips with basically the same measurements.

Am I missing something obvious here?

[Fender3D]

Sorry for the late reply, MNs chips usually are clock-hunger chips, than SADs...
Just remember, SAD512 is 512 stages BBD while 3007 is 1024 stages, but SAD got internal clock divider in order to provide the 2 clock phases needed. Then with the same VCO clock you should achieve the same delay time.
When I drew schematic with 3007, I breadboarded it and it basically worked, I didn't bother too much with clock capacitors and picked up the first pair I found in my drawer.
You may try and lower R22 and 23 to provide more current and check if clock restores, otherwise just look for a capacitors pair which do not stuck.