Help with General Guitar Gadgets (Big Muff Pi GGG Tuned Version)

[CRushing]

I acquired this project from a friend. He had most of the board populated and soldered but stopped working on it. I finished wiring up the switches and the few remaining components on the board using these two schematics

http://www.generalguitargadgets.com/pdf/ggg_bmp_ggg_sc.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a
http://www.generalguitargadgets.com/pdf/ggg_bmp_wiring.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a
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1.What does it do, not do, and sound like?

When powered up no sound unless effect is bypassed.

2.Name of the circuit =
Big Muff Pi GGG Tuned Version

3.Source of the circuit (URL of schematic or project) =
http://www.generalguitargadgets.com/pdf/ggg_bmp_ggg_sc.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

4.Any modifications to the circuit? Y or N

No

5.Any parts substitutions? If yes, list them.

None

6.Positive ground to negative ground conversion? Y or N

Not that I am aware of...

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? =>

10.01v

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 = 10.01v

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

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:

Following GGG's troubleshooting guide....http://www.generalguitargadgets.com/pdf/ggg_bmp_instruct.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

It looks like I have the right voltages on Transitors 1, 2 and 3 but Q4's values don't look right :/

Q1
C =4.0v
B =.64v
E =.04v

Q2
C=4.01v
B=.64v
E=.04v

Q3
c=3.72v
B=.61v
E=.03v

Q4
C=9.35v
B=1.81v
E=1.21v

Any insight into what might be wrong is greatly appreciated. I am not a complete noob but am also not skilled enough to know what the next troubleshooting step should be.

Thanks in advance.

Crushing

[dwmorrin]

You're correct about Q4... the collector voltage should be near 5V.
The base and emitter voltages look correct, and they are about 600mV apart, so Q4 is probably "on", but near cutoff for some reason.

If caps C12 and C13 are installed right, then the dc conditions are just set by those four resistors around Q4.  Since the base and emitter look about right, I would suspect R24, the 10k collector resistor.  Maybe a 1k was installed accidentally?

[CRushing]

dwmorrin

"If caps C12 and C13 are installed right"

These are film caps. They aren't polarized right? Best I can tell from the paperwork. They are the correct value. Assuming they can't be installed backwards like an Electrolytic, these look ok to me.



"I would suspect R24, the 10k collector resistor.  Maybe a 1k was installed accidentally?"

R24 is definitely a 10k Resistor. Just went and double checked.

Thanks for the help. Any more thoughts or things I can do to narrow this down are much appreciated.

[dwmorrin]

You've got 4 resistors and a transistor to check for Q4.  Check all 4.  Check connections.  Check the transistor out of circuit.  (Use diode tester on your DMM)

[duck_arse]

post some photos of your board, both sides.

[CRushing]

Front
http://i21.photobucket.com/albums/b254/Wireneck/back_zpsd82195f9.jpg~original

Back
http://i21.photobucket.com/albums/b254/Wireneck/boardFront_zps03a2d0ef.jpg~original

[dwmorrin]

Hard to see the color codes on the resistors... hate to say it, but I'm still suspicious of R24.  Did you measure the ohms with a meter, or did you just visually check the resistor?
Looking at the picture, the 4th band could be brown or red.  Red would be correct (10k) and brown would be incorrect (1k).
Sorry if I'm barking up the wrong tree still.  Did you check the other resistors?

[PRR]

> I'm still suspicious of R24

+1.

1K would give just those (wrong) voltages.

With power OFF, meter on Ohms, across R24, does it say 1K or 10K? (Yes, many auto-range DMMs are hard to tell what range they are on.)

--- R23 as 220 ohms might also give this result.

[CRushing]

Hey guys thank you for the feedback. I haven't had a chance to sit down with it and my multi meter since my last post. I only checked the color band code before.  I will report back with measurements. Thank you for your patience and your help!

[CRushing]

Ok I tested R24 and it was only reading 120 ohms on 200 ohm range. (My meter isn't an auto ranging). So I replaced it with the correct 10k resistor. After doing so Q4 is now reading

Emitter 1.28v
Base 1.87v
Collector 3.93v

Which isn't as close to the reference numbers from General Guitar Gadgets data sheet as the other transistors. Are these numbers ok now?

I went ahead and tested the other surrounding resistors as well

R21 81.5 ohms on 200k range
R22 .09 ohms on 20M range. (That's the next step above 200K on my meter) This number doesn't seem right to me?
R23 2.20 ohms on 20k range

A quick test reveals that I am still getting no output from the effect.

[dwmorrin]

Q4's voltages look like it will pass signal now.
So you have more than one problem perhaps.
I would trace the signal next.
Inject some kind of audio signal and trace it from input to output and let us know where it gets lost.
Use either an oscilloscope or a simple audio probe to trace the sound.
A simple audio probe can be just a cap alligator clipped to the input of an amp.
Trace from input jack to base of Q1, then collector of Q1, sustain control next, base of Q2, collector of Q2, etc.

[CRushing]

Ok built an audio probe today after work. I am new to this method but I watched a few videos on youtube and read the audio probe article on this site.

I have audio on Q1 but when I get to C3 I am only getting audio from one side of the capacitor. Could this be my first issue? A quick test revealed I didn't have any signal on any other transistor. Like I said still a newbie working through this. Thank you for your help.

I will have more time tomorrow to probe a little more in depth.

[dwmorrin]

I have audio on Q1 but when I get to C3 I am only getting audio from one side of the capacitor. Could this be my first issue? A quick test revealed I didn't have any signal on any other transistor.

Great.  This is good progress.  C3 should have audio on both sides of the cap.
If there's no audio on the "SUSTAIN" pot, then nothings going to get to Q2, Q3, Q4... all the transistor stages are in series, chained together through what's called "coupling capacitors."  C1, C3, C4, C7, C12, C13 are all coupling capacitors and that's where the audio gets passed from stage to stage.

Check resistance from the pot side of C3 to ground.  Should be R6+R7, or about 100k.  If it's not, maybe the pot is wired incorrectly.  C3 goes to the clockwise terminal, R7 to the counter-clockwise terminal, and C4 is on the wiper.

If you're probing the underside of the board, it's possible there's solder below one leg of C3, but no connection... legs can break... might want to desolder C3 to double check it's OK.

[CRushing]

I will check all the other stuff you suggested after work today.


"C3 goes to the clockwise terminal, R7 to the counter-clockwise terminal, and C4 is on the wiper."

Sorry but I am not 100% sure which is clockwise and which is counterclockwise. If I am looking at the POT with the post facing up at me and the three terminals are listed 1,2,3 from left to right......which one is which?

Thanks again!

[duck_arse]

hold the pot, and this applies to all pots, with the shaft pointing towards you. turn the shaft in a clockwise direction, until it stops. the wiper (always in the middle of the three lugs, often referred to as "w", and sometimes "c") is now shorted to the "clockwise" lug, often called "cw". the other, counter-clockwise pin/lug, "ccw", is at the other end of the wipers travel.

when you "turn the volume down to 0", you are turning the pot anti- or counterclockwise. the ccw lug is nearly always connected/closest to ground/earth/0V in a volume/level control.

I never use "1,2,3" to name the lugs, as my idea of 1 might be your idea of 3.

[dwmorrin]

And for the purposes of getting signal at C3, just make sure none of the terminals have a low resistance to ground.
As long as you have the middle lug going to C4, a backwards CW/CCW, or "1&3" will only result in the control operating "backwards" (low gain at max rotation, high gain at min rotation).

[CRushing]

OK the wiring on the pot looks correct per your description and the wiring diagram but a quick check shows......I am getting 0 resistance from the sustain side of C3 to ground , Counter Clockwise Post is also reading 0 ohms to ground and the wiper terminal appears to be grounded as well.....so that's obviously a problem lol.

[CRushing]

I got it sorted out. one of the pot leads at the board was touching the case. Now the pedal is fully functional. Dwmorrin thanks for all your help!