Ustomp.com Unicord (Univox) Super-Fuzz clone...problem.

[Jeast78]

1.What does it do, not do, and sound like?  Ok, using the pedal with 'non-scooped' selected on the tone switch, it works ok...the issue I'm having here is the scooped tone works but with very, very low volume and the volume (balance) pot has no affect, it just introduces more noise, but works as it should when switched the other way. 

I'm getting good octave content, but there is a very strange clipping characteristic at high-gain...very smooth up to about 12 o'clock, but starts to break-up beyond it.  Sound kind of glitchy.

Used combo of NTE123EP and 2SC828 trannys, germanium diodes (1N34), alu film resistors; electrolytic, ceramic and poly film caps.

This is my second attempt...my first attempt worked perfectly (but still with strange clipping), but I was unhappy with my soldering and decided to start again.  Wishing I hadn't bothered!

2.Name of the circuit = Unicord Super-fuzz

3.Source of the circuit (URL of schematic or project) = schematic - http://ustomp.com/files/superfuzzschem.gif, PCB layout - http://ustomp.com/files/superfuzzbuild.pdf

4.Any modifications to the circuit? No.

5.Any parts substitutions? 47k linear pots instead of 50k.

6.Positive ground to negative ground conversion? N

7.DC adapter voltage = 9.38v

Voltage at the circuit board end of the red battery lead = 9.35v
Voltage at the circuit board end of the black battery lead = 0v

Q1 - NTE123EP
C = 5.61
B = 0.69
E = 0.137

Q2 - 2SC828
C= 9.35
B= 5.58
E= 5.05

Q3 - 2SC828
C= 6.23
B= 3.38
E= 3.07

Q4 - NTE123EP
C= 2.06
B= 1.89
E= 1.31

Q5 - NTE123EP
C= 2.06
B= 1.89
E= 1.31

Q6 - NTE123EP
C= 3.83
B= 1.15
E= 0.55

[R.G.]

Does Ustomp offer any customer support for their products?

[Jeast78]

Does Ustomp offer any customer support for their products?

http://ustomp.com/ - it's not a company, it's just a website with a few PCB layouts and schematics.  Think it started of as a project but it hasn't been updated since 2007.

[Thecomedian]

it looks very particular about it's battery input. Dont see any AC adapter alternative in the gif.

are all those Transistors direct replacements for the ones in the file? All the ones are 2SCxxxxx in the actual device.

[Jeast78]

it looks very particular about it's battery input. Dont see any AC adapter alternative in the gif.

Hmmm...true.  I just added a DC socket into my build.  Could that be responsible for the glitchy clipping?

To be honest my main issue is that half the circuit appears not to be working!  From my very weak understanding of the schematic, the switch appears to introduce or remove this part of the circuit...



Which I assume is responsible for the scooped-mid EQ (just a guess, I really have no clue!).  Could a faulty component in this assembly be the cause of my issue?

[Mark Hammer]

If memory serves, there are some issues regarding the proper orientation of the 10uf electrolytic caps.  Depending on the schematic being used as a reference point, one or mroe of the caps might be oriented wrong.

[Jeast78]

If memory serves, there are some issues regarding the proper orientation of the 10uf electrolytic caps.  Depending on the schematic being used as a reference point, one or mroe of the caps might be oriented wrong.

Hi there, thanks for your reply - it's actually the second time I've built this using the exact same layout, first time it worked perfectly, so there are no issues with that.  I can only assume it's a faulty component, but I wouldn't know which component would be responsible for volume.  Any ideas?

[Mark Hammer]

Sometimes it's not the component but how you've soldered it.  I made myself a John Hollis Omnidrive a couple years ago and was underimpressed.  I was getting my castoff pedals ready for a musicians' swap this weekend, and learned that I had a couple of bad solder joints for one or two diodes.  Made a world of difference.  Still not my favourite pedal, but at least it now does what it is supposed to.

Check your solder joints.  Sometimes joints that look good aren't as good as they look.  I find it is often a good practice to scrape the sides of component leads so that they're shiny.  Lots of us end up with dull or tarnished leads on components, solder them in thinking that everything is fine because, after all, they're new, and the contact made isn't the quality it appears to be.  Shiny makes better contact.

[Thecomedian]

http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html

I was just reading this website today, maybe it's the fixed base bias on Q2 causing an issue, if the transistor isn't an exact replacement for the schematic transistor.

Fixed bias bases sound like they're very finicky.

Q2 - 2SC828
C= 9.35
B= 5.58
E= 5.05

um, yeah... the fixed bias transistor has Collector voltage sitting at max rail voltage. There's your problem.

Fig 11a and 11b shows a Common Emitter stage with fixed base-bias. This stage produces the maximum voltage amplification but it is very difficult to "set-up" because the value of the base resistor will either make the collector voltage nearly zero or full rail voltage. It is very difficult to get the collector to sit at mid rail.
If the base resistor is a high value, the collector will sit at rail voltage.  If the base resistor is a low value, the collector will sit a 0v.
If a transistor with a different gain is fitted, the collector voltage will change completely.

Anyone feel free to correct me if Im thinking of it wrong, although I believe that I've had this same issue in spice a lot, where huge Vc actually DEamplifies my signal..


transistors actually have variable capacitance and resistance based on volts/currents, but that's an ugly ball of wax to open up, just stick with trying to fit the right gain (or right approximate gain range) transistor in there.

Have you tried emulating a 50k pot on that base by adding a few K in resistors and seeing how it affects the voltages around Q2?

Disregard the above. It's because of your Q1. I couldnt find any info on a NTE123EP, but a NTE123AP shows minimum gains of ~20 or more, while the 2sc539 has a minimum gain of 70-250, depending on what site or datasheets you find. That will affect the volt/current.


Whats the gain situation for the transistor that did work in the previous one, the transistor that's in the current one? Have you tried replacing it?

[Jeast78]

Disregard the above. It's because of your Q1. I couldnt find any info on a NTE123EP, but a NTE123AP shows minimum gains of ~20 or more, while the 2sc539 has a minimum gain of 70-250, depending on what site or datasheets you find. That will affect the volt/current.


Whats the gain situation for the transistor that did work in the previous one, the transistor that's in the current one? Have you tried replacing it?

Yes, my apologies, I meant NTE123AP.  I've used exactly the same components in both builds, but for some reason the new one is exhibiting this issue.  I've listened to it again and in actual fact, the volume pot does increase the volume - from very, very, very quiet to just very, very quiet - and does not introduce more noise as I originally posted.  This only occurs when the tone switch is selected to include the portion of the circuit I posted above...surely this means it's a fault in that part of the circuit?

Does anyone know if that element of the circuit would act as a frequency filter?  Could a faulty cap cause the volume to drop or are they purely there to filter frequencies?

This is really starting to annoy me! 

[PRR]

> Could a faulty cap cause the volume to

Yes; and a tone-circuit is a frequency filter.

Replace the caps. Check values and joints. Check resistors also. While I hate shotgun diagnosis, 2R 2C is cheaper than getting annoyed.

[Jeast78]

Well, probably too predictably, it turned out to be nothing but a back-to-front switch connection! D'OH!!! 

Thanks for everyone's assistance, the pedal now works very nicely.  The over-clipping can be controlled by rolling back the guitar volume or edging off the 'expander', which I can live with.  It has very nice octave content too, so all-in-all, I'm very happy with my second (ever) build attempt!