Vintage Pow troubleshoot

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Hi everyone!

So I started putting together my first overdrive kit this weekend, a Vintage Pow by Fuzzdog, basically a Coloursound Overdriver clone with a volume knob.
http://shop.pedalparts.co.uk/Vintage_Po ... 48353.aspx

I checked it briefly before wiring up the footswitch to make sure that the pcb worked and noise came out so I didn't pay much attention and just wired it up.

However once I finished it up and plugged it in I realised that something was wrong.
Basically I have to turn the volume at least around to 3 o'clock to get anywhere near unity level and I really have to dime both drive and volume knobs to get somewhere near an overdriven sound. And even then the sound that comes out really seems wrong, kind of like a broken speaker but not in a nice way, with little sustain and a sound that quickly disappears in a kind of farting noise... The treble and bass knobs on the other hand seem to work fine.

Any ideas what might be wrong? A resistor mistake? A transistor or diode that might have overheated during soldering?
I really struggled recognising the different resistor values, so that's probably what I would go over first, but if any of you more experienced builders can tell what this noise means that would be a great help.

Here are a few pics of the beast. All suggestions are welcome!

[crochambeau]

Sounds like a case of improper bias to me. If you have a multimeter measuring the voltages at the transistors would be the best test.

Have you improved regarding identifying resistors? I ask because the very next thing I would do in this situation is confirm resistor values ~ based on the base leg circuit that tested off...

..if you do not have a multimeter, you'll just have to double check them all. It's educational.

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Thanks for the reply!
I haven't gotten around checking the resistors yet. I will probably do that this weekend.

I came across two big problems when trying to identify the resistors in the first place. First distinguishing blue from purple, and red from orange (and brown ) wasn't always very obvious for me... second when checking the chart when entering the color code, when the colour lines are symmetrical drawn how does ones know in which direction to read them?

I apologise in advance for the novice questions... but when you say a case of improper bias, do you mean improper bias of the transistor? What does that mean, how can I check if that is indeed the case, how could that have happened (if I used the transistors that came in the kit - BC109's) and how can I fix it?

[crochambeau]

*Typically* values are going to fall in certain color combinations. Example: 47. You're usually going to see 4K7 instead of 5K, or 47uf instead of 50. 4 & 7 are yellow purple, you will run across yellow purple about a hundred (or more) times more often than you run across yellow blue.

Here's a common value chart:



Regarding which way is start, it can be difficult, getting familiar with the color patterns work to key it into place quicker. The last values are usually tolerance (which introduces a few colors into the mix that are never first, like silver & gold) and sometimes temperature compensation. Usually those latter bands are offset a bit, but it can get really hard to tell.

Regarding bias of your transistors, resistors are the DC path to set that, so I'm guessing you've got some values swapped.

My favorite thing to do in biasing resistors on unknown or unproven designs is to watch the pass through on an oscope while I sweep the bias around - but you probably have a road map (which resistors go where), I'd use it.

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Thanks, things are starting to get a little more clear.

Here is the schematic



The gain seems to be the problem. Am I correct in saying that the Q3 transistor (edit: I meant Q1, duh ) is most probably the one that is improperly biased, and the resistor in cause is most probably R2 ?

I had a doubt on that particular resistor when mounting the pcb. FIY R2 is the resistor located at the very top in the center of the PCB. It reads brown/brown/black/blue, which doesn't exist. I had assumed, after going through all my resistors several times, that it must be blue/grey/black/brown/brown which would be 6K8 (which is the correct value for R2), and the grey either doesn't show on the light blue background or wasn't correctly painted on.

Do you think R2 is the faulty resistor or should I be looking elsewhere?

[crochambeau]

Q1 does the initial gain, 6K8 does sound reasonable in that position.

Look at the gain control and contemplate the following points:

1) The actual gain control is wired up as a variable resistor, and *FULL GAIN* amounts to a short circuit across the pot.
2) Since there is a capacitor in line (C2) this is an AC bypass section only, so it should not fuck with DC bias. Please note that the orientation of the electrolytic capacitor is important, you want the "+" on the higher DC side (Q1 emitter, as drawn) otherwise they tend to leak which could tank bias as well.

Can you confirm that those conditions are met?

I'm assuming you haven't got a multimeter, great tool purchase if you're going to stick with it..

R3 is completing a localized feedback path that will also sway bias on the first stage. Spreading out R1, 6 & 15 also play important roles.

Final stage is R11 & 12, that voltage ladder is going to wedge the base at a certain point, whereas the rest of the transistors are more or less propping each other up.

Hope my rambling does not muddy the waters at all.

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Sorry, yes, I meant Q1...
6K8 is what is the correct value of the resistor, I am not sure the one I installed is actually 6K8...

1) The gain pot is wired up like the other pots, so I am going to say yes, but I am not quite sure what you mean.
2) Yes I can confirm that the capacitor C2 is correctly oriented

No, no multimeter at the moment... depending on how this goes I might consider investing in a cheap one...
I started to lay out a diagram with the correct resistor values and check them one by one... But again the one I really had (have) a doubt is R2. Do you think the resistor I installed is 6K8 acoording to the colours you see in the picture?

Thanks again in any case for taking the time help me out.

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Ok, I just checked all the colour codes of my resistors.
The only one where I have a doubt is R2 because the grey is completely invisible on the light blue background and R1 that could either be 120K (correct value) or 13K depending on the which way you read it.

[crochambeau]

1) The gain pot is wired up like the other pots, so I am going to say yes, but I am not quite sure what you mean.

The gain pot is electrically different than the other pots, two of those legs must be shorted together.

[crochambeau]

Do you think the resistor I installed is 6K8 acoording to the colours you see in the picture?

Thanks again in any case for taking the time help me out.

I can't tell which one is R2, but it looks like R1 is unpopulated?

That could be your problem as well.

edit ~ looks like an R4, different build I guess.

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Yep, R4 is unpopulated... for the power boost build (different build same pcb)
R2 is in the very middle at the very top, just above the 6 holes. Admittedly the colours are not clear at all in my picture, but from left to right they read blue / blank (I am supposing grey) / black / brown / brown.

Could the problem be coming from something else? Like solderings connecting or overheating diodes or transistors?

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1) The gain pot is wired up like the other pots, so I am going to say yes, but I am not quite sure what you mean.

The gain pot is electrically different than the other pots, two of those legs must be shorted together.

No, I definitely did not connect two of the gain pot's legs together, and there is no such reference to this in the fuzzdog's instructions... could this be the problem? Which two legs should I try connecting?

edit : FYI the gain pot is a 5KC, if that might mean that the legs are shorted internally... ?

[crochambeau]

I wouldn't worry about overheated components yet, you're still passing signal.

I honestly can't tell what that resistor is, though a 6, an 8, and two zeros does fill the value out the color transfer on the pic is enough to maintain a second guess position. So: I cannot say. When in doubt, measure.

If you're looking at the back of the gain pot with the legs pointing down, you'd want to connect the center and right hand leg together (alligator clip to test), C is probably just taper, which is a reverse log if I remember correctly. In this case it's because gain is a function is decreasing resistance feeding an active, and not increasing the resistance between your signal line and ground.

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just tested, connecting the two legs of the gain pot simply maxes out the gain...
wil see about the multimeter tomorrow... thanks again for all the help

[crochambeau]

Sounds like I might have gotten the legs opposite then, that's a head cold for ya.

If you're getting something like proper full gain I wouldn't concern yourself with that section.