Big Muff Pi not much gain.

[stickninja]

So im just getting in to messing around with pedals. today was my first real endeavor.  A freind of mine had a Russian Big Muff Pi that had not worked in years.  He asked me to mess around with it, he has no attatchment to this pedal so it would be a good learning project.  After opening it up, ad taking a look around, I found the +9V lead, and the ground had both somehow been disconnected from the board.  Voila, fixed.  So next I decided I would try my hand at modding this little guy.  I found a cool article (http://www.premierguitar.com/Magazine/Issue/2009/May/Electro_Harmonix_Russian_Big_Muff_Pi_Pedal_Mods.aspx) and figured I would give it a whirl.  I changed out C1, R8, R13, R22, and C9, and then removed R19 as he describes.  I did not put in a pot for the mids, i figured that could wait for another day.  The pedal sounds great, however, with the volume cranked, I still get less gain than I do running it in true bypass, by a considerable amount.  I have never used one of these pedals before, as I am a drummer, not a guitar player, but I hear most people run the volume pot at 25%.  can anyone see a reason for this in what I have done?  I apologize for being such a noob, any help would be appreciated.

[maarten]

I assume the gain was all right after the first fix, before you went on to the modifications.....
If so, then the mods are responsible for the loss of gain:
1   make sure you got the right parts (sometimes sellers make mistakes and give you the wrong value; e.g check colour codes thoroughly - depending upon the light source red and orange and blue and green easily can be mistaken)
2   make sure you got them in the right places (e.g. exchanging R23 instead of R22 could explain for a loss of gain)
3   make sure your soldering is clean and making no wrong connections
4   make sure the new parts are causing no false connections (e.g. long threads you forgot to cut and that may connect to the casing when you close it)
5   if you worked from the photographed layout, make sure you have the same layout. when in doubt, trace the layout back to a schematic to see whether your mods were done at the right spots

Maybe there are other points to consider, but this should keep you away from your drumkit for long enough, I suppose...
Maarten

[stickninja]

so I took the thing back apart today to inspect it again.  all the components check out, and there are no shorts that I can see.  until today I had been testing the unit giving it a line level signal, then feeding it into a preamp in the studio.  today I plugged it into a blues Jr., and the gain is so low, the amp has to be maxed out to even faintly hear the dirty signal.  So my question is this, could I have shorted something out (ie Fried) in the gain stage that would explain this?  I will bust out the multimeter and measure voltages at all components and post them in hopes someone will recognize something wrong.  in the mean time, thanks for the help.

[twabelljr]

This is just a crazy idea but see what happens if you put R19 back in. Not sure exactly what it does but is in the two revised schematics in a different configuration. It would be simple enough to try.

[Meanderthal]

 Is r19 one you're supposed to replace with a jumper wire? Did ya (if so actually sub the jumper in)? That would severely misbias that gain stage without it...

Looked at the schematic... nope, removing R19 disables half of the tonestack... yuck...

Well, there's always solder bridges and cold solder joints to look for... so maybe try that?

[twabelljr]

Looked at the schematic... nope, removing R19 disables half of the tonestack... yuck...

Thanks, I see that now. I actually read the article this time. Oops!
Something is changed/modded before each gain stage. So what stage is losing the gain?? Do you have an audio probe? Easy to build then you could test various points along the audio path and hear what is happening all through the circuit and hopefully find your loss. If you can't see any problems visually this may be your best bet. I read a modding tip that recommended testing your circuit after each component was swapped to verify it worked. Kinda late now but something to remember in the future. Good luck. I hope you get it going!

[petemoore]

  Debugging page.
  I like this order:
  Measure the transistors votlages, looking for operational bias's. Find one that's off...figure out why.
  All voltages look like each stage should be operational ?
  Audio probe, each stage output should be higher gain version of it's input [since they're all gain stages], 2 and 3 have clipping/distortion diodes so look for some of that probably in stage 2, more of it in stage 3.
  Tonestacks passively remove. Frequency specific 'subtractions' = output will be 'somewhat' lower than input [depends on settings and etcetera's] of tone control.
  Most of the BMP mods are aimed at either being voicing related or increase gain related, a few for gain reduction.
  The 4 gain stage transistor design = relatively strong distortion and output level.