Small issue with my SWAH

[Jeremie]

Hello, fellow sound enthusiasts.

 

I’m coming to you because I think my SWAH doesn’t sound exactly as it should.

 

And I can’t figure out where this is coming from, apart from voltage measurements that aren’t quite consistent with the expected readings.

 

I find that Decay really doesn’t influence the WAH response very much.

 

Also, I have to set the sensitivity to the minimum to be able to trigger the WAH at the right attack.

 

Here are my voltage readings:

 

Power supply :9.5v

All pot at minimum (full ccw)

<hr />

Q1: 7.40 / 4.90 / 3.35

Q2: 4.97 / 1.46 / 1.07

Q3: 4.97 / 1.07 / 1.30

<hr />

IC1:

1. 1.31
2. 0
3. 3.69
4. 3.69
5. 4.88
6. 0
7. 4.88
8. 3.67
9. 3.70
10. 4.93
11. 7.41
12. 4.93
13. 3.69
14. 3.69
15. 0
16. 1.30

<hr />

Ic2:

1. 1.48
2. 3.69
3. 3.69
4. 7.41
5. 3.68
6. 3.69
7. 3.69
8. 3.57
9. 3.69
10. 3.69
11. 0
12. 3.69
13. 3.69
14. 3.69

<hr />

Ic3:4.97/0/7.40

If i have forgot some detail, i will give you all.

Thank you in advance for any help you can give me. Have a nice evening!

[Playsforfun]

I have to question D3 placement. It looks backwards to me. If you do change it, I would recommend using sockets if you have them, in order to keep the GE diode from the extreme heat of soldering. From what I can see your values are ok for resistors and caps. I can’t double check Q2 or 3 but your voltages seem off. Posting photos of the soldered side may help. I’ll be able to check voltages on mine shortly and will post.

[Barry]

Regarding D3 if that is a Russian diode then the Cathode of the diode is opposite of the stripe.

I would mention that this circuit really likes D3 with a forward voltage of about .35 to .38

[Jeremie]

Good evening,

Thank you both for your answers.

Concerning D3 , I think I have it well established, because I had already read a message from Barry answering this question.

The cathode (opposite side to the two lines) corresponds to the negative of the diode on the printed circuit.

 

I’m sending you a photo of the solder side of the PCB.

 

As for the operation of the diode itself, it seems to be fulfilling its role.

After a continuity test, the current flows from – to + with a reading of 270 ohms, but not in the other direction.

 

Could the problem be due to a chip or transistor fault?

 

Good evening to you, musically 

[Barry]

The one you are using should be in the correct range:

 

For the most accurate reading, it’s best to desolder one leg of the diode from the PCB and test it again. This ensures other resistors or capacitors aren’t affecting the measurement.

If it were me I would socket D3 and then get a few different diodes after checking datasheets, then measure the forward voltage before installing it if you are not satisfied with the one you have.

[Jeremie]

Sorry i’ve forgot to say that i had disconected D3 and all work fine for her.

[brd]

There is a small pass through next to R23, that is suspiciously close to pin 1 of IC2. Check your work there to see if you may have bridged that by accident.

[Barry]

Here are some Proper Q voltages for comparison (all pots full CCW) based on a 9.3v supply reading.

It does not need to be exact but should be relatively close. Q2 and Q3 are suspect.
Q1: (7.27 – 3.71 – 3.23 – DSG) <- Drain Source Gate not CBE Q2: (5.05 – 3.75 – 2.90 – CBE) Q3: (5.05 – 2.90 – 2.86 – CBE)

[Barry]

Since you tested the forward voltage of the Germanium outside of the circuit and got the correct reading that is not it.

I would also say that this circuit it is not going to behave like a Distortion or Drive where every setting is going to offer you something. It is designed to find the setting that works with your guitar. Many people have discussed this regarding the commercial pedal over the last 15 years.

One thread of many:

https://www.thegearpage.net/board/index.php?threads/help-with-snow-white-auto-wah.1667948/

[Jeremie]

Hi, guys,

 

The photos may be misleading because of the blur, the angle and the lighting.

But I don’t have a false contact since R23.

 

Okay Barry, I’ll try to dig on the transistor side, and I’ll let you know if I find the problem.

Or I’ll call you again for help if I do ^^

 

Anyway, thanks again

To be c ontinued!

[Playsforfun]

I found these pages from an older build doc that may help make things clearer.

[Jeremie]

So great !

Thanks man!

[Barry]

FYI: Regarding the R18 “mod” you already have a 47k resistor installed.

I did not want you to think that was an option here.

[Jeremie]

<p style=”padding-left: 40px;”>Hi Barry,</p>
 my R18 is 22k in relation with the doc of the SWAH.

You think that i must replace it with a 47k?

 

[Barry]

Unfortunately, this is confusing because the VERY OLD schematic with Mods posted above, R18 is the same as your CURRENT build schematic location of R17.

Look at the schematic location of the Sensitivity Pot and you will see.

OLD

 

CURRENT

 

You currently have 47k installed in that location, do not change anything

[Jeremie]

Hi

Thanks Barry for thé précisions.

 

I thought so, but with my limited knowledge of electronics, I haven’t managed to solve the problem.

 

I have a question, at what voltage should be va, vb, and vc.

I have the impression that the whole circuit may be overcharged.

 

Here are my voltages:

Va – 7.4v

Vb – 3.7v

Vc – 4.97v

 

What do you think?

 

Have a nice evening

[Barry]

I do not have one to test as I sold it a long while ago.

Maybe this thread will help?

SWAH v5 Build Notes & Mods

In particular he discusses this modification:

The BOM calls for 1N914/1N4148 silicon (@ ~0.7v Fvd) at D1 & D2 and changed them first to 1N34As and the overall effect went to hell. It seemed to result in an extremely minimalist auto-wah effect and the ‘quack’ was definitely less defined. So I went in the other direction with regard to the Fvd of D1 & D2 and socketed a pair of red LEDs @ 1.88v Fvd (matched). I’m convinced that using LEDs for D1 & D2 is a superior choice as they seem to open up amount the ‘quack’. Thinking I was fooling myself, I swapped back & forth a few times between the 1N914/1N4148s and the red LEDs at D1 & D2 to be sure. I definitely like the red LEDs in D1 & D2 better than 1N914/1N4148s. The difference is subtle – but noticeable.

[Author]

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