Mangler isn't mangling ...

[brucer]

Help please!  I'm still new.  I've got a bad feeling that I messed up.  I just can't figure out where ...

I've populated a Madbean Mangler PCB, mostly following the build document:







The PNP "All American" Ge transistors came from Small Bear, with the following values noted on the package:

Q1: 91, L=169
Q2: 136, L=120

The Ge transistors came with the carbon comp resistor values used to bias them to Fuzz face specs and these were my only deviations from the standard BOM.  I made a solid newbie effort to read the "Bear Face" (https://www.smallbearelec.com/Projects/BearFace/BearFace.htm) and Mangler (http://www.madbeanpedals.com/projects/Mangler/docs/Mangler.pdf) schematics to match the Small Bear resistor numbers to those of Madbean and came up with the following:

Small Bear (resistor # and biased value)      Madbean (resistor # and stock value)
R3=27k                     R2=33k
R4=680R                     R4=470R
R5=9.1k                     R3=220R
R6=91k                     R1=100k

Again, I used Small Bear's resistor values in the build ... and checked and double checked all values with a multimeter (even had them re-ship one resistor because they'd provided a very different value than what was recorded on the spec sheet).

ANYWAY ... I tried to rock it before I boxed it using a test rig:



I could hear my guitar!  That seemed good!  But, it kinda went downhill from there:

There was no "fuzz" when I played through the test rig.  Hmmm.

Then, when I attempted to bias the build per Madbean specs, I got +3.3V to +0.37V at the extremes of the bias pot.  The readings were supposed to be negative.  Oh, oh ...

Finally, I noticed the Fuzz and Volume knobs have no effect on the guitar tone to the amp; it just sounds like a bypass signal.  Urggh!

Here are some voltage readings.  I reversed lead placement due to reverse polarity of the circuit.  Hopefully this is correct:

Voltage readings (red lead to ground):

9V in at board: -6.14V
Ground in at board: zero

Transistor Voltage readings (red lead to ground):

Q1 (2N527):

C: +0.31
B: +0.39
E: +0.29

Q2 (2N404A)

C: +0.22
B: +0.03
E: zero

I'm not sure what to do next.  It seems like such a simple circuit.  Any thoughts or suggestions would be appreciated!

Thanks, Bruce.

[dondougie]

Try connecting the green ground clip from your testing rig to the board ground (you can use pin 1 of the volume pot). That should give you signal.

[midwayfair]

I see a few problems.

Finally, I noticed the Fuzz and Volume knobs have no effect on the guitar tone to the amp; it just sounds like a bypass signal.  Urggh!

1) The volume pot is a voltage divider at the end of the circuit -- even if your signal isn't getting amplified, the volume pot should affect the signal level.

2) Check the schematic. The emitter of Q1 is connected directly to ground. How, then, can you have positive voltage on it?

3) You got as much as +3V when you adjusted the trimmer, so the trimmer is working and voltage is flowing to and probably through Q2. Check your battery disconnected from the circuit. You're only getting 6V at the board input. The circuit will still work at that voltage, but it's something that you should fix, and it's certainly not going to sound right at that voltage.

[brucer]

Thanks for your replies guys.  I really appreciate it.

Try connecting the green ground clip from your testing rig to the board ground (you can use pin 1 of the volume pot). That should give you signal.

This was a good tip!  I wondered where the ground connection was (for "all" my other 3 builds there was a ground wire coming off the board that I used).  My newbie optimism had me thinking "Oh well, it mustn't need one!"  Connecting the green ground clip as suggested gave me fuzz!  Unfortunately, it's broken/intermittent, splattery fuzz at all points on the fuzz dial.  No smooth, sustained fuzz at all.

I see a few problems.

Ooohhh yeah!  But thanks for helping!

1) The volume pot is a voltage divider at the end of the circuit -- even if your signal isn't getting amplified, the volume pot should affect the signal level.

It does now ... with the ground connection made as suggested, but it only approaches unity volume (guitar to amp with PCB disconnected) at the far right of the volume dial.  Maybe that's normal?

2) Check the schematic. The emitter of Q1 is connected directly to ground. How, then, can you have positive voltage on it?

This, I don't know.  Updated transistor voltages (with the ground connection made as suggested and red lead to ground at pin 1 of the volume pot):

Q1 (2N527):

C: +0.39
B: +0.55
E: +0.38

Q2 (2N404A)

C: +0.54
B: +0.1
E: zero

Maybe I'm doing something wrong?

3) You got as much as +3V when you adjusted the trimmer, so the trimmer is working and voltage is flowing to and probably through Q2. Check your battery disconnected from the circuit. You're only getting 6V at the board input. The circuit will still work at that voltage, but it's something that you should fix, and it's certainly not going to sound right at that voltage.

Good suggestions for troubleshooting.  My disconnected battery reads 9V.  It also reads -9V with the ground at either the board ground or pin 1 of the volume pot.  The -6.14V was recorded when I was using an empty solder pad beside C3 as ground ... at least I thought it was ground. 

Unfortunately (with the ground connection made as suggested and red lead to ground at pin 1 of the volume pot), my trimmer no longer seems to vary the voltage recorded at the test pad.  I'm now getting a constant reading of +0.38V, regardless of trim pot position.  This doesn't seem right.

So, I've got working pots and power, but not-so-good fuzz sounds and my transistor and trim/bias voltages don't make sense.  Can you suggest any more tests or fixes that might help me sort this out?  Again, I very much appreciate any help folks can offer!

[dondougie]

The voltages do look odd... have you tried turning the trim pot completely down?

R3 is in series with the trimpot and sets the minimal resistance for the bias pot, so with your Smallbear selected resistor of 9k1 you should get around 4,5V - 5V at the test point with the trim out of the circuit.

On another note, Q1C and Q2B should read the same because they are tied together, maybe you have damaged a trace? The components and the soldering all look ok on the pictures, but it doesn't hurt to check and reflow all solder joints.

[midwayfair]

Put your multimeter on its continuity setting (beep).

Check for continuity to ground on the [edit: emitter*] of Q1, and check that the base of Q2 is connected directly to the collector of Q1.

*I'm a derp.

[brucer]

Thanks for the suggestions guys.

I pulled the trim pot and placed a jumper between trim pot solder pads 2 and 3 to maintain continuity of the circuit.  I figured you'd be right, Andre, that the Small Bear resistors should give me the correct bias point anyway.

Jon, I've got continuity between the Q1C and Q2B, but no continuity to ground from Q1C.

Perhaps that's the culprit?

[midwayfair]

Jon, I've got continuity between the Q1C and Q2B, but no continuity to ground from Q1C.

Oh jeez, I'm sorry, I meant the emitter, not the collector.

[brucer]

No problem Jon.  Yes, I do have continuity to ground on the emitter of Q1.

I just tried jumpering all 3 solder pads of the removed trim pot.  I'm now seeing a bias voltage of 3.75 and hearing smoother fuzz.  Maybe there were problems with my Tayda trim pot?

I'll pick up a replacement trim pot over the next few days, swap it in and see if that cleans things up.

Thanks for all your advice and direction.