Big Muff problems

[greg]

Hello

I have a 70's Big Muff that was dropped and has lost its good sound. I get sound, but the symptom is this:

When rotating the Tone pot, the output goes from very low (volume) at one end of the rotation to very loud and crappy sounding as it moves toward the "bright" end of the pot's sweep.

I have checked all of the solder joints (resoldered them all - man, they did some crappy work) and swapped out the tone pot itself. I also recall that I replaced the .01 cap going to ground off the tone pot.

Unfortunately, it has been years since I heard this thing, so I can't really say with certainly whether or not the sound that I am getting out of it with the tone knob up into the "bright" region is as it should be. It sounds pretty bad, so I have a feeling that something other than the tone pot and its neighboring parts are the culprit.

Any ideas?

Thanks,
Greg

[Mark Hammer]

Possible the drop fractured a copper trace on the board?

[greg]

It is certainly possible, but being that the traces are so wide, I've gone over all of them with solder just for grins.

I do have a replacement board purchased through Gen.Gtr. Gadgets and a schematic, but I'd really like to fix the existing board and keep it original, if possible.

I suppose what I am looking for is a tip on what "could" be causing this symptom of missing output volume when the tone pot is being turned toward it's bassy side. I simply don't have enough electronics knowledge to understand what function the components perform.

I suppose it would be prudent to replace all of the components which I don't know how to check (namely, the caps) and to track down a schematic which shows the voltages, but I simply have not had time to do so.

Thanks,
Greg

[frank.clarke]

You could sell it on EBay and get good money, I see non-working fuzzboxes going for over $200 often enough. That would probably be better than replacing all or any of the parts on a collectable. Mark could probably get it working in 10 mins in person, but if you don't have the debugging skills you might want to sell it to someone who does.

Otherwise:

It would be worth disconnecting the tone control middle lug and attaching an alligator clip to said lug. Then connect the other end to the input, or one of the legs of the last transistor just to see if the output transistor is working. Check out "Audio probe" here: http://www.diystompboxes.com/pedals/debug.html

Desoldering and replacing parts optimistically could go either way, so if you can narrow it down a bit that would help.

[Mark Hammer]

One of the other things that can happen when you drop something is that dirt and dust gets loosened and finds itself between the wiper and the resistive strip inside the pot. Sometimes, though not always, it is a good idea to remove a pot from the pedal, take the back off (by prying the tabs....GENTLY) and cleaning the resistive strip with a Q-tip. Personally, I like to apply a substance called Stabilant to pots (www.stabilant.com) to enhance contact quality, but this can sometimes be hard to find. Certainly making sure that all gunk is removed from the resistive strip and contacts is a big step in the right direction, even if you can't score fancy-schmancy chemicals.

Other things that can happen to pots as a consequence of jarring is that the rivets holding the resistive strip in place can be loosened a bit, or the bump to the pot shaft knocks the wiper contact off-axis such that it makes much better "springier" contact with the resistive strip at one extreme of rotation and poorer contact with thte strip at the other extreme. Occasionally you can re-tension the spring-contact that provides the physical "wiper" so that it touches the resistive strip in a more positive fashion. For the rivets, I find that a gentle pinch with needle nose pliers on the rivets can sometimes bring errant pots back to life. You will likely find that removing the back of the pot helps in getting the pliers seated atop the rivets at a better angle for pinching.

[greg]

Thanks for the suggestions - I'll give it a shot. At this point, the pot has been replaced by another. The new pot is the one displaying the symptom and it has been in place for so long, I can't recall if the original did the same thing, as I replaced the pot at the same time I touched up all of the solder and swapped out the .01 cap to ground.

Again, thanks for the tips

Greg

[52 Bill]

Just so we're all on the same page, which version of Big Muff do you have? Transistor or IC? Does your version of the pedal have a tone bypass switch? If it does, how does it sound with the bypass on?

I'd have to look up the schematic, but if I remember correctly, the tone control is a fairly simple circuit. Just a few caps and resistors, and the pot. Is the replacement pot the correct value? Was it wired in correctly? If it's been a while since you did the original repair, maybe you should now double check your work against the schematic.

In general loss of signal and more importantly, loss of low frequencies in a old pedal can often be traced to dying electrolytic caps. So if everything else is correct that's where I'd start looking.

[greg]

Yes - you are absolutely right. I need to get back and refresh myself on the situation. I actually forgot that I even had the thing until I was looking at this site a few days ago.

Mine is not the Tone Bypass version. It is a standard, red/black 70's version as seen in the following schematic:

http://www.generalguitargadgets.com/...bmp_l70_sc.pdf

Thanks,

Greg

[David Schwab]

One problem with those old EH pedals is the circuit board is supported by the pots. The connections break if and when it's dropped.

I always unsoldered the boards from the pots on my Big Muffs and ran short wires from the pots to the board.

I bet that's what happened. Check the solder joints between the pots and the board.

[edit] See what happens when you don't read everything! I see you said you re-soldered the connections.