Big Muff on the fritz!

[jbeasley]

I built a BMP and it worked fine but I realized it wasn't what I wanted so I loaned it to a friend for 6 or so months.  The pedal was returned to me a couple of weeks ago in a barely working condition.   This thing is driving me crazy!  I'm about 15 hours into it and still no fix.


1.Name of the project:
General Guitar Gadgets BMP Violet Rams Head version



2.Links to the source of the project web site with the schematic or project, layout and wiring diagram.
http://www.generalguitargadgets.com/projects/17-distortion/108-big-muff-pi?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a



3.Some candid admissions about how closely you followed the suggested layout, if any.
I followed the instructions perfectly - The pedal sparked up first try and worked well.

It worked fine, I played around with it for a couple of weeks but I was not satisfied with the sound as it didn't offer up much that I could use. 
I realized that I wanted something more novel sounding.



4.Any parts substitutions or modifications you made to the original.
I received some suggestions from a forum member a while ago - maybe 6-9 months... (Bumblebee) to change a few capacitor and resistor values to give it better range in bass and treble, less mushy. 
He suggested:

1 - a treble cap toggle switch to switch cap values on C2 & C5 from 470pf to 100pf. 
2 - change resistor values R5, R11, R17 & R22 (to 10k, 10k, 10k & 470k)
3 - change C1, C3, C6, C7, C9 & C13 to 1uf

I performed these changes and it worked perfectly for about 6-9 months while a friend borrowed it to play his bass through - He loved it and wanted to buy it from me.
When I asked for it back the pedal was returned in a barely working condition.  The sound cuts in and out on certain tone and volume settings (see #6).



5.Whether or not it's a positive ground circuit like a PNP fuzz or a Rangemaster that has been hacked to work with negative ground.
No, nothing out of the ordinary.



6.What it does and does not do how it works; partially or not at all. For instance, if it lets no sound through; or if it makes a harsh, distorted sound only when you strum hard on the strings at full volume; or if everything is really, really low volume.
When the Volume is past 1/2 to 3/4 up to max there is no sound at all.  When the volume is turned up 3/4 AND the tone is past 9:00 there is no sound until the tone is turned up past 3:00 and then it cuts in and out.  Sometimes the pedal doesn't work at all or just cuts in and out constantly.  Sometimes I only get sound when I hit the strings hard and the sound cuts out as the guitar is still sustaining at lower levels.  The sound cuts in and out if I "wiggle" the board around.  Sometimes when I wiggle the board the sound changes from fuller to thinner to squishy to raspy to nothing... 

I went over the entire board and:
1- made sure there were no cold solder joints
2- removed all wire and replaced with teflon coated stranded wire
3- Removed the volume and tone pots and replaced with new ones
4- Looked for cracks in the PCB or broken traces - NONE

I also thought that *maybe* the last transistor was bad so I replaced it. 
I'm sure I did a weird thing by using a 2n222A instead of the 2n5089 it came with but Radio Shack didn't carry them and I wanted to try a replacement.
Regardless, the pedals behavior didn't change and when the sound does come in for a minute or two I actually prefer it to having all 4x 2n5089's so if it is possible I'd like to keep the 2n222A in there.  I will change them all to one type if it has to be that way - I'm sure mixing transistor types is taboo for one reason or another.  The only ones I could find at radio shack are 2n222A, 2n3904 & 2n4401 and I have no idea how they sound in this circuit.  I have enough of each to try them but no sockets unfortunately.  I feel weird admitting that I did this because, admittedly, I don't know what the hell I'm doing but like I said, the pedals behavior didn't change after the transistor switch and I promise that wasn't what caused the problem to begin with.  I switched that Q4 transistor as a last ditch effort so I can assure you that it wasn't the beginning - I'll pinky swear.

Anyway,
None of this worked.  The pedal still behaves exactly as it did two weeks ago.



7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage?
=>8.18

Now insert the battery into the clip. If your effect is wired so that a plug must be in the input or output jack to turn the battery power on, insert one end of a cord into that jack. Connect the negative/black meter lead to signal ground by clipping the negative/black lead to the outer sleeve of the input or output jack, whichever does not have a plug in it. With the negative lead on signal ground, measure the following:

Voltage at the circuit board end of the red battery lead = 8.09
Voltage at the circuit board end of the black battery lead = 0

Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn. Report the voltages as follows:

Q1 2n5089
C = .04
B = .64
E = 4.06

Q2 2n5089
C= .04
B= .65
E= 4.09

Q2 2n5089
C= .04
B= .64
E= 4.04

Q2 2n222A
C= .92
B= 1.51
E= 3.77


D1
A = 50
K = 64

D2
A = 64
K = 51

D3
A = 51
K = 64

D3
A = 64
K = 50

(Anode, the non-band end)
(K= cathode, the banded end)

[Hides-His-Eyes]

Transistors look fine, check the switch is shorting where it should be and not where it shouldn't be. 3PDT switches are notoriously fond of breaking for no reason.

[petemoore]

Q1 2n5089
C = .04
B = .64
E = 4.06

Q2 2n5089
C= .04
B= .65
E= 4.09

Q2 2n5089
C= .04
B= .64
E= 4.04

Q2 2n222A
C= .92
B= 1.51
E= 3.77
  All  the voltages seem to have common problem, reversed power supply or missed ground come to mind. The Emitters would be 'near' ground, and the collectors should be 'middlin' voltage bias.

[jbeasley]

Thanks, I'll check the 3PDT switch.

      "All  the voltages seem to have common problem, reversed power supply or missed ground come to mind. The Emitters would be 'near' ground, and the collectors should be 'middlin' voltage bias."

I understand the reversed power supply part but when you say "missed ground" & "The Emitters would be 'near' ground, and the collectors should be 'middlin' voltage bias." I'm at a loss.  I'm pretty new to transistors and diodes as most of my background in electronics is in guitar guts.  As far as I can tell everything is grounded properly so I'm not sure where the ground might be missing in the layout.  Is there a way to test where the ground might be disconnected or missing?  Is there an online reference that might help me to troubleshoot this transistor issue and learn about transistor voltage bias? 

[petemoore]

  Si Transistor, consult data sheet, check/verify pinout is correct.
   Base above emitter by 1 Si diode drop [there is Si Diode in Si transistors between an NPN:  Base-->lEmitter.
  The Collector's DC 'idle-bias' voltage needs to be near the 'center' of supply [with a 9vdc supply: ''1/2v'' would be 4.5v].
   When the base sees input - or +, the collector swings + or -, and needs room to do so. Say it wants to swing 2volts + but the bias is 8vdc, it'll 'hit' the 9v power supply 'rail' limit. Same thing for if it wants to swing negative from it's bias point: Say collector is biased at 2v and wants to swing 2v negative from there, it'll run into ground.
  Transistor requires a bit of 'elbow room' so even when it swings near to a rail the response changes or it acts like it hit a rail...some get closer to rail before clipping than others.
   The emitters DC bias should largely be determined by the small resistor to ground [emitter resistor]. Can be DMM'd...set the meter to 20k range, test from transistor emitter pin to battery clip button, the meter should read close to the marked value on the schematic through several solder joints etc. [quick way to run tests in 1 probe/meter reading.

[jbeasley]

Thank you for your time, I appreciate the response immensely. 
I checked the 3PDT and all of the poles are switching as they should.

I found a datasheet online for the 2n5089 and it appears that the pinout is correctly oriented.  I tested continuity from pin 1 emitter to the 100ohm and 2k resistor junction - positive on all counts.

When I look back at my original values in my first post for the pin values I had E and C reversed, I apologize for the confusion (facepalm).  So the original values should have read:

Q1 2n5089
C = 4.06
B = .64
E = .04

Q2 2n5089
C= 4.09
B= .65
E= .04

Q2 2n5089
C= 4.04
B= .64
E= .04

Q2 2n222A
C= 3.77
B= 1.51
E= .92

When I test the emitter pin of each transistor to the positive battery button (with battery connected) I get the following values.

Q1 2n5089 = 7.98
Q2 2n5089 = 7.99
Q3 2n5089 = 7.98
Q4 2n222A = 7.11

[tiges_ tendres]

Your battery seems a little low.  Try a new one and see what it does.

[petemoore]

  I can't keep focused, but read more closely.
   At first I saw the whacky voltage readings and decided to start where it couldn't work as focus on that.
  ok...reading more carefully the original post...
  Test the Volume pot. Read the R value as the shaft is super-slowly or 'notched' up in super-small incraments...looking for open spots where the wiper disconnects from touching the Rwafer.
  open ? IIRC it's De-Oxit or something like that, I find RS: Tuner Cleaner does fine job on pots that can be cleaned up...
   Outside...don't breath in..wear Safety Glasses.
  I shake, spritz through the pot-slot [hopefully] or detent [otherwise may be difficult to get fluid in there..I like to tip the can upside down and let a little compressed air get in there [seems the air is 'high speed' compared to the fluid, and pushes/evacuates the fluid], it's possible to just shake the can gently while pinning the tube into the slot between 2 lugs, this prime-loads a spritz of fluid so fluid-air-fluid-full range twist of shaft 3? times, another spritz/air.
  doesn't take much to get flow across all internal surfaces, wait 1 second for draining/evaporating to finish, the idea is to evacuate any dirt/oil/  .
 

[jbeasley]

I installed new pots... didn't help.

[jbeasley]

Ok, got it fingered out!

It was a bad tantulum cap. 
According to the diagram I had it wired in correctly but when I changed it out for a new non-polar cap the circuit acted like nothing was wrong all along
- If it were a person I'd punch it in the eye.

Thank you petemoore, tiges_ tendres & Hides-His-Eyes for your input, I appreciate it.

~Justin