September 25, 2020 at 9:59 pm #14932
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
StuSeptember 26, 2020 at 10:02 am #14936
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 beSeptember 26, 2020 at 6:34 pm #14942
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 BillySeptember 26, 2020 at 7:31 pm #14945
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, 1.3September 27, 2020 at 9:30 am #14958
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 IC2DSeptember 27, 2020 at 11:40 pm #14978
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??September 28, 2020 at 3:15 am #14980
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 faultySeptember 29, 2020 at 12:39 am #15003
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!!
StuSeptember 29, 2020 at 4:41 am #15006
Some info here Stu
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 kitSeptember 29, 2020 at 10:50 pm #15020
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 helpSeptember 30, 2020 at 4:01 am #15021
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 voltagesOctober 1, 2020 at 2:02 am #15032
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
StuOctober 1, 2020 at 3:11 am #15034
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 crackedOctober 1, 2020 at 10:36 pm #15040
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
StuOctober 2, 2020 at 12:10 am #15042feedbackParticipant
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.October 2, 2020 at 8:55 am #15046
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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