Before unsoldering anything, I’d first check the contacts of the input jacks for the expression pedals.
It could be that they’re corroded or not making good contact when unplugging the expression pedal.

• This reply was modified 6 months, 3 weeks ago by j_flanders.

BillFoster:
The Sovtek Deluxe Big Muff Pi is nowhere near what he’s looking at, which is a vintage big box (opamp version) Big Muff Pi with attached power cord. See image below.

fuzzbuzzfuzz:
Analogman can probably do a conversion. He does a similar conversion for the Deluxe Memory Man with attached power cord. https://www.analogman.com/
If you want to go the DIY route, a better place to ask and get detailed instructions is probably https://www.diystompboxes.com/smfforum/index.php?board=2.0

https://i.postimg.cc/ssng2cPh/Screenshot-2022-05-01-225816.jpg

3PDT true bypass schemes:

True bypass wiring schemes

Personally I wouldn’t drill a hole in a ‘vintage’ Green Muff.
With my Russian Big Muffs I use a ‘battery eliminator'(DC-wall wart to battery snap adapter) and run that through the battery door on the back.
Make sure the one you buy has the correct polarity.

Besides the difference in layout and box caps vs greenies, the only ‘real’ difference is the transistors.
Rev A has 2N5088 and rev 2 (and rev 3) has BC550B. The silkscreen on the pcb says BC550C, but the actual transistors are BC550B.
The seemingly missing 47pF ceramic cap in rev 2 is soldered on the back of the pcb.

I remember there’s a slight difference in tone but I’d have to A-B them again to see where they differ. It was subtle though.
Rev A is on my board. Rev 2 which came with a wooden box is in my ‘collection’.

The effect is essentially 2 silicon Fuzz Faces in series.
Put it first in line and roll back your guitar’s volume control for excellent clean up.

Since it comes with an attached three-pronged AC plug as the power source you don’t have to worry about voltage and mA draw.
Just plug it into the wall socket and play.

1.6V instead of 9V on the LED is fine. I see the current limiting resistor is before the LED, so 1,65V is the voltage from the LED.
Do you have any idea on what could have happened in the mean time? You’re now experiencing the opposite: it works in effect-mode but not in bypass-mode, while originally you had bypass-mode but no effect-mode. How can the LED suddenly start to light up?
You’re not by accident plugging your guitar into the output of the the pedal? (Given that it has its input at the left and the output at the right, whereas on most pedals it’s the other way around;)

I posted a follow up but it does not show up. :s
Maybe I cannot post twice in a row?
Trying again:
You can use this wiring diagram.
http://www.kitrae.net/music/Images_Secret_Music_Page/V8_Wiring_Diagram.pdf
As far as I can tell it’s correct.
It is not the one for the true bypass version but for 99% it will be the same.
It shows where on the board and its traces you should read 9V.

Connect the battery.
Insert a guitar cable in the input of the pedal.
Make sure the pedal is not in bypass mode.
Set the multimeter to DC (somewhere in the 0 – 20 V range). The symbol is as straight line with a dashed line beneath it.
Put the black probe on a ground point. Sleeve of the jack for example or any non painted spot on the enclosure.
Put the red probe on either leg of the LED. One leg should read 0V, the other 9V (more or less).

Since the LED doesn’t light up, most probable both legs will read 0V.
Work your way back and find where the 9V connection to the board is interrupted.

• This reply was modified 3 years ago by j_flanders.