Tonepad Corrall - MN3007 Issue - Your thoughts please ??

[scuzzphut]

Hello all !

Just built a Tonepad Corrall Chorus (CE-2 Clone)  http://www.tonepad.com/getFile.asp?id=101 and have been debugging.

I have audio all the way through the direct path to the output.

On the chorus side , I have audio all the way to pin 3 of the MN3007, but nothing coming out of pins 7 and 8.

BBDs are new to me, so the community's thoughts on where to check next would be most welcome !

[Govmnt_Lacky]

Nothing coming out of the BBD does not sound good 

Try posting voltages and we can see where that leads us. 

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[Mark Hammer]

The layout and board are beautiful, and all the needed information is provided, except for....what to do about the bias trimpot.

The audio signal needs to sit on top of an appropriate DC bias voltage at the input of the BBD in order to be able to make it to the output.  As you adjust the trimpot, you will go from no delay signal at all, to a distorted and modest-volume delay signal, to a clean and audible delay signal, back to dirty and quieter and finally no delay at all.

In a perfect world, one would feed the circuit with a signal generator, look at the result on a dual-trace scope, and use a 10-turn trimpot for precision.

We live in the rock and roll world, so use your ears, listen under favourable conditions, and just tweak until it sounds right.  Ideally, you would lift one end of R21, such that you hear ONLY delay signal (when, indeed, you start to hear something).  That will help you to nail the best sounding trimpot position with greater ease.  Once you've found it, just solder the lifted end of R21 back in.

[scuzzphut]

Mark / GL  Thanks for your input.


Adjusting the trimpot has yielded the following results.


0-10%   Distorted output on 7&8 of the BBD. No sound out of the direct path.
10-50% No sound out of either path.
50-100% Clean sound on direct path - no sound from BBD.

Seems to be counter to expectations so  - here are my voltages :

3101

9.3    1 ----8    0.7
4.7    2-----7    3.7
0.0    3-----6    6.7
4.7    4-----5    2.0


3007

9.3    1-----8    3.6
4.6    2-----7    3.6
3.8    3-----6    4.7
0.7    4-----5    0.0


Going to bed now - Celtic vs Barca has left me emotionally drained ( I actually had to go to the toilet 3 times during the match - thank goodness for live pause)

More investigation tomorrow !!

[Mark Hammer]

At first I thought there was something amiss with your board since many other chorus units would direct the bias voltage output of the trimpot specifically to the BBD input (pin 3), which ought to have no impact on the clean signal whatsoever.

But then I took a quick look at the schematic and realized the trimpot set a universal bias voltage, that is blocked at several points and reintroduced.  So, C2 and C5 block any DC voltages introduced via R2 and R7.  The DC bias is again re-introduced via R8,and since there are no obligatory caps the signal has to pass through, on the way to the BBD input, that bias stays in effect, until it is once again blocked by C10.

However, we have now verified that at least the BBD functions, and the trimpot setting makes a difference.  Play with the trimpot in that first 10% and see if you can't nail a cleanish delay sound.  If you can't find one anywhere on the dial, then that suggests your issues lie elsewhere in the circuit.  So, for example, is there any possibility that one or more of the 5 transistors have the wrong pins inserted in the wrong place?

[scuzzphut]

Mark / GL ....... success ! 

That was easily the most intense debug I've done yet - many thanks to you both for setting me on the right track.  On top of the CE-2 being a fairly complex build, this was also the first PCB I had etched mysefl, so that compounded things.


For the record ,  I had ....

One leg of the trimpot for the bias voltage had a cold joint.  My bias voltage was swinging around 8v where it should be 4.5ish.

An issue with no oscillation - a static voltage at pin 7 of IC4 and pin 3 of IC2.  THis was traced to a solder bridge at C19 to the 9V rail.

An issue with the 9V supply to IC3 and IC4 dropping to 8V when the ICs were inserted - a quick scrape around their pins sorted that out.



And now I have a great sounding chorus - thanks again guys.

[Mark Hammer]

Excellent! 

For a full-featured unit, I recommend all the mods in the Corrall document.

[scuzzphut]

Just another note for future reference.

One of the problems I had was that all of my transistors were back-to-front.  I followed the orientation of the layout , but later discovered that the 2N5088s I had bought were the opposite pinout.

It's amazing how much of the circuit will function with this error - leading you to believe that they are ok.

Always check !!!

[Mark Hammer]

 

NEVER encountered that in my entire life!  Not once, not ever. 

Yep, as I'm annoyingly fond of reminding people, with just 3 pins it is a mathematical miracle that there are 837 ways of getting the pinout of a transistor wrong.