mn3207 chorus problem

[chovi]

Hi everyone, a couple of weeks ago I decided to build a chorus based 3207 according to this youtube.com/watch?v=ykvRbAgyyRs&t=106s but I only had a chip from aliexpress (probably fake) so I bought a new one at retroamplis. Even with the retroamplis chip, the effect had only a clean tone. So I built a new one small clone from effectslayouts based on 3007, but with the conversion board 3007 to 3207, the effect again had only a clean tone. Finally, I decided to build a mxr chorus, but it still has a clean tone. I checked each pcb many times, values, traces, positions... etc but effects have still clean tone.

pins voltage 3207 mxr chorus:
1-gnd
2-4,0v
3-4.7v
4-7.3v
5-7.9v
6-3.9v
7-0
8-0

is it possible that i have bad chip from retroamplis? does anyone have a similar experience??

[ElectricDruid]

It's *possible* that you have a bad chip from Retroamplis, but it's far from being the *most likely* problem.

It looks like you etched your own board, so did you find a PCB image somewhere, or did you design one? Where's the schematic you used? We need to see the circuit to be able to help you. I can find ten versions of the Small Clone online, so it's important that we're both talking about the same thing!

The Small Clone circuit (like the one in the video) uses a CMOS 4047 astable as the clock oscillator. I notice your circuit doesn't include that chip. So how is the clock generated? (This is why we *really* need to see a schematic).

[chovi]

I dont have any schematic, I just found verified pcb layouts.

[ElectricDruid]

I dont have any schematic, I just found verified pcb layouts.

So can we see them or are they top-secret? Where did you get them?

(Lordy, this is like pulling teeth... )

[chovi]

http://effectslayouts.blogspot.com/2022/09/ehx-small-clone.html
https://www.dropbox.com/s/3w16vrdui5ai8m8/MXR%20MICRO%20CHORUS%20MN3207.pdf?dl=0

[ElectricDruid]

Ok, so there's the EHX Small Clone which includes the 4047 clock I mentioned. That's the first one you built, is it?

The second one you built is the MXR Micro Chorus, which looks to have come from a different page on EffectsLayouts:

http://effectslayouts.blogspot.com/2018/09/mxr-micro-chorus.html

And here's the schematic for it:

http://www.wizardinside.it/foto/schemi/m_chorus_schem.pdf

Many of the comments say you have to swap over two of the resistors (to adjust the Vgg supply) for the MN3207 rather than the MN3007. Did you do this, or have you tried it? Since this is something that should have been included on the adaptor board, this would affect both circuits.

[chovi]

yes i did, last pcb layout mxr chorus have swapped resistors.

[ElectricDruid]

Ok, so which one do you want to start with? I don't think we should try and debug both at once, we'll just end up confused. If we fix one, perhaps it'll give a clue for the other.

[DrAlx]

Regarding those reported 3207 voltages:
Looks like Vdd is about 8v, Vgg is 14/15 of Vdd, and the clock pins are showing about 1/2 Vdd (which makes sense 'cos of the duty cycle).
The bias voltage at pin 3 is in the right ball park too. So all those look about right, but 0V at the 2 output pins is not correct at all.

1) See if changing the bias voltage level  (pin 3) does anything.

2) Take the BBD out of the socket and see exactly what pin holes 7 and 8 lead to on your PCB. Check for short circuit to ground.
If those pins are not shorted to ground, then either your BBD is bad/fake, or the clock signal is no good (but I don't know how you test that without a scope).

[ElectricDruid]

The other thing discussed in the comments under those layouts is whether the output resistors need swapping around. The MN3207 typically has 100K's to ground. The MN3007 typically has 100K's to Vcc. You can check this in the datasheets.

I didn't get a clear sense of whether this was important or not. Some people seemed to say they'd done it, others seemed to say it didn't matter and it worked anyway, even if they were wrong. Personally, I'd follow the datasheet just to make sure.

[chovi]

Thanks for your advice. I measured the IC with oscilloscope and the output had no signal, so I tried to measure the IC from aliexpress and this one had a signal on output. I changed IC in the effect and it started working.