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basstard · Joined: 23 Oct 2010 Posts: 9

Hi all Gentlemen, hi Paul.

Some time ago I wrote you to ask for some suggestion on a wah I was about to build, don't know if you remember.
Anyway, I promised to write on the forums as soon as I'd finish it.
Faithful to that promise, here I am, pity that instead of writing just to show my build, I'm here to ask for help since I got problems with it!

I wanted to build a wah without using an ordinary, premade shell/mechanism. I was like: "the circuit is very simple, the hard part will be building a decent mechanism". Now I built my mechanism and used a gcb-95 replacement pcb, but I have problems with the electronics! Shame! Here's some pics:

During the build:

Inside:

Finished [well, almost Very Happy

]:

So, to cut it short, everything works ok except for a noise I can hear in a certain range of the pot sweep. It's really similar to a scratchy pot, and I mean REALLY, but then I went thru three different pots [all of them new and never used] and they all had the same issue. Changed the 4u7F cap, changed the two .22uF ones, changed the transistor near the pot,,, still nothing. I'll go on and try to change other stuff, in the meanwhile, any of the gurus having any idea?

Thanx a lot in advance!

Paul Marossy · Posted: Sun Oct 24, 2010 2:53 pm Post subject:

The only thing I can think of that could be causing that problem is that you have DC on your pot somehow, but theoritically that shouldn't happen.

What transistors did you use in this project? Maybe it might have something to do with that?
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basstard · Joined: 23 Oct 2010 Posts: 9

PCB is a stock Dunlop replacement one, an original GCB-95 item.

Transistors are stock MPS-A13 and MPS-A18. I changed one for a BC550 with no results.

I should be able to meter DC on the pot if there's some, right? Any suggestion? Just metering between ground and the legs of it should do I guess,,,

Thanx a lot.

Paul Marossy · Posted: Mon Oct 25, 2010 1:36 pm Post subject:

The other possibility is that the pot is wired wrong. I remember hearing similar symptoms from other people and it turned out that the pot was wired incorrectly. Double check that, maybe that is the problem.
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basstard · Joined: 23 Oct 2010 Posts: 9

About the DC, as I changed the 4u7F electrolytic, the other one is the 220uF, which I still have to change. Do you think a leaky cap in that position could let DC on the pot?
On another note, both the wiper and one leg of the pot have caps before them [0.22uF, both of which I already changed with no results] so the only leg that has no cap on it is the grounded one, do you thing DC could be "coming" from there?

About the pot wired incorrectly, I'll triple check that, but IIRC I got one leg wired to the pin that comes from one of the two 0.22uF caps and then that same leg to the true bypass switch; the wiper is wired to the other .22uF and the other gets ground from the PCB [the only ground the PCB gets since, as you can see from the pics, I removed the jacks from it. Also in this way I can avoid ground loops] and then goes to the output jack sleeve which is the main ground star for this circuit.
Any obvious error in this?

Appreciating your help a lot!

Paul Marossy · Posted: Mon Oct 25, 2010 2:05 pm Post subject:

Could possibly be a leaky cap, but I doubt that is the problem. My bet is on either the pot being miswired or a bad solder joint on your pot.
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basstard · Joined: 23 Oct 2010 Posts: 9

Changed the 220uF and nothing.

Checked the pot again and it seems wired ok [well it's late, I'm tired and just came back from the vet as the dog is sick - so I'll check again tomorrow after work, just to be sure].

I've re-done all of the pot solder joints on the pot various times in the last weeks and the problem stayed the same so I'm not for that.

On the other hand, I'm measuring a steady 1.68Vdc between the two pot legs [the one that goes to the output jack and the grounded one] but I can measure the same voltage across the jack that I use to connect the output of the wah to the pc soundcard,,, if I disconnect it from the pc there's no Vdc there,,, btw, I tried the wah also on a marshall amp and it did just the same so it ain't just a thing of the pc soundcard.

One good thing is that I now have an original crybaby to swap the pcb with,,, tomorrow I'll make some other test,,, will keep you posted.

basstard · Joined: 23 Oct 2010 Posts: 9

Either I must be dumb or I'm missing something,,, I just connected the other Crybaby I got from my guitarist and it does exactly the same thing!!

To this point I'm asking if this is normal behaviour on Crybabies,,,

Paul Marossy · Posted: Mon Oct 25, 2010 10:07 pm Post subject:

Don't have an answer for you on that one. Try it with another guitar maybe?
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basstard · Joined: 23 Oct 2010 Posts: 9

It does that even with the guitar unplugged!! Very Happy

I'll try it with some more serious amp when at the reharsal room next time,,,

Paul Marossy · Posted: Tue Oct 26, 2010 1:25 pm Post subject:

basstard · Joined: 23 Oct 2010 Posts: 9

Ok, placed a 2u2F cap before the output and the noise is gone!

Thanx for the heads up on dc on the pot! One never stops learning,,,

Paul Marossy · Posted: Tue Oct 26, 2010 9:01 pm Post subject:

Huh, that is odd. Glad you figured it out! Cool

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basstard · Joined: 23 Oct 2010 Posts: 9

Well not that odd if you think about it: the GCB-95 isn't decoupled from the output which is a pretty basic feature almost every pedal has,,,

Again, if you wouldn't have pointed dc out, I wouldn't have solved this so I owe you a beer nevertheless! Very Happy

[and as you're on the other side of the pond, the least I can do is craking one open for you! Very Happy

]

basstard · Joined: 23 Oct 2010 Posts: 9

Oh and BTW, if I can give back to the forums answering questions about this pedal I built [if anyone is ever interested!], here I am!