SWAH manual WAH only (solved)

[Anonymous]

Hi guys,

just built a SWAH and am having similar issues with an old post, the WAH works when turning the Bias pot but not automatically, well it seems to work but mildly, not full on quacking.

VA=8.3 VB=4.1 VC=5

my values above Barrys

Proper Q voltages for comparison (all pots full CCW)

          8.3        4.2     3.4

Q1: (7.27 – 3.71 – 3.23 – DSG)  Drain Source Gate not CBE

         5        1.4        2

Q2: (5.05 – 3.75 – 2.90 – CBE)

         5          1         1.3

Q3: (5.05 – 2.90 – 2.86 – CBE)

 

Not sure why my Q2-3 voltages are low, would this cause it to not work,?

Maybe TL074 fail like previous post?

Thanks in advance

Stu

[Billy]

Stu post all your IC voltages to see if there’s any indication of where the fault lies

Your Q2 E should be the same as Q3 B

Q2 B voltage is too low but the problem may precede it so IC voltages will hopefully point us to where it may be

[Anonymous]

I think your onto something!

IC1 all pins read 4.1V except pin 6&11 = 0 pin 4 =8.3V pin 14= 1.6V

IC2=1.3, 0.3, 4.1, 4.1, 5.3, 0, 5.3, 4.2, 4.1, 5.4, 8.2, 5.3, 4.1, 4.0,1.8, 1.3

I’m sure when I measured IC1 yesterday I was getting voltages that seem comparable to what I saw somewhere, but today theyre mainly 4.1v

Thanks Billy

[Anonymous]

I got my IC values around wrong chip read

IC2 all pins read 4.1V except pin 6&11 = 0 pin 4 =8.3V pin 14= 1.6V

IC1=1.3, 0.3, 4.1, 4.1, 5.3, 0, 5.3, 4.2, 4.1, 5.4, 8.2, 5.3, 4.1, 4.0,1.8, 1.3

[Billy]

Pin 14 at 1.6v is low can’t see R24 and C8 left joint looks like it could be dry

Reflow all your components around IC2D

[Anonymous]

I replaced IC2 and now get 3.5V on pin14 instead of 1.6 (Why are all other values 4.2?), I soldered Q2 and 3 in as the sockets I am using are old and now get Q2 B=3.5 Q2 E =3.2 and Q3E=2.8, these voltages jump around a bit so I thing theres something going on somewhere. I still don’t get any auto wah though, perhaps what damaged IC2 also damaged IC1??

[Billy]

The 4.2v on the other pins is VB half your power supply as it should be

You’ll see all the VB points on the schematic where you’d expect half of the power supply voltage give or take, that tells us your voltage divider R29 and 30 is working and providing the correct bias voltage to IC2B

Did you reflow the solder joints on C8

I wouldn’t change or alter anything unless it’s verified as faulty

[Anonymous]

Yeah Billy I have reflowed all solder joints. Do you have a list of expected voltages on all pins of IC’s I saw one where you filled in Feedbacks incorrect values, a friend said if the wah works with the bias pot then the LM13700 is ok. the issue must be around Q2-3 . Is 3.5V good for pin 14 on IC2?

Getting beyond my skill set to get this going, have spent so much time on it, I think I might get a new kit and start again but it feels like I’m close!!!

Thanks soo much for your help!!

Stu

[Billy]

Some info here Stu

https://guitarpcb.webs.com/apps/forums/topics/show/13409170

http://revolutiondeux.blogspot.com/2012/07/mad-professor-snow-white-auto-wah.html?m=1

I’m having to work for the first time in 6 months today so no sitting around on my phone doing nothing today for me haha

If the problems not prior to  Q2 and 3 I’d check all components around the transistors bear in mind bias will alter things turn your bias pot all the way down for the least amount of resistance going into IC1 then very gradually increase it pins 1 and 16 of IC1 look low but should now be better so I’d try adjusting bias to see if it improves the wah’ing

Unfortunately I don’t have the voltages for IC1

You’ll get it going just takes a bit of time it’s basically a logical process of elimination where did you get your kit

 

[Anonymous]

Sorry to here you have to go back to work!

I get my kits from Pedalpartsaustralia as I’m in outer Sydney, this is my 7th pedal so far and the only one I’ve had dramas with. Sometimes Pin 14 reads VB sometime VA,checked all component around it to no avail . I just ordered another kit and will  try again!

Thanks for your help

[Billy]

I wouldn’t give up yet Stu

If it works moving the bias control it’s a case of finding out if there is enough voltage going into pins 1 and 16 looking at your Q2 and 3 voltages in comparison to  the working voltages yours are low specifically Q2 B Q3 B,E

So it’s trying to find out why check what voltage you get both sides of D3 and update us with your transistor voltages

[Anonymous]

You solved it Billy!, D3 was in the wrong way, I assumed the green band on the diode was the cathode so matched the silk screen, swapped it around and away we go!!,  the board is so butchered now I have another one coming tomorrow, won’t be making that mistake again!

Thanks so much

Stu

[Billy]

Ah was it a D9 series Russian diode? they have the band on the anode side

If that wasn’t clear I’d perhaps mention it to the vendor they may not know

Just a shame we couldn’t see the damn thing we’d have got there sooner, but it’s all a good learning experience

We’re always harping on about nice clear pics so we can see everything clearly I suppose this vindicates it but I could see the end didn’t see a band so thought it’s correctly orientated

I actually thought when I realised it was germanium the glass was cracked

[Anonymous]

Yeah it was obscured by C8, if I was smarter I would of known something wasn’t right! Very steep learning curve. Such a cool sounding pedal! thanks again

Stu

[Anonymous]

Ive just updated my post about this issue as well. Ive done the rebuild of the SWAH and it works as expected. It really does sound fabulous so its all worth it.

The issue with my original build was the reversed  Russian diode i got from the reseller. That wasnt picked up from my photos here either, but its such an odd case that im not surprised no one thought of it. Ill let the reseller know, they may not even be aware of the issue.

[Billy]

That’s what it’s all about Stu learning

I’ve been building for some time and still learn

I would say to the vendor he may offer something it doesn’t actually say on the listing

As Feedback says it’s not a well known fact that most but not all Russian diodes have the band on the anode + side

you could see enough in your pic to see it matched the silkscreen but as I say when I realised it was a germanium is when I thought is it Russian?

We may have got there sooner if we’d seen the entire thing but it’s good to get in the habit of fault finding in a logical manner, find your fault then check what’s causing it rather than change anything, in your case a reverse biased diode causing less output and having a knock on effect, so it’s a case of finding the cause and checking everything around the fault to hopefully lead you to the problem, you immediately recognised that Q2 and 3 voltages were wrong so you’d then trace it back to see what’s going into them and so on, sounds easy but as you know isn’t always!

 

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