"King Tut" Fuzz Debugging

[Vitus]

Dear people of DIYstompboxes.com! This is my second attempt at a DIY-pedal. While the first one (a Mastodon-fuzz) worked like a charme, this one is starting to drive me crazy. Unfortunately I'm still a total noob in terms of electronics (albeit willing to learn). I don't want to waste anyones time but any help on this one would be greatly appreciated!


"King Fuzz" (Pharoh-clone // modded Big Muff Pi)
Project website: http://www.rullywow.com/product/king-tut-fuzz-pharoh-clone-pcb/
Build doc: http://www.rullywow.com/build_docs/King%20Tut%20Build%20Doc%20v1.1.pdf

What I changed for my build:
- replaced all the 47nF caps (C4, C6, C7, C9, C12) with 470nF ones - as per recommendation from this blog entry http://fuzzypedals.blogspot.de/2014/12/pharaoh-fuzz-king-tut.html
- socketed all four transistors (Q1 - Q4)
- socketed diodes D1, D2, D5, D6, D7
- added a GuitarPCB 3PDT-wiring board for the foot-switch

Polarity: it's negative ground (uses a center negative DC adaptor)

What works:
- True Bypass works fine
- foot switch seems to work
- Power LED used to work fine until I unsoldered it and put it in again because I initially used the wrong terminals (LED on when in Bypass-mode), I may have damaged the 3PDT-PCB because now the LED isn't working anymore
- Volume-poti works
- Tone-poti seems to work
- High-poti seems to work
- Hi/Lo-switch seems to work / does something to the tone
- diode-switch (SI/OFF/GE) does something to the tone although not very much and OFF/GE seem more or less the same


What doesn't work:
- no fuzz at all
- fuzz-poti acts like a volume-poti (changes volume up/down) but by turning it up, it also adds a lot of noise (almost like white noise) to the audio signal but without changing the incoming audio signal istself (nothing like fuzz or distortion) - just adding a layer of noise, almost like a weak radio signal
- audio-signal is going in and out of the board but is weaker and thinner then when it's bypassed
- Power-LED (see above)

What I already tried in terms of trouble-shooting:
- triple-checked the component-list against what I put on the PCB
- triple checked the polarity of the polarized caps, diodes and transistors
- checked and reflowed most solder joints including the wiring
- checked the voltages before R7, R12, R18, R23 and R22 (all read ~8,75V)
- wiggled the socketed elements but everything seems to have contact
- used an exacto knife to make sure that there aren't any unwanted bridges between the solder spots
replaced C7 because I wasn't sure if I damaged it when soldering the wires from the potis
- build an audio probe and probed around a bit but I'm not really sure what to look for

- ordered some other Germanium-diodes 1N34A since the ones I have now look unlike the ones I see pictured elsewhere and the handwritten label on them read 1N34 (without the A), still I don't think that's the root of my problem because switching to SI or Off doesn't give me any fuzz either

I think this is as far as I can get on my own. This is why I now turn to you...





(Sorry for all that cat hair there...)

Some DMM readings:

---------
Diodes:
---------

1N914
D1: A=0 K=719
D2: A=0 K=719

1N4001 (<- are the differences between these three tolerable?)
D3: A=0 K=673
D4: A=0 K=691
D8: A=0 K=680

1N34A (or 1N34, not sure)
D5: A=0 K=246
D6: A=0 K=245
D7: A=0 K=246

I guess these readings are indicating that the diodes are all good? Is that correct?

---------------
Transistors:
---------------

When pulled out and checked for functionality via the diode-setting, all came out good I think:

(all are NPN)
Q1: C=874 B=0 E=887
Q2: C=898 B=0 E=900
Q3: C=893 B=0 E=900
Q4: C=892 B=0 E=890

These is what I get when the pedal is hooked up with the 9V power supply and I probe the transistors for voltage:

MPSA18
Q1: C=5.44 B=0.92 E=0.32

2n5089
Q2: C=4.01 B=0.65 E=0.05
Q3: C=3.96 B=0.65 E=0.05
Q4: C=4.72 B=1.48 E=0.89 (<- are these ok?)

-------------
Resistors:
-------------

I actually checked each resistor before I soldered it on the PCB and everyone read like it should (or very close to it)

----------------
Potentiometers:
----------------

checked all four before soldering them, all were working fine (and still are, as far as I can hear)

[Maroslav]

On project website one diode is backward

[Vitus]

Thank you for replying, Maroslav! I actually noticed that (and wondered about it) too but that's not what's causing my problem since I have soldered this diode like it is supposed to be (you can see it on the pictures at the end of the initial posting).

[Gus]

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

Looks like a BMP type circuit.

[Vitus]

Hey Gus, does "BMP type circuit" stand for "Big Muff Pi"? Then yes, afaik the King Tut/Pharaoh is a modded Big Muff. At least that's what I read, wouldn't really know it from looking at the schematics.

[Vitus]

Just checked all the diodes with my DMM:

- edited the results into first post for better readability -

[duck_arse]

vitus - yr transistor volts look about right for some sorta peeps out the end.

what did you do with the audio probe, and what were your results? usually, you apply signal to the input of a stage, and monitor the output of that stage, to see if you get more out than in. if the first stage works, for inst, listen at the out of the second stage, and so on down the line. you can also start at the last stage, and listen at its out, and then work forward (backwards?) to the input.

at some point, if you lose all yr signal, you know you're in the problem area.

[Vitus]

Hey duck_arse! Thanks so much for replying!

I think I get it so far. The incoming audio signal is getting amplified in the first two gain stages (that the correct term?) and stays like that at least until after C7 but somewhere between that and C10 it loses the amplification - and stays like that for the rest of the circuit. So now, we know that the problem is in that area. Not sure how to proceed from here, especially with the diode switching and stuff. The different switch-positions (SI/OFF/GE) make for a slightly different sound but neither gives me amplified/fuzzed output before and after C10.

SI -> very weak audio on D3, D4   
        amplified audio on D1, D2, D5 (Anode amplified, Kathode weaker), D6, D7 (Anode weaker, Kathode amplified)
       
OFF -> very weak audio on D3, D4, D5, D6, D7
          amplified audio on D1, D2

GE -> very weak audio on D5, D6, D7 
        amplified audio on D1, D2, D3 (Anode very weak, Kathode slightly louder), D4 (Anode weaker, Kathode louder)

Q3: C= weak   B=louder   E=weak

C8: one leg is weak, the other loud
C9: both loud

[duck_arse]

well, vitus, you've pretty well mastered the audio probe. stages, yes.

generally speaking, you'll find 'weaker' at the base (to the left) and 'laouder/amplified/stronger' at the collector (to the right). below the centre line, in a circuit like this, the emitter is not so interesting to the probe.

try isolating Q3 collector and its diodes and resistor and all that carp from the outside the yellow box, the tone stack. does that get you big signal at the Q3 collector? if yes, your problem is probably the off board wiring, or wrong value in the tone section. if no, there is something wrong around the diode switch. (I can 'see' your images, but can't save them to local to 'see' them in all their megabyte resolution glory, so can't see much detail.)

over to you!

[Vitus]

Dear duck arse,

could you please clarify the following for me:

try isolating Q3 collector and its diodes and resistor and all that crap from the outside the yellow box, the tone stack.

By "isolating" you mean probing it with the audio probe? Negative wire clamped to the case for ground and positive wire tip for probing (the Q3 collector, the diodes and everyting that's located on right side of the yellow box)?

does that get you big signal at the Q3 collector? if yes, your problem is probably the off board wiring, or wrong value in the tone section. if no, there is something wrong around the diode switch. (I can 'see' your images, but can't save them to local to 'see' them in all their megabyte resolution glory, so can't see much detail.)

I did probe the Q3 yesterday and it came out like this:

Q3: C= weak audio (but still louder then when I probe the PCBs' output terminal or anything after it) 
      B= louder audio (clearly amplified)   
      E= weak audio (just like the collector, I'd say

If I understand you correctly this hints at a bad diode switch then, right?

Is it possible to position the DPDT (with center off) wrong on this PCB? One of the 6 terminals/holes was marked with a square which is indicating ground connection, I believe. Before soldering it in, I tried to look that up but came away assured that it doesn't matter - other then flipping around of which position does what - "GE-OFF-SI" or "SI-OFF-GE"). I'll probe the switches' lugs and the individual elements of the tone stack tomorrow and post the results here though (have to go to work now).

As for the pictures, I'll try to find a way to provide you with a way to make them downloadable for you - or could send them to you directly, if you wish.

Thank you very much for taking the time to help me!

[duck_arse]

dear? that's a new one.

isolate, cut them off. look on your circuit, see where the yellow box draws, on the right it cuts the lines to C10 and the 25k pot. those parts connect to the Q3 collector, I want you to isolate the tone stack (C10//25k and to the right of) from the Q3. I'm not sure how the board wires these together, so I can't say what to lift or seperate. see if Q3 C is then louder than Q3 B, as it should be.

so's you knows, this type tone stack has an insertion loss, it eats signal to make it work. that's why Q4 is there, it makes up the loss, and also presents a constant, know load to the tone section output. this means it isolates the tone section from interacting with unknown loadings that might follow, another pedal or amp input.

hmmm, I posted "carp", yet you quoted "crap".
you can also test the makeup stage, inject your test sig into that Q base, and it should be louder at the C again. switches do what you tell them, they don't come with assigned ground. the layouting person probably used an IC package, which would indicate pin 1.

if the switch has only 6 pins, being dpdt-centre off, it surely would take some effort to insert it to the board wrong. it is board mount, yes?

[Vitus]

Mighty duck arse,

thank you for the reply. Unfortunately, I still haven't had the time to do further debugging. I think it'll have to wait until the weekend.

dear? that's a new one.

isolate, cut them off. look on your circuit, see where the yellow box draws, on the right it cuts the lines to C10 and the 25k pot. those parts connect to the Q3 collector, I want you to isolate the tone stack (C10//25k and to the right of) from the Q3. I'm not sure how the board wires these together, so I can't say what to lift or seperate. see if Q3 C is then louder than Q3 B, as it should be.

so's you knows, this type tone stack has an insertion loss, it eats signal to make it work. that's why Q4 is there, it makes up the loss, and also presents a constant, know load to the tone section output. this means it isolates the tone section from interacting with unknown loadings that might follow, another pedal or amp input.

I think I understand most of what you said there, thanks for the clarification.

hmmm, I posted "carp", yet you quoted "crap".
you can also test the makeup stage, inject your test sig into that Q base, and it should be louder at the C again. switches do what you tell them, they don't come with assigned ground. the layouting person probably used an IC package, which would indicate pin 1.

I took the liberty to edit that before quoting, yes... I was pondering wether or not you were actually referencing local wildlife but ultimately decided that there's a much higher probability for you meaning to say "crap" - as that is what I myself keep thinking when fiddling around with this non-working pedal build... After further meditation I agree that there is indeed something fishy about it though.

if the switch has only 6 pins, being dpdt-centre off, it surely would take some effort to insert it to the board wrong. it is board mount, yes?

It is, yes. Suspected something was wrong, when I had to drill the extra holes. Kidding...

[duck_arse]

Mighty duck arse,

can I quote you on this?