Parapedal halfway working.

[Radamus]

Hello, forum. It's been a while since I've posted, and, coincidentally, built a pedal. I got the urge yesterday to put something together, and I build the paralooper pretty quickly with minor debug. I guess I was in some sort of Para phase, because I then started to put together the parapedal from the geofex website. Printed out the schematic a while ago, and used the bottom layout (http://www.geofex.com/FX_images/newparapedal.pdf). I moved the bias voltage power section around a little bit, as I didn't have a perfboard in the right size, but I was very careful to keep everything the same.

First things first:
Battery: 9.03 in circuit

IC1: TL072
1 - 7.7
2 - 2.6
3 - 4.8
4 - 0
5 - 4.8
6 - 4.8
7 - 4.8
8 - 8.3

IC2: TL072
1 - 4.8
2 - 4.8
3 - 4.8
4 - 0
5 - 4.8
6 - 4.5
7 - 5.4
8 - 8.3

Symptoms:
When i used the audio probe, I have sound at pin 2 of IC1, but it is not coming out of pin 1. I say that it is sort of working because there is a hum generated somewhere in the pedal that goes through the filter sweep when I move the pedal and listen at the output. I know that this has to be a biasing issue, but I already fixed one problem with the bias, and the voltage seems to be about right compared with some others I found. Then again, I may not be converting positive ground to negative ground very well. THis is the negative ground version, and I built it with all the connections specified in the layout. There is one difference from layout to schematic in that C6 is either in parallel with R13 or R3 depending on which version. I tried it in both places, bit it didn't help.

Also, the battery went from 9.03 at the beginning to 9.00 by the time I was done checking voltages. Seems a bit fast. I did most of the debug with a homemade power supply, but I know how you guys like your batteries.

Let me know if you have any ideas. I don't think I can work on it any more today.

Thanks!

[PRR]

> IC1: TL072
> 1 - 7.7
> 2 - 2.6
> 3 - 4.8

Lift C1 and/or R2 (input cap and resistor). These voltages suggest ~~300K leakage to ground here. Find it.

This error leaves U1A slammed, unable to pass signal; yet the rest of the plan still filters whatever crap is sneaking in.

[Radamus]

I'm working on finding the problem there. From what I can tell, the input cap and the first resistor after are very cleanly soldered and not touching anything else. I know that's no excuse, so I'll pull them out and try again anyway.

By leakage to ground, do you mean positive or negative ground? Come to think of it, I didn't check the +V. I'll check that. What is it specifically about those values makes you think that this is the problem? I read that chapter of the art of electronics, but I think I'd need a few more reads to get everything.

Thanks for the help. Back to work!

[Radamus]

Okay, so I tinkered about for a bit. There is continuity between the 100k resistor coming out of the output of IC1A and the bias voltage, but only when there's power and the chips in. Not sure if that's significant.

With the input resistor and capacitor out of the circuit, pin 1 is still 7.6v, and pin 2 is 1.3. Pin 3 is 4.8.

Hope I narrowed it down a little more. What do you think?

[PRR]

A happy opamp has both inputs at the SAME voltage, and all inputs and output somewhere "between" the supply rails (often about half-way, with exceptions).

Your output is 7.7V which is "stuck" against the 8.3V supply (there's about 0.6V loss in the chip). It's like trying to dance while leaning on the wall... not so good.

The "-" input pin 2 has (should have) no DC path except R1 650K from output. So it too might be up at 7.7V. But it isn't. So something is sucking on it.

[Radamus]

Okay. Let me make sure I understand what you're saying:

Both inputs should be at around the same voltage, so pin 2 and pin 3 should be the same. That means that when I get pin 2 to about 4.8v like everything else, it's probably working again. The output voltage is too high, leaving the signal no room to move. It needs the distance between the rails to fluctuate (being that it's a wave), and it doesn't have that now. This is caused by the reduced voltage on pin 2, so the debugging needs to start there. Or are there two problems, one being the increased voltage on pin 1 and the low voltage on pin 2? Do these have separate causes, or are they probably related?

I pulled both the input cap and the input resistor out of the circuit, but the voltages were still wrong, so maybe my soldering on the socket wasn't clean enough. I'll chekc that later when I get a chance.

Thanks for your patience, PRR. I think I've had this problem a couple times before, but I usually just find where I accidentally connected something, then it works, then I stop thinking about it. So it's good that I'm learning a bit more about it this time.

I'll let you know how it goes.

[Radamus]

I got it working. I'm a bassist, and I replace the input cap so often that I don't think of mentioning it. I did replace all of the first three parts, but I think that the problem was most likely that I had the 10uf cap pointed the wrong way. With the positive side to the pedal in, it works. Now I'm having some issues with it in the case, but I think it's probably a loose connection or some accidental grounding. It was one of those things where I had it working, but the wah was backwards, and then I turned it around and something went wrong. I think I'll probably get it with a little more tinkering.

Thanks for the help.