Small issue with my SWAH

[brd]

Using the schematic and the readings that you have already supplied I think your Va, Vb, and Vc reading are fine.

Long explanation:

Va should be the same as your Q1 drain voltage, which was 7.4v. The expected value given by Barry was 7.27v. That is close enough.

Vb should be half of Va because of the voltage divider R29 and R30, which it is.

Vc should equal to your collector voltage on Q2 and Q3, which it is, and it is close enough to the expected voltage of 5.05v

What doesn’t look correct is your base voltage (middle pin) on Q2. Barry’s number is 3.75v you are at 1.48v.

Recheck that voltage measurement and be sure you have the Decay pot turned fully counter clockwise.

Also, Just using a little math, you have a diode D3 that should be dropping at least .25v so you should expect your voltage at pin 1 of IC2 to be at least .25v higher than your base voltage on Q2. Your readings are nearly identical, that doesn’t seem correct either.

Just to be sure, Are Q2 and Q3 BC550’s?

[Jeremie]

Hi brb,

I’m sorry guys , i ‘ve not seen the last message because it’s on seconde page… Oups!

Thanks you man for your research with my SWAH.

 

Yes all m’y pots are full at Counter-clockwise.

 

 

So for Q2 and Q3 , i think there are the good transistor.

I can read on both “ctbc 550 ls”.

 

 

Your mention is exacte , the pin 1 of IC2 send 1.48 v and i read on the base pin of Q2 1.45.

These measures seem too close in regard with your analysis.

 

 

Maybe for help i read this:

On R17, R18, R19, R20, R21, i measure 3.68V at both side, is it normal?

The 3.68v seems to come to ic2 pin 12 13 14 that i measure this tension at eatch.

Do you think than the problèm come upstream?

 

And on R23 , there is 3.68V and 1.48V

On R4 : 4.89v and ground

 

Thanks in advance

[Playsforfun]

I’ve built a few of these and each acted the same. I’ve been known to use cheap parts but I’m sure my circuits are working as intended and don’t expect that anything would have been different if I had made different choices. That said I found the one that I thought I still had and took voltages. They are posted below. Mine are very close to yours, Jeremie. I think you may have recorded IC2 pin 1 and pin 14 reversed. See mine. What I find about this circuit isn’t real different from what I read that I should find about this circuit or very different from what you have said. I find it hard to sync with at times. Like I can’t find a spot on the sensitivity. And when I do I have to play with more pick attack accuracy than I think I actually even have, to keep the wah behaving as I think it should. I think it can be more difficult with vintage spec single coils, which I like. Once I get going it’s ok but I have to say that I believe this swah requires more talent than I have frankly to operate at it’s full potential. That being revealed, I think that you have a working pedal. You have indeed said so. Maybe it’s not what you were expecting. I’m not saying that your playing stinks like mine, I’m just saying that maybe you just haven’t found the sync spot with your rig yet. The bias is probably better left alone. Don’t starve the circuit. Give it plenty of controlled pick attack and you should get some consistent quack. Let us know what you think.

[Author]

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