Danelectro french fries with very little effect and strange pot

[eve-the-frog]

Hi everyone,
I'm here again with my noob questions...

This time I'm breaking my teeth on the Danelectro French Fries autowah :


Here is my work




With the resonance pot maxed I have a very little effect. I guess it should be like that with the pot at 9 o'clock.
There is two other issues :
- When I fully turn the resonance pot CCW the sound become muddy (with no effect at all) and seems to act like a tone pot
- all of this happen with the switch in position A, but when I switch to B the sound is very low in volume, very trebly and if I play a chord I have a strong distorsion. When I play a note the sound volume increase to normal after a few seconds (I'd say 2 or 3).

Of course I double checked my board and for components misplacement.
I also found the sweet spot on the trimmer, and try other positions with no better effetcs.
Eventually I tried with different IC I have in stock : TL072, LM833 and TL062 and different transistor...
The sound is louder and the effect seems better with LM833 but nothing drastic.
I tried to replace the 2M2 resistor with a 3M9 (tips I found on another autowah project) : still the same.

The muddy pot issue sound like a capacitor problem... but not sure.

If you have any idea, I 'd really appreciate your help.
Thanks !

[PMowdes]

Is your ic upside down??  

Can you measure the voltage at each leg of the chip?

[eve-the-frog]

The IC is not upside down, my picture is...

But another strange thing is that the effect works the same way with transistor in both positions.

I'm not at home, but I will measure the voltage  at each leg of the chip this evening (which mean the middle of the night for you : I'm in Europe).

[Marbles]

If the IC is not upside down.. your transistor is I think...

Which one did you use? Cause it seems to be the opposite from the layout.

[eve-the-frog]

I use 2N3904, and I found that it works better with a reversed transistor... But I'm not sure : maybe the distorsion occur only with a reversed transistor...? I'll check it this evening too.

[eve-the-frog]

I checked the voltages and here is the result :


I've tested the effect again and didn't find that it's better with reverse transistor. With the right position the distorsion issue disappeared, but all other issues are the same.

[Marbles]

I'm no expert, but the 0v on the collector of the transistor I would suspect to be something wrong with.

I'm trying to see if it's connected to ground through a link or something, but nothing yet.. so.. maybe it's not getting any voltage? I'm sorry, i'm just using your debug for learning purposes :D

[eve-the-frog]

No problem, you're welcome.
We're all here to learn, right ? Well, at least you and me

Anyway, if I follow the +9V from the battery I have 7,7V after the 47R resistor, then 3,79V after the 10K resistor, and 0V after the second 10K resistor placed before the E of the transistor.
So I guess it is normal to have 0V on the E, isn't it ?

[Marbles]

The E is fine. That should be 0. It is directly connected to ground on the lowest rail. It's the C i'm trying to figure out :D

[eve-the-frog]

I'm definitely the one who needs to learn...
I didn't notice that E is connected to ground !

[PMowdes]

Eve

A few words of advice,

if your build doesn't work and you have to look for a problem it really helps if you can find a schematic.

Look for the power supply to your transistors / IC's and make sure that you have the right supply to the right pins / legs. And do the same for the GND.

Next try to follow the signal path through the schematic and relate that to the vero.  Make an audio probe and see if you can follow the signal through the build from input to output, you usually find a place where the signal disappears.

There's definitely something wrong with the voltage at your transistor on this build, i'd concentrate on the components that link the collector to the 9v supply.

There's some useful info here  

http://www.diystompboxes.com/wiki/index.php?title=Debugging

http://www.geofex.com/fxdebug/fxdebug.htm

[PMowdes]

I've got a feeling that the french fries is very similar to this (read the thread on the build page it mentions the Doctor), it is probably a good place to start.

[PMowdes]

There are three components which could supply the collector, the two 4n7 caps and the 47K to the left of the IC.  check these and also check for solder bridges around the jumper that connects the bottom row GND to leg 4 of the IC.

[eve-the-frog]

Thanks for the advice... and the links ! These are goldmine for me !

I have to admit that I don't know all the signs that appears on a schematic, and I don't really understand the process that come from a schematic and end witn a vero layout...

I'll check the caps and the resistor this evening.

[eve-the-frog]

I replaced the two 4n7 caps and the 47k resistor, I also replaced another resistor which had a wrong value (47K near sw3 and sw2). Nothing wrong around the jumper from GND to leg 4 of the IC...
I heat up all the solder to make sure there is not a cold one and checked again for a solder bridge between strips.

I alos try with a fresh 2N3904. And a new JRC4558.

The problem remains : still 0v on the collector of the transistor, and still the same issue when I test the effect.

I didn't audio probe, but as I already know what is my problem (but not how to solve it) I'm not sure it could help.

There is "bad magic" around autowah for me : I already built Seamoon funk machine, both versions (with and without buffered blend) and none worked :(

[Marbles]

Sorry to hear :/

You provided a lot of info. I never post a schematic either. But it can be handy. You know there is a problem with a part of the transistor only getting 0v.

With a schematic it's easier to see what is directly connected to that part (or what should be).

It looks like all your cuts are in place too.. Any way to upload a higher resolution photo?

[rocket88]

Just did a quick glance. Follow the current with your DMM the current from the DC jack up to the transistor. Since you're not getting voltage at the collector you should be able to see where it disappears, which should help isolate the issue and we can go from there.

[eve-the-frog]

Here is a better resolution pic :


And the result of further investigations :
- I checked my battery and it's only 8v, so I plugged the board in a wall-wart 9v power supply (not regulated) that give me 12,5v : the result is the same in A position of the switch, but no sound at all in B position.
- if I follow the current that feed the C of the transistor I have 6,15v before the two 4n7 caps and 0v after (is it normal ?), but more important I have 0v before and after the 47K that comes from sw5 wire (in both switch position). If I follow the wires of the switch there's something I don't understand : the sw5 wire should have curren from sw4 or sw6, right ? sw4 and sw2 are linked to leg 3 of the pot and sw6 is linked to ground...

[eve-the-frog]

Any idea ?
Would you try to build again from scractch at this point ?

[Marbles]

I'm sorry, I meant a pic from the bottom..

I always find it easier to see bridges, accidental cuts, part not soldered in easier in a high res pic from the bottom than having the real thing before me..