OD Clipping diode always on help

[suou44]

Hi all,

I've been a long time lurker, but this is my first post. I've been having some trouble with this circuit: http://revolutiondeux.blogspot.com/2012/01/mad-professor-sky-blue-overdrive.html

Whenever I turn on the effect, the indicator led lights up but the only things that happens to the sound are a volume drop and some compression, which is a far cry from the overdrive I'm expecting. I haven't made any changes to the schematic, and I've had to rewire it a few times already. The first time I turned it on, it sounded great and all the controls responded in the ways I expected. However, when I screwed in the pots and knobs, it just kind of stopped working. When I strummed really hard it made a "splatty" sound that reminded me of the very last note of "Bleak" by Opeth. I figured something was biased incorrectly, and my strumming was overcoming a threshold voltage, so I checked all of my semiconductor orientations but they were correct. I assumed I had blown one of them up, so I replaced my IC, transistors, and clipping LED's. It looked like the IC fixed my initial problem, but now it just does the quiet compression thing I described earlier. I did notice, however, that LED D1 lights up and stays on, whether the effect itself is on or off, as soon as I plug in my battery or 9V adapter. Anyways, like I said before, the thing that really bothers me is that it was] working until I screwed in my pots, which was hardly a modification at all!

If anyone has a problem with the image I linked or the brand itself, let me know, and I'd be happy to take it down/get the thread closed. I didn't see anything about this problem in previous threads, so hopefully I'm not beating a dead horse. If anyone has any help, it would be much appreciated. I've also tried RG Keen's debugging suggestions, which fixed the splatty sound, but I couldn't get the "No or very little effect" link working.

Thanks,
John

[Mark Hammer]

"It worked before I boxed it up" comes up a few times a year here.  Some of the common causes are component leads shorting out against the backs of pots, or pots whose lugs are shorting out against the chassis because they twisted during nut-tightening.

I find it handy to slip a bit of heat-shrink tubing over the pot solder-lug and last little bit of wire.  It insulates and protects against such shorts, and also provides a bit of strain-relief to wire to prevent fracturing during the inevitable twisting around that tends to accompany one-off builds.

[suou44]

"It worked before I boxed it up" comes up a few times a year here.  Some of the common causes are component leads shorting out against the backs of pots, or pots whose lugs are shorting out against the chassis because they twisted during nut-tightening.

I find it handy to slip a bit of heat-shrink tubing over the pot solder-lug and last little bit of wire.  It insulates and protects against such shorts, and also provides a bit of strain-relief to wire to prevent fracturing during the inevitable twisting around that tends to accompany one-off builds.

Sweet, first reply! I'm wondering what a short would even do , since I've replaced almost all of my semi-conductor components, and I think resistors or capacitors would be pretty obvious if they were busted. I did put electrical tape over the pots, so I don't think they shorted anything, but I'll look at my passive components a little closer... Thanks for your help!

[Brisance]

your components are most likely fine, but the short is preventing it from working, remove the short and they should work. Alternatively something came loose and needs a resolder.

[samhay]

Indeed, you almost certainly have a short that is grounding the feedback loop of the first op-amp.
Take the circuit back out of the box and it will probably work properly again. The trick then is to figure out how to get it back in there without shorting, which often involves stategically placed pieces of plastic and/or cardboard.

[Brisance]

sometimes bending the leads out of way will be enough without using any plastic. A few layers of electrical tape beneath the pot leads can help as well

[suou44]

Indeed, you almost certainly have a short that is grounding the feedback loop of the first op-amp.
Take the circuit back out of the box and it will probably work properly again. The trick then is to figure out how to get it back in there without shorting, which often involves stategically placed pieces of plastic and/or cardboard.

Hmm...I've taken the circuit out of the box a few times, but it does the same weird thing with the LED always on. The only thing still attached to the case was the input jack that served as a common ground.

[suou44]

sometimes bending the leads out of way will be enough without using any plastic. A few layers of electrical tape beneath the pot leads can help as well

Yeah, I tried wiggling leads to see if one was loose/would make a weird noise but nothing happened. I also pretty much coated the interior of the top of the case in electrical tape just to make sure, but that didn't help. I've started to ask myself stupid questions like "Should pot cases be grounded?" because I figured is was something small like that. I can't wait for someone to solve the problem by saying that the pots do need to be grounded.  

[anotherjim]

Something to the left of D1 cathode is going to ground. Either a short or... what have you used for the 1uF C4? The scheme shows a non-polarized cap. You can use an electrolytic there IF it's negative wire goes to ground.

[suou44]

Something to the left of D1 cathode is going to ground. Either a short or... what have you used for the 1uF C4? The scheme shows a non-polarized cap. You can use an electrolytic there IF it's negative wire goes to ground.

I think...I think you may have solved it. Looking back at my order history, I see a lot of Vikiin tantalum caps, which look like they're polarized. I guess I didn't even pay attention when I was populating the board; I just stuck 'em in. When I get home later I'll check, but I have a feeling that's what it is.

Now I'm puzzled over why it would even work in the first place...

I'll give an update after work. Thank you so much.

[suou44]

Something to the left of D1 cathode is going to ground. Either a short or... what have you used for the 1uF C4? The scheme shows a non-polarized cap. You can use an electrolytic there IF it's negative wire goes to ground.

Also, if you don't mind, could you explain your thought process that led to you asking about C4 as opposed to, say, C2? I'm not that great at tracing through schematics. Thanks.

[anotherjim]

1uF is often a really large cap if it's a non-polarized film cap, so some folk default to a polarized when they see a cap in the uF range without realizing the implication -  you'll note there is no + mark on C4. Like I said, polarized will work so long as it's the right way around  - negative to ground. The fault if it's wrong way around is leakage current, which will drag the circuit negative (and the diode comes on all the time and the bias is wrong on the op-amp. You can have faulty caps though that leak current even the right way around.

[suou44]

Well I just got home from work, and, sure enough, four of my caps that should be unpolarized are polarized. I feel like an idiot, but I guess it's one of those things where frustration made me blind to obvious problems. Thanks to everyone, especially anotherjim, for the help. Now I just gotta wait a week for a dollar worth of parts before I can fix it, since I don't really want to jury-rig the pedal...

Thanks again.

[PRR]

> puzzled over why it would even work in the first place...

Electrolytic caps (even Tantalum) "may" work "backward" for a short while, until the reverse voltage eats-through the factory oxide-film.

Which is annoying/confusing in this case, because the fault "seems" to have happened "when I screwed in the pots and knobs", which is a very common problem, and usually means a mechanical fault, short-to-case or busted-wire. That your cap held-on until boxing-day seem to be coincidence.