Author Topic: Big Muff Replica has low ouptut  (Read 1091 times)


Big Muff Replica has low ouptut
« on: October 14, 2018, 08:10:06 AM »
Hi everyone,
I spent a few hours building and trying to debug a Black Russian Big Muff Replica, and now I am stuck.
I have reached the point where Bypass works, and when I hit the footswitch:
  • LED lights on
  • output is lower than through Bypass
  • All three knobs have an effect: no sound when either volume or sustain are at the lowest, tone works fine too
  • Tone switch seems to work too
  • Thre is almost no effect on the sound

All info on the circuit can be found at the provider's store, but i took a screenshot of the schematic:

As far as debugging goes, I double checked component values, wiring and measured some Voltage values.
Code: [Select]
Q1: C 7.60V - B 0.65V - E 0.13V
Q2: C 6.92V - B 0.54V - E 0.01V
Q3: C 7.34V - B 0.55V - E 0.01V
Q4: C 7.65V - B 0.54V - E  0.01V

D1: A 6.96V - K 6.60V
D2: A 6.60V - K 6.96V

D3: A 7.01V - K 7.37V
D4: A 7.37V - K 7.01V

D5: A 8.64V - K 8.12V

Battery : 8.66 V

I also checked all solder joints, but as I had not done any soldering for a few years I can't be sure of the result (and I am not proud of the quality).

Also, I took some pictures as I worked (first to show my pal who offered me the pedal, but then it helped me to double check the component placement)

With capacitors and transistors. Some are bend, as I did not understand the instruction "Mount rotated 180 degrees compared to PCB". I checked and the emitor is every time connected to the matching resistor on the schematic.

Can you help me having this damn thing to work ? The more time I spend on it, the more frustrating it gets, and the more I want it to work correctly !

Edit: I decided to have a look at the transistor datasheet, and remarked that the C/B/E annotations do not match those on page 5 on the instructions. I'll look at that (should have used sockets...).
« Last Edit: October 14, 2018, 08:38:46 AM by Eustache »


Re: Big Muff Replica has low ouptut
« Reply #1 on: October 14, 2018, 10:07:27 AM »
welcome to the forum, Eustache.

unfortunately, I see no resistor values on your circuit diagram, so I don't know what should be where. BUT! you need to power off and measure the resistance of each of your Emitter resistors, and each of your Collector resistors, and compare to the circuit values.
"He thinks that growing hair is hard work."


Re: Big Muff Replica has low ouptut
« Reply #2 on: October 14, 2018, 11:54:35 AM »
First I should mention that I tried to revert the transistors, and now the PCB is a mess... I think I wrecked the whole thing.
Still, here are my measures:
Code: [Select]
R22: 388Ω (expected 390)
R21: 390Ω (exp. 390)
R10: 387Ω (exp.390)
R4: 2.68kΩ (exp. 2.7k)

R9: 24.5kΩ (exp. 470k)
R13: 11.66kΩ (exp. 12k)

R17: ? (exp. 470k)
R18: 11.6kΩ (exp. 12k)

R15: ? (exp.470k)
R11: 11.59kΩ (rises slowly) (exp. 12k)
R8: 20kΩ (rises slowly) (exp 22k)

R6: 10.02k (exp. 10k)

R15 and R17 both start around 200k and very slowly rise. Maybe to reach 470, maybe not. R9 is same as R15 and R17, on the schematic and according to colors, yet it has a wrong value...

After reverting the transistors, I had a few measures: this does not look good (a lot of zeros), maybe the orientation was right the first time.
My unsoldering skills are even lower than the soldering ones the PCB has taken quite a shock... I am still curious of the reasons it did not work, but I think it would be a lost of your time to continue helping me... Sorry about that.

On the plus side, now I want to build an other one, maybe simpler.

EDIT : The pedal is not dead. Not output is lower than before (barely audible), but the effect is more obvious (as expected, but since output is so low I have to max up the volume on the amp :icon_lol:)
EDIT 2: The transistors were indeed in the wrong position ! Just patched my pcb with jumpers all over the broken legs, it looks awful but I have a working effect ! At least, better than earlier !  I really have to go now, but I wanted to give you that last update.
« Last Edit: October 14, 2018, 12:42:24 PM by Eustache »


Re: Big Muff Replica has low ouptut
« Reply #3 on: October 15, 2018, 09:26:46 AM »
go? what do you mean, go? nobody gets out!

good work on the fixx. when it comes to transistors, find the manufacturers datasheet for the part in your hand, and stick to it.
"He thinks that growing hair is hard work."


Re: Big Muff Replica has low ouptut
« Reply #4 on: October 16, 2018, 07:25:49 AM »
go? what do you mean, go? nobody gets out!
At least, NOT without posting the correct voltages..!!  :icon_redface:
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..


Re: Big Muff Replica has low ouptut
« Reply #5 on: October 21, 2018, 10:17:55 AM »
Ahah ok, here we go (transistors only):

Code: [Select]
Q1: C 5.22 B 1.23 E 0.72
Q2: C 4.39 B 0.63 E 0.09
Q3: C 4.48 B 0.70 E 0.10
Q4: C 4.50 B 0.70 E 0.10

I do not know the normal values, but I've been playing with that pedal whole week, and I like what I hear ! I'm still not sure why I did not manage to measure some resistors, though.