My Newest Fuzz Project (New Post/Schematic)

[italianguy63]

OK--

Starting a new thread to clean things up.  Got this board done-- all seems to work alright, but with some minor issues.  Looking for help for improvements; and they are probably simple.  

Please review the schematic:  (NOTE, don't be freaked out by extra ground pads, or the "duplicate" bias resistors, or the error in it).  They are drawn so I can have the optional pads on the PCB's).



Anyway-- I have 2 minor issues:

1.  The Treble Bleed control is generally ineffective.  I can't figure it out.  I have tried different values in C9 with no real change (1nF, 100nF, and 500pF).  What gives?

2.  The Sag control is "scratchy" when turning it to full power (i.e. no resistance).  As the wiper goes to the end... I get some noise.  I thought it might be the pot, but tried another with the same effect.  Is there a fix for this too?

Thanks in advance--
MC

[PRR]

Aren't the transistors drawn backward? The _E_ of Q1 goes to ground.

Backward connection may work, but isn't right.

The Bleed pot can't do anything if Level is turned way up.

The Fuzz pot *will* scratch if turned rapidly. C10 has to charge to the higher DC level some-how.

[italianguy63]

Duh.  Yes.  It is PNP.  Drawn wrong.

I have had the level pot up.....  maybe my issue.

Is there a fix?  Like a small cap on the pot?  Or just "crackle OK?"

Thanks Paul.

MC

[PRR]

[italianguy63]

Thanks Paul-- I'll play with that...  Backfilling, working on a Rev B. now.....

[italianguy63]

ATTENTION ATTENTION ATTENTION!

I apologize profusely to my "test builders."

The diagram on the PC board is backwards for Q1 and Q2!!!  Emitter and Collector are swapped.

Seems I didn't have a batch of bad trannies after all.  (How many hours spent?!)



Sorry guys.

MC

[italianguy63]

i.e. put the trannies in the opposite direction of the diagram, and you are good to go!  MC

[italianguy63]

Here is the corrected schematic so I don't lead anyone astray.  The trannies are fixed, and I fixed the minor trace issue on the parallel bias resistors (you choose one path or the other).  By the way the 3K9 fixed with the 5K variable as a "Sunface" bias control works awesomely.  MC

P.S.  I am also having some better luck with substituting Q3.  It does not always want to engage.  I think I may have had a bad batch of 2N3906's.  

[italianguy63]

(removed)-- Obviously, a part of my brain is defective.  Probably that AC/DC concert in 1981.

[italianguy63]

Some other late thoughts--

1.  The treble bleed is generally ineffective still.  Any suggestions for changes in component values welcome.  I am considering moving it to R3 location as I have seen others do.
2.  Some scratchyness (crackle OK!) on the SAG pot.  I added a .1uF across it, helps some.
3.  Still having random problems with Q3 engaging.  Seems maybe faulty components.  Sometimes replacing/reseating IC1 fixes.  Sometimes replacing Q3 fixes it.  And, seems using 2N4403 for Q3 works.  Perplexed.

[haveyouseenhim]

Man. I still haven't had a chance to finish this one. I'll let you know if I come across anything when I get my iron unpacked.

[italianguy63]

Thanks Mike-- I am VERY happy with it.  Just minor tweeks here.  Hoping somebody will chime in and help with them from their experience.

I'm not an EE.  On the bleed, wondering if C6 is filtering more than C9 already making it not work.  I dunno.  The transistor switching thing is being a pain.  I have zero idea why it works ONLY 90% of the time.  My only thought is I have gotten faulty components due to purchaing from my ultra-discount source (i.e. you get what you pay for?).  And, the pot thing is understandable, just an annoyance, and wondering if there is a "bandaid."

[Govmnt_Lacky]

@Mark

Why the 2 voltage bias controls for Q2 Collector? Are you putting one as an internal trimmer and the other as an external pot?

[italianguy63]

Yeah Greg.  You use one OR the other.  either the on-board trimmer OR the user-controlled pot plus the 3K9. 

I built a bunch of bells and whistles into the basic Fuzz circuit, so, you can build a 2 knob, 3 knob, 4 knob or 5 knob depending on your mood.   

[Govmnt_Lacky]

Got it! 

I just saw that they were parallel resistances and was wondering if dialing in the internal trimmer would be a pain in order to get a good usable range from the external pot.

[italianguy63]

Cheaper with the fixed value! $0.01. 

Found the 3K9 work pretty darn well.  It gives a nice sweet spot assuming Q1 and Q2 are within desired specs, and not too high or low Hfe.

[italianguy63]

Bump, and update.

After a bunch of trial and error, it seems I have the Q3 problem licked.  I am not a EE, so I can't give the technical reason for the problem or the solution.  Just observations.  It seems the combined large size of C2/C3/C4, in an effort to keep noise down was keeping Q3 from engaging or the charge pump starting.  Most spec. sheets show C2 at 10uF.  Anyway, by reducing the size of these-- I am now using 47uF instead of 100uF, it seems this has been corrected.

The crackle on the sag pot has been fixed mostly by adding a .01uF or .1uF across the legs.  Less scratchy anyway.  This is a Crackle OK scenario.

But, I still can't figure out the treble bleed.  I would appreciate if someone could take a look.  I understand it is a low pass filter.  But, I can't seem to make it do much regardless of the setting of the Bleed pot, the Level pot or the guitar volume pots.  Do I need to change the value of C6, C9 or the Bleed pot?

MC