EHX - Big Muff Pi (new-ish, 2014, EC-3003 REV_E)

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fast_luck
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Post by fast_luck »

Hey guys,

I didn't see the newer Big Muff schematics up anywhere, so I traced mine while I had it apart. I bought it sometime in 2014, PCB reads EC-3003 REV_E. Ref Des on the schematic should match the PCB. Level pot may be wrong (Silk says 500kA) as I replaced the pots almost immediately after getting it because one of the awful, awful ones that came stock broke.

Hopefully this will help people looking to mod theirs.

Joe
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BMP_EC-3003_REV_E.pdf
2014 Big Muff Pi
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Nocentelli
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Post by Nocentelli »

Interesting... huge coupling and clipping caps. Just a tiny change to your schem - lug 1 and 3 on your tone pot need swapping if you want the standard CW = brighter
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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fast_luck
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Post by fast_luck »

Didn't even look at that to be honest, just wanted to get the values and reference designators down so I could give it a look and figure out what to swap out. I'll flip the pot on the schematic. Thanks!

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Post by fast_luck »

Fixed the Tone pot, added the power supply filtering from the board.
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BIG_MUFF_PI_EC-3003_REV_E_.pdf
Revised: Flipped tone pot, added power supply filtering.
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Cub
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Post by Cub »

Hey Fast, thank you for sharing this ! Funny to see how this is already different from a 2013 Revision E Muff. It's almost exactly the same as a traced Revision C from 2008. Almost, as the only difference is the huge value for C3. In all the Muffs and clones that I've seen, I've never seen that one go over 1µF. Could you please check again if this is really 2.2µF ?

500k Log for the volume is almost certainly right, it's been like that since at least Rev. C. Possibly even B, but I have yet to see a schemo of a B. Do you still have the original pots somewhere in a parts drawer to check, or did you chuck them in the bin right away ?

Another little remark, R16 and R21 are swapped with how the components are labelled in the standardised KitRae schematics.
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Post by fast_luck »

Yup, double checked...it is definitely a 2.2uF cap going between the clipping diodes and the base of Q3. I agree...that's a bit weird.

As for R16 and R21, I went with the parts numbering from the PCB so it would be easier to find the parts I plan on pulling out to mod it. I just double checked to be sure, R16 is definitely a 120 ohm on the emitter of Q2, and R21 is definitely a 100k from base to ground on Q3.

Naah, I don't have the pots to check. I pulled the knobs off when I got the thing because I wanted to take a look at the board, and the shaft of the tone knob came out with the knob. All 3 went in the garbage after that. I had some 24mm alpha PCB mount ones, and I put those in.

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Cub
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Post by Cub »

Thanks for clearing that up ! :applause:
fast_luck wrote:Yup, double checked...it is definitely a 2.2uF cap going between the clipping diodes and the base of Q3.
That's would be C7, it's been 2.2µF since Rev. C.
The one I'm wondering about is the coupling cap between the wiper of the tone pot and the Collector of Q4 in your schematic (C3). 2.2µF Seems rather large.

But I just noticed there's an extra 2.2µF coupling cap between the second clipping stage and the tone stage (C16). Doh ! :slap: That might have something to do with the value of C3.
Hey, it's a Muff schemo, I usually stop counting after 13 caps. :lol:
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Post by fast_luck »

Cub wrote:The one I'm wondering about is the coupling cap between the wiper of the tone pot and the Collector of Q4 in your schematic (C3). 2.2µF Seems rather large.
Seems like you're right there! I took another look, C3 is actually a 0.47uF 50V electrolytic. I updated the schematic with that and the 500kA pot (since that was probably there before I tossed the old ones).
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BIG_MUFF_PI_EC-3003_REV_E_SCHV1.2.pdf
Updated 7/13/2015
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Post by Cub »

Great ! Glad we got that sorted for more accurate schemo. :thumbsup
Got any ideas yet on how you want to mod yours ?
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Post by fast_luck »

Cub wrote:Got any ideas yet on how you want to mod yours ?
Not yet...There's not necessarily any one specific model that I want to do. I'll probably will just build one on the breadboard and keep swapping parts until it sounds good to me. Only thing I've totally settled on is putting in a little loop on a momentary footswitch to get it to oscillate, and a tone stack bypass switch.

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Post by crguti »

C8 must be 0,01uF, right?

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Post by Nocentelli »

Yep
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by erfci »

I recently got a new big box bmp, and the board says REV_F. I checked it compared to the rev E schem here and it is the same. Kind of strange that the rev letter is different, maybe they changed the LED resistor value (mine is R23 = 10k, LED draws 0.733 mA @ 1.622V) or the switch connector wiring or something, or the voltage ratings on the caps? With a new 9V batt, the collector voltages are:
Q1cv = 4.20
Q2cv = 4.12
Q3cv = 4.11
Q4cv = 4.88

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