MXR Carbon Copy Repair

[nate_noise]

Hi Folks,
Recently picked up a broken Carbon Copy for $20. Original owner says "all of a sudden one day, the output became extremely quiet."
I get a clean signal in bypass mode, but when the pedal is engaged it's extremely quiet but I am able to hear delays (if I crank my amp way up). With Regen cranked it won't self-oscillate though. (A hint as to where in the signal the gain drop is happening?) I've tried with both battery and adapter.

I've attached a photo of the PCB and also the only source I could find online claiming to be a Carbon Copy schematic (low res, kinda hard to read).
Any tips on troubleshooting what the problem might be? (I'm still a novice pedal builder.)

Thanks!

-Nathan

 

[Guardians of the analog]

That's not the right schematic my guy. That's from the big green box analog delay. The schematic is marked 77 so that's from the 70s.

 

[jimilee]

See, what had happen was….

 

[Laundryroom David]

My only tip is to close it up, slowly set it down and then RUN.

Kidding. I’m guessing somebody played with the trimmers, but if it isn’t that, then … RUN.

 

[KR Sound]

I have a couple and even tried to get info from MXR, but no luck. I know someone played with the trim pots as they were all turned CCW. It’s been in a box for a year now.

 

[ADAOCE]

Holy Christ what’s with all the trimmers?

 

[Laundryroom David]

RUN
AWAY

 

[KR Sound]

RUN
AWAY

Seriously.

Unless you are sleeping with an MXR tech, you ain’t getting it.

 

[Fama]

If someone has a working Carbon Copy, maybe they could take a picture of the trimmers and that could work as a starting point? I assume they can't be all set up the same (obviously wouldn't need trimmers then), but with what, 12 trimmers it's going to be tough to be tough to try and set them correct otherwise.

Plus there's also the chance of something else being broken, if something broke and someone decided to mess with the trimmers to try and get that fixed, you could have a complicated issue.

 

[PJS]

I am sure it is doable (I mean, anything can be repaired). It will not be an easy task though. The first thing to do is to measure the voltages on the ICs and see if they are close to what the datasheets say they should be. If your supply voltages check out then you are going to need to trace through the audio path from the input with an audio probe and see where the signal is lost. Without a schematic that means you are going to have to trace out the circuit as you go. I suspect this is a long term project.

 

[jimilee]

My guess is, if anyone can measure their voltages at all the test points, you can get it calibrated.

 

[Betty Wont]

 

[PunchySunshine]

View attachment 52625

Wife said "she should change her name to Betty did"
nice

 

[RetiredUnit1]

I had a cat pee on mine, and I didn't notice for a while. When I opened it up little parts started falling out, the PCB was totally corroded.

Cats..... You have to love them, otherwise you would kill them..... My cat damaged electronics bill is over $1000, easy........

The cat in the picture is George, his brother Newbie is the guilty party. I could not get the smell out of my pedal board and ended up tossing that too.

 

[nate_noise]

Thanks all for your input! That diagram of the trimpots is real nice, I may begin by adjusting the ones for level/balance (and marking where I started before I do!)

I also came across this old post on another forum "I've repaired a few of these that had the (quite common) switching fault where cooked SMD FET's result in no audio when the effect is switched in, or out, or in either mode. Apparently the SMD FETs are susceptible to static electricity and can be easily destroyed while plugging and unplugging the pedal into your signal chain. If this is your problem, you can try replacing the SMD FET Q10 with a J176 or J177 FET, as per the yellow arrows on the photo. MXR issued a service bulletin a few years ago recommending that SMD FETs Q10,Q11, Q12 and Q13 be replaced with J177 FETs if the delay effect can't be turned on."

I do get ~some~ audio though, so maybe not quite the issue. Hm...

 

[PJS]

If you suspect a switching FET you can temporarily short the source and drain. If the signal goes through after that then replace it.

 

[ReinoutVrijhoef]

Not trying to be captain obvious here, but will reflowing the big solder joints help? The joints on the switch don't look too well, and there's little harm in making sure these joint are indeed connected.

 

[Laundryroom David]

Not trying to be captain obvious here, but will reflowing the big solder joints help? The joints on the switch don't look too well, and there's little harm in making sure these joint are indeed connected.

Which joints do you think need reflowing?

 

[ReinoutVrijhoef]

I would start with the larger solder joints. All that are easily accessible. Especially these look quirky in the photo.

 

[Inducted]

Thanks all for your input! That diagram of the trimpots is real nice, I may begin by adjusting the ones for level/balance (and marking where I started before I do!)

I also came across this old post on another forum "I've repaired a few of these that had the (quite common) switching fault where cooked SMD FET's result in no audio when the effect is switched in, or out, or in either mode. Apparently the SMD FETs are susceptible to static electricity and can be easily destroyed while plugging and unplugging the pedal into your signal chain. If this is your problem, you can try replacing the SMD FET Q10 with a J176 or J177 FET, as per the yellow arrows on the photo. MXR issued a service bulletin a few years ago recommending that SMD FETs Q10,Q11, Q12 and Q13 be replaced with J177 FETs if the delay effect can't be turned on."

I do get ~some~ audio though, so maybe not quite the issue. Hm...

Hey mate we wondering if you got anywhere with your fix / troubleshooting?

I picked up a damaged unit yesterday, assuming it suffered esd damage and it may have, I'm not convinced that much static can build up in the claimed way but....that's for another time.

Long story short it looks like mine came with suppression across q11, 12, 13 just not q10.

I get audio both switch positions, just around 25-50% less when engaged, no delay.

I've only had one night of visual inspection and testing (just with guitar) but my initial thoughts are my symptoms don't quite line up with everyone else's.

Anyway figured your a couple of months in the lead and was hoping you've had some luck.

P.S. Sorry for the long rant, from a newbie hijacking your thread