# MXR Stereo Chorus 18V DC Supply Issue??



## hollowbody (Jan 15, 2008)

I just picked this up in a trade and the pedal works fine, I can't find anything wrong with it, but I'm experiencing a strange issue. I tried the pedal with an 18v wall wart adapter and no problems at all. Then I put it on my board and fed it 18v from my PP2 and I get strange noises from it. Sometimes it a harsh, continuous white-noise sound, other times it's a digital artifacts that change pitch in time with the clock on the pedal (I'm guessing). Unplugging the 18v adapter and re-plugging it in solves the issue, but sometimes it takes a couple tries. Also, when the pedal is acting up, it affects the tone in bypass mode as well, making the signal sound harsh (for lack of a better word). 

Polarity shouldn't be an issue, because I'm also feeding 18v to my OCD, Timmy and MC401 from the same tap. Amperage shouldn't be an issue either, since all pedals together have less draw than the 150ma the PP2 supplies and I've also experienced the same issue feeding 18v to the Stereo Chorus along without anything else daisy-chained to it. 

I have a bunch of gigs coming up and I'd love to get this solved before then, so any help is hugely appreciated. What am I missing???


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## mhammer (Nov 30, 2007)

Well, first, thanks for the very thorough bit of troubleshooting until now, and complete detailing of it. Helps a guy like me from wasting everyone's time on pointless suggestions.

The fact that the problem takes some time to occur and the pedal can be "reset" by unpugging and replugging, suggests that the source of the problem either relates to something that is heat sensitive, or to one or more bad electrolytic caps.

However, the fact that the problem does NOT occur when using a source other than the PP2 suggests that whatever bad caps or heat-sensitive things there might be are to be found in the PP2 itself. 

There is a very good chance that the total current draw of the Timmy, OCD, and MC401 is still less than the current draw of the Stereo Chorus alone, such that any problem in the PP2 on the 18v supply line may not reveal itself, even with those 3 pedals in use. I'll note in passing that those individual PP2 outputs have a 150ma limit because the regulators used to down-regulate the "master voltage" are only rated to withstand passing that amperage before going "poof!". National Semiconductor regulators can manage 100ma (again, a commonly found spec on individual power brick outputs), but Toshiba makes a slightly larger 3-pin regulator that stays healthy up to 150ma.

Although it is obviously an inconvenience, not to mention a source of frustration to spend big bucks on the PP2 for the convenience of one monolithic power source, but trouble-shooting and repair of the PP2 is likely to be above both our capacities (I gather the PP2 uses a switching supply as the core), and pricier than using the 2nd wallwart. My advice is to stick with the inconvenience if it does the trick.

I would also suggest, as an experiment, running the Chorus and the PP2 with the PP2 open to dissipate heat better, just to see if that makes a difference in how long it takes for the problem to occur.


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## hollowbody (Jan 15, 2008)

Thanks, Mark! I actually have a 2nd PP2 on hand that I'm going to try the board with soon. I'm gonna also see if I can't actually duplicate the issue with the wall-wart adapter - just because it hadn't happened yet doesn't necessarily mean it won't happen. I'll get to the bottom of this yet!


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## hollowbody (Jan 15, 2008)

I've actually just confirmed that I get the same issues with the 18v wallwart adapter. I guess the first couple times I plugged in the pedal, it just didn't happen to display the issues. Again, they are intermittent, but occur at least 50% of the time.

Regarding heat, etc. I'm not sure if that's the case, because when the pedal is plugged in, upon engaging the effect with the footswitch, if it doesn't display any issues it won't until the power supply is unplugged and plugged in again. If it DOES display issues, it will continue to do so no matter how many times the effect is turned on/off with the switch. Very strange.


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## hollowbody (Jan 15, 2008)

Gah, it's the pedal for sure. I just bought some 9v batteries and tried it with batteries and experienced the same issues. I guess I go hosed in this deal. I tried the pedal at the dude's house and it worked perfectly. Boo-urns.


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## mhammer (Nov 30, 2007)

Well, at least we've determined it's the pedal. Again, if it is something that changes with time, thenit is typically something that is heat sensitive, or one or more electrolytic caps. Since it is hard to imagine that there is anything either generating that much heat inside the pedal, that directs our attention to electros.

Happily, those are cheap and easily found.

In the meantime, I have a schematic for the several different issues of the 3-knob unit. In the interests of trying to suss this out, long distance, is yours 3 or 5-knob? What delay chip/s does it use?


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## keto (May 23, 2006)

Huh, isn't that wierd....my buddy stepped on his same pedal at our jam today and got....exactly the same issue. First time it's happened. Will follow this thread, I hope you find a solution I can pass on!


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## hollowbody (Jan 15, 2008)

It's the 3207 chip, a CD3207GP to be exact - and boy is this thing annoying to take apart! Mine is the 5-knob version. Here's a couple gut-shots.

None of the electrolytics seem to be obviously bad.


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## mhammer (Nov 30, 2007)

Many thanks for the pics. Unable to find any on-line.

1) What is the oversized flat 16-pin chip under the Treble control? (_EDIT: Saw a gutshot of a different Dunlop-produced pedal, and realized that the chip is a 571 or 570 compander chip.)_

2) I see a bunch of tantalum caps around the board (the amber drops). Any possibility that one of them is fractured at the base and only *look*s like it makes contact?


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## hollowbody (Jan 15, 2008)

It's an SA572D.

All the tantalum caps seem to be making solid contact under a bright light. Nothing looks burnt out or out of whack. I'll see if I can borrow a lens to take closer pics with.


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## mhammer (Nov 30, 2007)

Okay. 572 is a _little_ different than a 570/571, but from the same family and does essentially the same thing.

I started looking for some info on what might cause failure of tantalums, since I sort of remembered some vague echo of reverse voltages or something like that, but I couldn't find anything clear. Not that I looked THAT hard. I just didn't find any sort of in-your-face warnings about what not to do with tants.

So, as much as I wish I could help more at this point, I'm stumped. My sense is that it needs to be heard, tested at a variety of relevant points, and possibly scoped.

Sorry.

Is there any more diagnostic info you might provide, such as what control settings it gets better and worse with? Or even how long it takes, post plug-in, for the problems to emerge.


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## mhammer (Nov 30, 2007)

A note from RG Keen, in response to another thread in another forum: "_One could debate that for a while, but electro caps are damaged by reverse current flow, and while they do withstand some minor reversal, it's small - a volt or so depending on the manufacturing. So a reversed battery would not make the caps explode, but it could well make them leaky and then unable to support their normal voltage. Depends on how much current flows when_. "

So, the ugly question is whether at some point an adaptor with reverse polarity was applied to the pedal. If so, you might have one or more suspect electros.


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## hollowbody (Jan 15, 2008)

mhammer said:


> A note from RG Keen, in response to another thread in another forum: "_One could debate that for a while, but electro caps are damaged by reverse current flow, and while they do withstand some minor reversal, it's small - a volt or so depending on the manufacturing. So a reversed battery would not make the caps explode, but it could well make them leaky and then unable to support their normal voltage. Depends on how much current flows when_. "
> 
> So, the ugly question is whether at some point an adaptor with reverse polarity was applied to the pedal. If so, you might have one or more suspect electros.


I can't really speak to that with any real sense of certainty as I just picked it up used from someone else. I've certainly never fed it anything other than 18v dc centre negative, but who knows what's been fed into it more than 2 weeks ago


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## mhammer (Nov 30, 2007)

Well, the good news is that a trip to any of the decent electronics places in your city, armed with a few dollars and a list of the electros in the pedal, plus a few hours of unsoldering and soldering, would completely eliminate any concerns about "risky" caps. Heck, you'd likely spend more on gas or parking than on the caps themselves.


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## hollowbody (Jan 15, 2008)

mhammer said:


> Well, the good news is that a trip to any of the decent electronics places in your city, armed with a few dollars and a list of the electros in the pedal, plus a few hours of unsoldering and soldering, would completely eliminate any concerns about "risky" caps. Heck, you'd likely spend more on gas or parking than on the caps themselves.


Yup, once things settle down and I have some free time, looks like that's what I'll be doing.


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## hollowbody (Jan 15, 2008)

For the sake of closure, just an FYI - I ended up sending the pedal to MXR and they replaced the entire PCA. I only paid for shipping to MXR/Dunlop. They covered return shipping and didn't ask for a dime for the repair, which is outstanding, IMO! They may have felt bad for me, because the post had lost my package for a little while. I know they had mentioned a flat repair rate when I initially contacted them, but I was never charged anything, so it may have been an oversight, or an act of generosity. Either way, I'm thoroughly pleased.


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## mhammer (Nov 30, 2007)

That's a VERY happy ending to the saga. Stands in bold contrast to the recent Fulltone thread here, doesn't it?


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