Dr. Quack Problem - Need help

[Lucianorg]

Hi guys. I'm new to the forum, and also a noob in building stompboxes.

I started my firts project recently, a Doctor Quack (maybe too much for me at the moment). I made the PCB (ok), got the components (ok), solded it all in place, but I got no sound.
With the switch in one position I have a Clean sound, on the other I've got nothing. As I said, I'm new to this and don't know how to troubleshoot it. Some guidelines would be great.
Also:
1 - I've got the layout here: http://www.generalguitargadgets.com/diagrams/dr_quack_lo.pdf and couldn't realise what's the expression "to control pad, millenium bypass" so I just left that without conections.
2 - I didn't discover which 470k resistor is to be replaced by a 2.2M resistor.

Thanks everybody.

[Lucianorg]

Giving more details;

- The two leds don't light up (they're properly working, I tested)
- If someone could give me tips on where to test with a multimeter I could solve it probably.

[Mark Hammer]

1) The LEDs don't necessarily need to light up, and a great many probably won't.  They may emit light when the current is right, but they're being used as diodes here.

2) Get to know the AC voltage part of your meter.  If it isn't autoranging, set it to whatever range will let you measure AC voltages in the 2V range and under.  First confirm that the JFET buffer is working properly and that the JFET isn't yet another one of those that has the pins where you don't expect them to be. 

With the guitar plugged in, measure the AC voltage on the negative side of the 10uf cap.  A handy spot would likely be the input lug of the sensitivity pot, which is connected to the cap.  You should probably see something in the 30-150mv range when you strum.  If you do, then the buffer checks out.

Does the envelope follower produce an envelope output?  Measure the AC voltage on the output of that op-amp.  You should get a coupla volts.  You should also get something in the 1V range or so (depending on how you pick) on the input of the 25k trimpot.  If you get something that shows a response to your strumming, then the envelope follower is good to go.

That leaves the filter.  Does the trimpot adjust the filter center frequency?  If so, good.  The filter works. 

[Lucianorg]

---- The LEDs don't necessarily need to light up, and a great many probably won't.  They may emit light when the current is right, but they're being used as diodes here.

ok to that.

---- Get to know the AC voltage part of your meter.  If it isn't autoranging, set it to whatever range will let you measure AC voltages in the 2V range and under.  First confirm that the JFET buffer is working properly and that the JFET isn't yet another one of those that has the pins where you don't expect them to be. 

I've tested the Transistors and the IC. They're all good.

--- With the guitar plugged in, measure the AC voltage on the negative side of the 10uf cap.  A handy spot would likely be the input lug of the sensitivity pot, which is connected to the cap.  You should probably see something in the 30-150mv range when you strum.  If you do, then the buffer checks out.

With the AC I tested the Cap you told and got a response, but the strumming made no difference.

Is it the issue?

[jasperoosthoek]

Read the DEBUGGING thread on top of the build your own stompboxes topic. Then report back

[Lucianorg]

Thank you very much for the help. But it turned out it was a beginner's mistake. Actually, two. The transistor substitute for the 2N5457 I used (MPF102) was installed backwards, and I was using as substitute for the TL072 the TL081. I changed it for the TL082 and it worked fine!

But, there's still one thing.
The 100K pot changes nothing. It seems that there's no sensitivity.
The 20K trimpot works good, but could I change it for a common pot? Would it be better a liner or log pot?

Thanks again guys.

[Mark Hammer]

The sensitivity pot may be fine, but one or both of the LEDs are mis-oriented (it isn't necessary that either lights up, but it doesn't help that they don't).

[Lucianorg]

One Led (on the side of the transistor) lights up. The other doesn't. I Don't know if it's ok.

About the pot and trimpot. As I said, it was all working fine. But I didn't like the trimpot, because it wouldn't give me the control that I needed (actually, I'm making this to use on a keyboard, so I need a easy knob to adjust as I play).

Because of that I desoldered the trimpot and replaced it by a 20K Log pot. I tested and got no sound, just as before the problems were fixed. I tried to put back the trimpot but it continued without sound. Could I have burnt a component? And also, what would be the right pot to replace the trimpot?

Thanks one more time.