Help with DOD250-type OD

[Wadey86]

to just put one LED in each slot and see if I get anything there.

The circuit should pass signal with no diodes loaded at all.
You might not want to pull the wires out as they might be hard to get back in.
You could just desolder the wires from the switches but just desoldering the centre pin on the switch will pretty much remove everything to do with the diodes.

Thanks for thta, should save me some time. I'll do that, but as noted there's a centre position on the switch that should effectively do the same thing...

[Wadey86]

Thought - should the 9v power and the signal ground be separate things? I'm having trouble visualising how the circuit would get to the guitar that way... Then again I'm new at this.

[Rob Strand]

Thought - should the 9v power and the signal ground be separate things? I'm having trouble visualising how the circuit would get to the guitar that way... Then again I'm new at this.

The -ve side of the 9V supply and signal ground must be the same.  Normally the battery -ve connects to the signal ground via the input socket.

[Wadey86]

OK thanks for clarifying that

[duck_arse]

I have a measuring request - please measure the resistance between the pad marked "in" and the one marked "gnd". if this reads low, less than say 1k, snip the link across the C1 position, and remeasure the resistance.

[Wadey86]

Ok so I swapped the batteries around, getting about 6.5 from the 9v powering the board now, and IC voltages of
1.8
1.5
1.2
0
1.7
1.8
1.8
3.5
Seems more realistic!

Desoldered the switches, no change.

Duck - measured at about 0.9 ohm, so I cut it, and it now measures infinite... Was that expected?

Good news is now I am getting signal! My 6 month old is sleeping in the next room, so i can't test at volume yet...
I'll repost when I have.

[Rob Strand]

Duck - measured at about 0.9 ohm, so I cut it, and it now measures infinite... Was that expected?
Good news is now I

Well spotted Duck!
It means the C1 cap (which is not on the schematic) appears across the IN and GND terminals.
When you loaded it with a shorting link it shorts the input signal.
It matches all your symptoms

And now you have signal - problem solved.

At 3.5V your battery is kaput.

[Wadey86]

Beautiful! Thanks so much for all your help. You guys are amazing.

One last question, more on the advice side, half the idea of this pedal for me was to experiment with the clipping diodes. The LEDs dont seem to be lighting up, even with a good battery, so I assume they're not doing much (if any) clipping... Is there such a thing as a socket (like for the IC) but for diodes so I can mix and match to find a good combination?

Thanks again for all your help!

[Rob Strand]

One last question, more on the advice side, half the idea of this pedal for me was to experiment with the clipping diodes. The LEDs dont seem to be lighting up, even with a good battery, so I assume they're not doing much (if any) clipping... Is there such a thing as a socket (like for the IC) but for diodes so I can mix and match to find a good combination?

The best thing to do is check them out in a pitch black area - like a dark room or under a rug.
Try two diodes/LED, one pointing in direction and one in the other.
If they are Blue or White LEDs they might not light.  Reds should light.

There's a few ways to do it.  Some people just break off pins from an IC socket and poke them in - OK for experiment but not robust enough to put inside a pedal.   Another way is to get a multipole switch, or even a small dip switch.

[PRR]

> the clipping diodes. The LEDs dont seem to be lighting up

Clipping diodes are not about light. We are not feeding "big current" to make light. We are clamping an *audio* signal, and there is no need to make a BIG audio signal for that.

Look at the plan. The one top-thread does not show a "light" LED, but we would normally feed full 9V to a 1K resistor to the LED, steady, to get light. Now look at the clipping LEDs. They come from an audio op-amp which is centered in the 9V supply so at-most 4.5V swing each way. Actually 3V due to chip losses. Already a-third of the "light" LED. Then instead of a 1K resistor we have a 10K resistor! So 1/30th the current. Even if you clip 100%, which is not possible on a plucked string at any sane gain. I suspect HEAVY playing in a DARK room may show a 1%-2% glow. You won't see this in the light. You won't see even this much with "normal" strumming.

Does it fuzz? Does it fuzz different for different diodes, '4148 and red and blue? Then it does what it is supposed to do.

[Wadey86]

Thanks, PRR.

I've seen some YouTube vids of LEDs glowing as they clip, but it's a different circuit so like I said it was an experiment. I'll try a bit more with switching between LEDs and if that doesn't yield results then I'll try sone different combinations of silicone/germanium diodes till I get what I'm looking for.

Thanks again for all your help all!

[ElectricDruid]

+1 agree with PRR. LEDs for clipping don't light up, in general. That's normal.

[duck_arse]

Wadey86 - it behooves you now to contact Paul at http://www.diyguitarpedals.com.au, [chromesphere on this very forum] and inform him of the errors and omissions in his build docs.

and well fixxed.

[Wadey86]

Duck - I was thinking just that. Good suggestion.

[Rob Strand]

LEDs for clipping don't light up, in general

I guess there's a difference between seeing it light up full brightness in a lit room and seeing it in the dark.
I used to do silly experiments on the how low the current needs to be before you can't see it in a totally dark room (or under the blankets!).    It's pretty darn low.

[chromesphere]

Hi guys, thanks for notifying about the error in the build doc. 

Build doc should say "empty" not "link" of course as you have worked out that component goes to ground.  Surprised this hasn't been picked up earlier....by me   Sorry for the error!

Please let me know if you find any errors like this I pride in the accuracy of my build docs.

Build doc updated.

*Edit: I just noticed the discussion, sorry I should address this:

The 1M feedback resistor and 500K gain pot + 4.7K resistor to ground are faithful to the classic DOD 250 circuit https://sites.google.com/site/snmavronis/NeoClassicFX/741overdrive/research.  That used a single 741 op-amp - buffering the bias is an addition.

Fair enough. I used to design distortion pedals with an eye to getting current drawn below 3mA. I was fifteen. TL062 or LF444 were my favorite chips and a 1M gain pot wasn't something to be afraid of.

Moving it to a dual op-amp and then not using the other half does seem a bit odd though, doesn't it?! I mean, it's not like the TL061/71/81 wouldn't have done the job.

T.

The error was not tying down the spare op amp correctly.  Trying to cater for the dist+, dod250 and Ross I was confronted with the choice of single or dual, I went with dual, in hindsight probably should have gone with single or just left off the ross! 
*edit

Cheers
Paul