Big Muff Pi diode mod troubleshoot

[Eddward]

Hi all, thanks in advance for your advice!

Modded a big muff pi. Circuit board states EC-3003 REV_C.

I've done the following,

- replaced the volume pot (like for like, the shaft came out of the pot when removing the knobs!)
- removed D1, D2, D3, D4
- placed 2 on-off-on dpdt switches in the D1/2 and D3/4 point, switching between LED and stock 1N4148 diodes

Result?
- Bypass signal as expected
- Circuit on, LED on, clean sound passed (exactly as bypass)

Checks performed,
- Bypassed sustain and tone pot (signal didn't seem to flow through them properly when tested), when sustain was fully up, the LEDs in the clipping stage were on, flickering. Brightness reduced as sustain rolled down (weird!)

EDIT - these are definitely not working, no connection between any lugs. Bypassed by linking hots together to get above result. New pots now on order.

- checked connections in the vacinity of soldering, all connected as it should be (including new switches). No cold joints discovered (unless you see any...)
- battery and psu used for tests

Points of note,
The modern dc jack is not yet wired in, to avoid the circuit being stuck before it works
D3 and D4 terminals used for a neater solder joint

I'm most curious as the pedal should generate effect regardless on D1-4 being present (like a no diode mod), my understanding then being that nothing can be wrong!

If you can work out what's wrong, please let me know!
Edd

[ElectricDruid]

Welcome Eddward!

- Bypassed sustain and tone pot (signal didn't seem to flow through them properly when tested), when sustain was fully up, the LEDs in the clipping stage were on, flickering. Brightness reduced as sustain rolled down (weird!)

This sounds about right to me. The "sustain" pot is a gain control, so more gain = more signal = more brightness from the clipping LEDs.

What do you mean by "bypassing" the pots? The resistance of the pot is an important part of the circuit and shorting it out with a wire instead won't make things work better.

EDIT - these are definitely not working, no connection between any lugs. Bypassed by linking hots together to get above result. New pots now on order.

A pot will have enough resistance that it won't make a continuity tester beep, unless the wiper happens to be right at one end or the other. Most continuity checks will fails if the resistance is over 100 ohms or so. You'd need to test on an "ohms" setting and check you've got the values you expect.

[MaxPower]

Replace the LEDs with si, ge, or schottky diodes and see if it works. It may be that the LEDs's voltage drop is too high for the circuit to clip.

[Eddward]

Thanks both, here's the results from your comments,

- pots definitely gone, moving the wiper fully to either side makes no difference in readings (always no signal), whilst the new volume pot does go from open to closed as the wiper moves.
- stock silicons on the other side of the switches, still no output (though they do pass signal as expected)

Hopefully the new pots will do the trick, but could I run a resistor in their place temporarily to complete the circuit and see if I've any more issues?

[DIY Bass]

You could run 2 resisters to get in between settings.  With 1 resister you could get full on or full off, but you would also need to short the middle terminal to one of the outsides (one would give you full on, the other full off)

[Eddward]

Good news, the pots were the only fault.

New working pots in, and it's working as expected/hoped.

For future reference, or if you're interested, the circuit won't function properly without pots (i.e. jumpering the lugs), as discovered in this mod fix!

Thanks for your support,
Edd

[pinkjimiphoton]

Replace the LEDs with si, ge, or schottky diodes and see if it works. It may be that the LEDs's voltage drop is too high for the circuit to clip.

put a ge diode in series with each led, and it will clip more musically at a lower volume, and the sustain part of the note will fade much more naturally. when leds stop conducting, they tend to gate off the ends of notes. the ge prevents that to a large degree, but ya still get that nice phat led crunch.