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

 

[jaimebcn]

Hello everyone!
Jaime here from Spain!
I've come here because I have a CarbonCopy (REV-F) that suddenly stopped working.

I mostly used it at home and occasionally at quiet gigs, but only sporadically.Yesterday, when I tried to use it again, I realized it wasn't working. The problem is: Even with the MIX potentiometer at 100% (and REGEN and DELAY around half, although it doesn't really matter)... I only get one repeat at a very low volume.

I tried adjusting the "Delay sound level" and "Delay time offset" internal trimmers, but nothing changed.

I'm working with a signal of approximately 1kHz and 300mVpp. I have measuring equipment (oscilloscope, multimeters, etc.) and a fine-tipped soldering iron; I can solder and desolder SMD components fairly easily.

I have another CarbonCopy (REV-C) for comparison, and its circuit board is practically identical. Most of the measurement points are the same, and in fact, I get the same readings on both (not 100% the same, but they can be considered equal due to component tolerances.). The REV-F board, the one that isn't working, has some tiny extra integrated circuits (Ux) that I assume are regulators since they're located on signal traces. But I can't read the part numbers, even with a magnifying glass, because they're so small, so it's just a guess as to the type of component.

I'm aware of the FET problem, but in this case, they seem to be fine, since I'm comparing the measurements with the Carbon Copy REV-C, and the measurements are exactly the same in both. I replaced C41 because I read that it could be the cause, and it turned out to be faulty. But I haven't fixed anything.

HOWEVER, right at the analog input of the V3208's integrated circuits (replacements for the obsolete MN3208) on the REV-F, I get a signal that's half as wide as the signal I get on the REV-C.Without the schematic, it's practically impossible to trace the signal path.

It's worth mentioning that I've checked the V3208 datasheets and the bias voltages are correct (Vdd and Vgg).

May thanks and thank you!
J.

 

[Juan Diego Enriquez Mendo]

Hello everyone!
Jaime here from Spain!
I've come here because I have a CarbonCopy (REV-F) that suddenly stopped working.

I mostly used it at home and occasionally at quiet gigs, but only sporadically.Yesterday, when I tried to use it again, I realized it wasn't working. The problem is: Even with the MIX potentiometer at 100% (and REGEN and DELAY around half, although it doesn't really matter)... I only get one repeat at a very low volume.

I tried adjusting the "Delay sound level" and "Delay time offset" internal trimmers, but nothing changed.

I'm working with a signal of approximately 1kHz and 300mVpp. I have measuring equipment (oscilloscope, multimeters, etc.) and a fine-tipped soldering iron; I can solder and desolder SMD components fairly easily.

I have another CarbonCopy (REV-C) for comparison, and its circuit board is practically identical. Most of the measurement points are the same, and in fact, I get the same readings on both (not 100% the same, but they can be considered equal due to component tolerances.). The REV-F board, the one that isn't working, has some tiny extra integrated circuits (Ux) that I assume are regulators since they're located on signal traces. But I can't read the part numbers, even with a magnifying glass, because they're so small, so it's just a guess as to the type of component.

I'm aware of the FET problem, but in this case, they seem to be fine, since I'm comparing the measurements with the Carbon Copy REV-C, and the measurements are exactly the same in both. I replaced C41 because I read that it could be the cause, and it turned out to be faulty. But I haven't fixed anything.

HOWEVER, right at the analog input of the V3208's integrated circuits (replacements for the obsolete MN3208) on the REV-F, I get a signal that's half as wide as the signal I get on the REV-C.Without the schematic, it's practically impossible to trace the signal path.

It's worth mentioning that I've checked the V3208 datasheets and the bias voltages are correct (Vdd and Vgg).

May thanks and thank you!
J.

Hi Jaime, were you able to fix your problem by replacing the FET transistors Q10-Q13? The problem I'm having with my pedal is that the sound comes through normally in bypass mode, but when I activate the effects, it doesn't work.

 

[jaimebcn]

Hi Jaime, were you able to fix your problem by replacing the FET transistors Q10-Q13? The problem I'm having with my pedal is that the sound comes through normally in bypass mode, but when I activate the effects, it doesn't work.

Hi Juan Diego,

Thank you for your reply.

In my case (Carbon Copy REV-F board), when the pedal is engaged, I can hear both the delay effect and the clean guitar signal. However, my problem is that the delay effect I hear is very, very quiet, and I can only hear two repeats (at a very low volume). I can only hear those two repeats when the MIX potentiometer is turned all the way up.

I haven’t replaced any FETs yet; I have another Carbon Copy (REV-C board) and I’ve been comparing signals and voltages all over the boards, and they’re identical on both. That’s what’s driving me mad.

Cheers.