I'm just about to the point of starting over again on this thing.  The voltage in my Nurse Quacky circuit appears to be getting sucked right down to nothing at R13...but R13 measures 47k just like it's supposed to. What should I be looking for? Here is my original Nurse Quacky saga-  http://www.diystompboxes.com/smfforum/index.php?topic=52305.0

I went through the whole board measuring every strip and it appears that my voltage is just too low right off the bat! I'm either missing something really simple here, or there's a higher scientific principle at work that's screwing me over.

Mother Nature's just trying to teach you something.

Let's divide and conquer.

If you (mentally) build just the section with the 100 ohm resistor and the 47K R13 and R14, you will drop a little battery voltage on the 100 ohm, but you'll get pretty close to half the battery on the junction of R13 and R14. Right? Well, yes, if R13 and R14 are the right values.

So if you build this and the voltage at the junction of R13 and R14 is too low then something must be pulling too much current out of R13/R14. What could that be? Well, it could be that cap, it could be the + input of the opamp it's connected to, or it could be an extra solder connection you didn't think you made, or a cut on a vero board conductor you didn't ...quite... make.

So to debug it, separate the junction of R13 and R14 from what loads them. Pull the ends of R13 and R14 up out of the board and solder them together in mid air. Now measure. Does the voltage equal half battery? If not, there is something wrong with R13/R14, as that's all you're looking at. If it's correct, then either that capacitor or the opamp input or an unintended short is pulling it down.

So remove the cap temporarily, then put R13 and R14 back in the board. Now is the voltage on the opamp input correct? If yes, it was the cap. If no, it may be the opamp input or it may be an unintended short.

You get the general idea.

Quote from: R.G. on December 21, 2006, 06:21:08 AM
Mother Nature's just trying to teach you something.

Let's divide and conquer.

If you (mentally) build just the section with the 100 ohm resistor and the 47K R13 and R14, you will drop a little battery voltage on the 100 ohm, but you'll get pretty close to half the battery on the junction of R13 and R14. Right? Well, yes, if R13 and R14 are the right values.

So if you build this and the voltage at the junction of R13 and R14 is too low then something must be pulling too much current out of R13/R14. What could that be? Well, it could be that cap, it could be the + input of the opamp it's connected to, or it could be an extra solder connection you didn't think you made, or a cut on a vero board conductor you didn't ...quite... make.

So to debug it, separate the junction of R13 and R14 from what loads them. Pull the ends of R13 and R14 up out of the board and solder them together in mid air. Now measure. Does the voltage equal half battery? If not, there is something wrong with R13/R14, as that's all you're looking at. If it's correct, then either that capacitor or the opamp input or an unintended short is pulling it down.

So remove the cap temporarily, then put R13 and R14 back in the board. Now is the voltage on the opamp input correct? If yes, it was the cap. If no, it may be the opamp input or it may be an unintended short.

You get the general idea.

Excellent! I was kind of leaning in that direction but had this vision of someone saying "Why would you desolder this and that when you could have just....". I measured that voltage with the IC out of the socket and got the same result...so...time to pull things apart a little.
I will dig into it when I get home tonight.
Thanks a bunch! 
Chris

OK, check this out!!! I desoldered the junction of r13 and r14, connect them together off the board...same low voltage. r13 by itself, no problem, ohm meter on r14 reads 47k. Desoldered c9, same low voltage. Desoldered lead to range 1 by mistake (thinking it was range 3) and my voltage goes up! Desoldered my short lead between range 1 and attack 2&3 and desoldered range 2&3...voltage goes down with only range 1 connected!!!! I had no idea that a pot could do something like that. Some little alien living in there sucking up 2 volts at a time to beam back to the mothership or some damn thing! Guess I'll see if I have another pot and go from there.
Chris

Quote from: greenacarina on December 22, 2006, 12:01:37 AM
OK, check this out!!! I desoldered the junction of r13 and r14, connect them together off the board...same low voltage. r13 by itself, no problem, ohm meter on r14 reads 47k. Desoldered c9, same low voltage. Desoldered lead to range 1 by mistake (thinking it was range 3) and my voltage goes up! Desoldered my short lead between range 1 and attack 2&3 and desoldered range 2&3...voltage goes down with only range 1 connected!!!! I had no idea that a pot could do something like that. Some little alien living in there sucking up 2 volts at a time to beam back to the mothership or some damn thing! Guess I'll see if I have another pot and go from there.
Chris

Scratch that. Don't know what I was smokin' but obviously didn't smoke enough.