Need troubleshooting help - MXR Classic Distortion

[ronh95]

I've enjoyed this forum over the years for DIY builds, but I have a different sort of problem that I hope someone will be willing to help with. The guitarist in my son's band handed me an MXR Classic Distortion pedal and asked if I could fix it. It is in mint condition and I agreed to take a look.

What I found inside was the stomp switch itself was completely broken in two, the threaded barrel separated from the base entirely. Great! I easily found the exact Alpha DPDT switch at Small Bear and placed my order. Well it came yesterday and I soldered it in, put the box back together and.... nothing! I was running it on my One Spot power supply and absolutely no sound came through... at all. The LED was not lit, so I stomped the switch. The LED still did NOT come on, but now at least clean guitar sound was passed to my amp. It sounded like pass through sound. I tried it with a 9V battery and had the same result.

The pedal is bone stock and mint, not abused. There is nothing amiss that I can see, but clearly something isn't right. I've checked the orientation of the switch and it's in the same way as the original was. With a DIY build, the troubleshooting process involves checking 'correctness', but hear it's got to be component(s) that are not working. Can someone give me some info to get started here? I surely appreciate any insight you might share.

Many thanks...

[darron]

I suppose it seems obvious, but I've you checked with a meter to see of there's correct voltage going to the board when it's all plugged in? Measure the voltage relative to the enclosure and you shoul get the +9VDC.

Doesn't sound like it would be the switch does it. Especially with the bypass working.


Failing that we really just don't have any information to help debug with except "it doesn't work".

Maybe someone fried it previously with an incorrect power supply and it otherwise all looks perfect.



I suppose check the voltage suppy to the board first and if that doesn't work I reckon people would like to see a photo of the wiring to double check it all.


Might be something stupid and easy I hope but they can be the hard ones to find

[aron]

Maybe there's a protection diode that is dead now?? Try and trace the power from the power input.

[ronh95]

Thanks for the responses. I know there's not a lot to go on, but they got me to thinking. I did notice a dark spot on a trace leading to a diode at the edge of the board on one of the traces from the 9V and external power supply jacks. I haven't looked to see if this is one of those circuits with the less common power input (positive ground?). The traces are under a transluscent top layer so it's not that plain to see. You're suggestions have led me to a place to start, that is, from the power supply in and see if the trace in question has continuity (i.e. not burned through) and then is the circuit getting any voltage.

Thanks again for your thoughts. Stay tuned...  = )

[R O Tiree]

If, indeed, someone managed to connect reverse polarity to it, that diode (1N400X, probably) might well have failed. If it failed "open circuit" then you will probably have twin problems of a fried opamp and fried electrolytic caps. Even if you replace the opamp and it works, I'd replace all the electro caps on spec... they may be working now, but they'll almost certainly fail in the relatively near future.

First step, if it was me, would be to de-solder that diode and test it. If it's OK, then we have to think again. If it's knackered, try the pedal (you might have got lucky). If there's still no joy, replace the opamp and caps as above.

[petemoore]

  Using a signal source injector [guitar or other], and audio probe, and a DMM settings for continuity and voltage...
   All kinds of tests can be set up to follow:
  A sourced Signal through it's path to listen for signal loss, signal amplification or partial loss [requires audio probe/source and usually power supply].
  DC supply lines to all points marked as connected to the +, then - poles of PS or battery [ and that they don't go anywhere else]. This can be done with continuity tester alone or by measuring that the supply voltage appears at the end of all supply lines.
  ...checking for operational bias voltage around any chips is always a good idea, and makes a good 'next thing' to do if there's signal loss at that stage.
  Quite a number of tests and measurements can be made with DMM black grounded, so it's nice to have an alligator clip on the black lead to let your other hand free.
  Run continuity tests through the switch in both switch modes, test every switched wire routing at the most distant points to which they connect.
  Some 'odd' resulsts may have as yet unknown explanation.

[aron]

> then you will probably have twin problems of a fried opamp and fried electrolytic caps.

Is it really necessary to change the electrolytic capacitors once the pedal works?

[R O Tiree]

If they have been subjected to reverse polarity, then they will have been damaged internally to some extent. Obviously, this depends heavily on the voltage level, temperature reached and time applied and those are all completely unknown quantities but, since he's doing this for someone else, it would be a shame for the electros to fail... let's say "early"... when, for a few cents/pennies each, they could be replaced and therefore relied upon. Resetting the clock, I suppose, given that they are "lifed" items.

If it was my pedal or I was doing it for money, or even if I was not being paid but there's no guarantee that the chap won't move away from my town, then I'd do it. They're cheap as chips, so why not?