OCD V3 No distortion

[Guzz]

I've been pulling my hair out for the last two days about this pedal.

Everything works, except there is no gain at all. The tone knobs does its thing (brightening and darkening the sound), so does the volume. But no matter what I try, there is no distortion, at all. Just the clean signal (the effect in on, of course). Even the SPST switch works fine.

Oh, one thing I didnt mention. I've actually built this circuit twice, with different components, and I have the EXACT same issue with both.

So far, here's what I've tried:

- 3 different pots for Gain
- Hacksaw everywhere
- Replacing the 2k2 and 18k resistor, and the 68nF cap aswell as the 220 pF cap at pin 1 and 2
- 2 sets of 2n7000, and a pair of BS170 with the correct orientation
- 3 different TL082, checked the orientation too
- Triple checked placement of components and their orientation and the cuts
- Checked the value of all the components

Here are my voltages for the IC:
1: 4.5v    8: 9v
2: 4.78v  7: 4.5v
3: 2.77v  6: 4.5v
4: 0v       5:4.5v

I'm running out of idea.

Pictures (with the BS170 instead of the 2n7000, for the orientation)

http://imgur.com/MJxhzJk
http://imgur.com/dtBCK3U

Thanks !

[Marbles]

From your picture of th bottom, it looks like in the bottom left there is a cut? Or does it just look like that?

I see in all the things you checked no mention of the other wiring. Are the pots grounded like mentioned in the notes? And the switch connected to volume etc?

[Guzz]

Yeah, there is no cut in the bottom left, it's just a weird reflection.

Everything is grounded and wired properly, including the switch. I've just re-checked everything for the fourth or fifth time.

Thanks for your reply !

[Ciaran Haslett]

Hi Guzz

Pin 3 isn't happy.  Have a look round there for any bridges/wrong valued resistors etc...especially the 470K to vref.

Take out the IC and measure the socket pins to ensure no shorts  (it happens)

The pots may be fine what what about the wire?  Continuity test from gain pot lug 1 to bottom of 18K.  do the same for lug 2-3 to IC pin 2.

Then the general stuff...reflow, clean traces etc.

[Guzz]

I've tried 3 different IC, but the one thing i couldn't bring myself to do is resolder the IC socket. Could that be it ? Something wrong with the socket?

I've check every resistor value, but I'll try another 470k just to be sure.

Oh, one thing I didnt mention. I've actually built this circuit twice, with different components, and I have the EXACT same issue with both.

[Guzz]

Ok, something is definitely funky with that 470k Resistor. I get 4.5v from S of Q1 going into it, but when I measure the other end of the 470k going into pin 3 of the IC i get 2.27v. Is that normal ? I've replace that resistor twice now.

[Ciaran Haslett]

Be methodical in your testing and always troubleshoot with a schematic in front of you.

1. Visual examination
Compare the layout with your build, looking for misplaced components, wrong values, missing cuts/links etc

2. Continuity test
Set a multimeter to Continuity mode (or resistor mode if it doesn't do continuity) and probe each connection by following the schematic.  Place the probes on the component legs, not the solder.  This will help find any cold joints.  This also includes any wiring

3. Variable components test
Test all your pots with a meter.  Set to resistance mode, place 1 probe on lug 1 and the other on lug 2.  No sweep the pot while checking the meter shows a rising resistance the full way up.  The the end of the sweep the meter should show the pots full value (within 20%).  Best to do this when the pot isn't connected to the circuit.

4. Voltage test
Measure DC at all the important places.  The IC, power, vref rail etc.

5. Audio Probe
Last resort before throwing the thing out and excepting failure.  Follow the schematic and probe for your guitar signal.  If its on one side of a component in the signal path but not on the other you either have a bad component for some short nearby.  But if you've completed the last 4 steps then the probability is high you've a bad component.

As for the IC socket...I have had a short in one before.  A small sliver of copper from the strip board found its way under a socket and it took forever to find as I was probing all over the place.  But I follow the above now if I ever have a problem and I haven't been defeated by a build in a long time. (jinxed myself now)

SO...try doing all of the above and if still no joy post some hi res pics of the COMPLETE build (top, bottom, jacks, switches pots etc.  Look hard at the pics before you post though...if you can't see what's going where in the pics...we can't see it either.

As for voltages  you should see 9V on pin 8, 0V on pin 4 and around 4.5V on the rest.  If any of these are different take a good look at what's connected to them.

Sorry if some of this sounds condescending.  I've no idea what your skill level is.

Post back however you get on and let us know.

Good luck.

[Ciaran Haslett]

Sorry...missed your last post as I was posting too.

Yeah there's something going on round there.  But you've proved vref (4.5V) is good so you've limited the search area.

Try the continuity test I mentioned above on the IC sockets and any components connected to pin 3.  Failing that change the 470K

[Guzz]

I removed the socket and soldered the IC straight to the board, same issue. I'm still getting only 2.27V to pin 3 of the IC.

I'm fairly new to pedal building, so correct me if I'm wrong, but my problem should either be from the 22n cap or the 10k resistor that are right after the input, right?

[Guzz]

Ok, I'm getting somewhere. I'm fairly sure I only have 2.77v out of the 22n cap. I say fairly sure because my multimeter is finicky around that part.

[Ciaran Haslett]

If your cuts are good then yes, just those 2 components.

Check for any DC on your input.  If that 22n cap is bad it won't block DC from going to your guitar...screwing up pin bias

[Guzz]

No go. I replaced both the 22nf cap and the 10k resistor and I still get 2.77v from input to pin3, but now I also get 5.5v for pin 2.

[Ciaran Haslett]

Did you do the checks I mentioned above?

As a last resort post some hi res pics of everything...good lighting, in focus and good angles.  While you wait for us to get a look build an audio probe and get probing.  Great little tool!

...and I'm sorry to say but we all have a nightmare build from time to time.  Sometimes the time wasted debugging just isn't worth it.

[Guzz]

Thanks for your help, i appreciate it a lot, but i've put this one on hold. What's bugging me is that i have the exact same issue on 2 different build of this same layout, and i seemed to have made the same mistake twice but can't find it. Oh Well, third time might be the charm.

[Ciaran Haslett]

Yeah good idea.  Get a successful build under your belt, restore the confidence then revisit this with fresh eyes.

All the best

[NickT]

I built this and have EXACTLY the same issue - almost no distortion even with gain on max, and voltage at pin 3 2.79v (other pins normal).  Voltage at junction of 470K resistor and Q1 was 4.43, dropping to 2.79v at the other end of the 470K resistor. I tried replacing this resistor, checked everywhere for bridges and shorts, ripped out the IC socket and checked underneath that the jumper wasn't touching a leg.  But still no dice.


I've put this one on hold for now, thinking to try again at a later date with a different layout.  Would be curious to know though if you ever did find out whet the problem was.