So we've done a fair few mods lately of Big Muffs for people, mostly to the V8 Russian Big Muffs. We've thrown up a few pictures here so you can get an idea of what you'll see inside, and what to expect. EHX make some great pedals, and I'm a big fan of some of their ideas.
Most commonly, people bring us the Russian Big Muff, asking for a true bypass mod. We normally throw in an LED swap for a high-brightness blue, which is much easier to see than the stock red LED, and let's face it, it looks pretty cool.
It is unfortunate that EHX tried so hard to save money on the modern V8 Russian Big Muffs. They're a great sounding pedal, much more of a warm overdrive tone compared to the NYC reissues, and they definitely have their own relevant place in the Muff lineup. However, the stock bypass is absolutely terrible, sucking big fat gobs of tone out of your signal. I don't know many people that would ever use one stock - the tone loss is incredible. However, with the true bypass mod, these things are fantastic.
Other mods we've commonly performed are boosting the gain of the circuit (we favour swapping the emitter resistors, tastefully, for lower values) and adding DC jacks to the rear panel (because who wants to buy 9Vs every six months?). They're a great pedal to work on, with only four screws to remove, and everything slides apart and is mounted to the front panel. We've also done pot replacements, normally to higher-grade Alpha units. If necessary, the stock knobs (which are a set-screw type) can be drilled out to accept larger-shafted pots if you wish to keep the original Russian look.
For interest's sake, we've also looked at working with the current models from EHX. These are much higher quality than the aforementioned Russian pedals. These can be had for under $120 AUD at the moment and sound pretty fantastic, with the classic high-gain NYC sound. I've taken apart my own Big Muff Pi with Tone Wicker to take a few pictures and to get an idea of the construction.
Note the two switches on the front panel. These are PCB mount DPDT types, of a kind not easy to find for the home builder. One is to disengage the tone circuit, a common mod often featured on boutique clones. This gives a slight volume increase, though not major, as the Big Muff has always featured a gain stage to recover the losses through the passive tone control. It gives a much fuller, more open sound when the tone control is disengaged.
The second switch is to engage the Tone Wicker. The switch controls a 4066 quad-switch IC to engage/disengage three high frequency filters that alter the tone of the pedal, giving a more brittle, fizzier sound. EHX clearly chose a cheap DPDT switch part and paired it with a switching IC, as they deemed this cheaper than trying to specify a 3PDT toggle. I can also only assume they specified DPDTs for both switches where SPDTs would suffice in order to add strength, as the DPDTs are soldered to the board with two rows of three pins, adding rigidity. The board is held in place by the right-angled PCB mount 16mm Alpha pots, which aren't visible from the side of the board shown above, and the Neutrik input and output jacks. Note as well the cutouts in the PCB board just above the jacks. These allow the board to flex for installation. This board is very difficult to remove from the case, however, as the pots, audio jacks, or the DC jack tend to snag on the case when one tries to remove it. All in all however, I quite like the construction of this pedal - it is very robust and probably very cheap to make thanks to the SMD parts.
I found the EHX pedals to be very educational as to pedal design and construction. It led me to realise many improved techniques in my own designs. Try disassembling some of your own pedals - you can learn a lot from the mistakes and successes of others!