Wow, nicely done. Now I see it’s all wood as well.

In case you experience too much noise, or interference from the outside, you could glue some copper or aluminum foil to the entire inside. That will work as a shield (if it’s connected to ground)

A small cap between collector and base reduces gain for (very) high frequencies. In general it will prevent fizzy top end distortion. But if the cap is (too) small or the gain rather low the effect is very subtle or inaudible.
Sometimes people use it to simulate germanium transistors when using silicon transistors. Germanium transistors have this extra capacitance internally. You can replicate it on silicons by adding it externally.

Normally that big electro cap across the power rails is used for power filtering (removing ripple) but in battery operated pedals there’s no ripple to remove, so I guess the only reasonable explanation is this one by PRR:
https://www.diystompboxes.com/smfforum/index.php?topic=113734.msg1053703#msg1053703
and this one by Teemuk:
https://www.diystompboxes.com/smfforum/index.php?topic=113734.msg1054057#msg1054057

Thanks a lot for the schematic!
Any idea what the original diodes were? (silicon or germanium?)

You can use it together with negative ground devices but you cannot daisychain them from the same power supply. It needs its own, battery or adapter.

Apart from the transistors, and diodes perhaps, most schematics online look to be accurate.

It’s a silicon Fuzz Face variant btw (but it has no bypass cap on the emitter of Q2 and therefor a lot less gain, but it has (clipping?) diodes on the output.

Are any of the online schematics correct when compared to your Muff Fuzz (apart from the resistor mistake)?
If so could you please post a link.

I love my Double Muff btw but the circuit is slightly different from being just two Muff Fuzzes cascaded as it is being advertised. Well, at least if I go by the Muff Fuzz schematics I find online.

The ‘tab’ that limits the rotation probably broke of.
http://www.geofex.com/article_folders/potsecrets/potscret.htm
[img]http://www.geofex.com/article_folders/potsecrets/pot4.JPG[/img]

Trying to repair the ‘broken’ pot is doable but a gamble as there’s a good chance you will break it beyond repair and then what?
Since it still works, I would just use it as it is. Now you can go from min to max or vice versa with only 60° rotation instead of 300°.

Happy hunting for a new pot.

Normally, for these niche items I’d suggest Smallbear but even he doesn’t have them for sale, so it’s going to be really hard to find one.
He does list a pot you could use after a tedious amount of modding to both pot and enclosure:
http://smallbear-electronics.mybigcommerce.com/poteniometer-for-russian-big-muff-work-alike/
http://smallbear-electronics.mybigcommerce.com/electro-harmonix-sovtek-spares-1/?sort=featured&page=3
[img]https://cdn10.bigcommerce.com/s-sxoup5r/products/3762/images/6147/Russian_BMP_Pot__93201.1523243561.1280.1280.jpg?c=2[/img]

With the volume up, the pedal should not be quiet or any quieter than other Big Muff Pi’s. So there might be other issues. This has nothing to do with old or new.

Black Russian BMP uses linear pots though, so you have to turn the volume up more, compared to a lot of other versions of the BMP which use log pots, to get the same output volume.
But with volume at max on both, they’re pretty much equally loud.

Lots of pictures of the Rams head inside here:
http://www.kitrae.net/music/big_muff_history1.html

schematic is in this thread:
https://www.ehx.com/forums/viewthread/7000/
Edit:
It’s on photobucket.
In case you cannot see it, here’s a repost: