Swah v4 – Static (cocked) wah sound, no sweep

[Steve]

I have seen that there are similar problems recently with Swah projects from Das Musikding.  I am having this problem too.  The pedal engages and I get a a decent sound, but there is no sweeping.  If I turn the trimmer, I can hear it sweep through frequencies, but there is no autowah sound.

One thing I will mention is that I did change the r17 to 47k and I put a 50kB pot in place of the trimmer.

As well, I have used one of my own 1n34a diodes as the one from Das Musikding shattered while trying to carefully bend the leads.

Here are the pin/transistor voltages

Q1
D : 7.25
S : 4.25
G : 3.32

Q2
C : 5.00
B : 1.228
E : 0.844

Q3
C : 5.00 
B : 0.843
E : 1.329
LM13700
1 : 1.32
2 : 0
3 :  3.62
4 :  3.62
5 : 4.83
6 :  0
7 :  4.83
8 :  3.62
9 :  3.62
10 :  4.83
11 :  7.23
12 :  4.83
13 :  3.62
14 :  3.62
15 :  0
16 :  1.32
TL074
1 : 3.62
2 : 3.62
3 : 3.62
4 : 7.24
5 : 3.611
6 : 3.62
7 : 3.62
8 : 3.403
9 : 3.62
10 : 3.62
11 : 0
12 : 3.62
13 : 3.62
14 : 1.526

As well, the voltage on D3 is 1.453 between it and R24 and 1.231 between it and Q2

The transistors are BC550B (not the 550C that is recommended)

I am sort of thinking there is something going on if all of a sudden there are many Swah projects from Das Musikding that are not working…

 

Let me know what info you need.  I am pretty good about component values and such, but could post board pictures if need be.

[Barry]

What is your power supply voltage to the project?

[Steve]

Supply voltage is 9.34 from a Boss power supply.

[Billy]

Post some nice clear images Steve your Q2 base voltage is too low it looks like it starts at pin 14 of the TL074

Did you check all your resistor values etc prior to installing them

[Steve]

If you think the problem starts at pin 14, to I’ll double check from there.  I usually check values, but might have installed some just based on how they were labeled from Das Musikding.  I’ll try to get some clear images later today.

[Billy]

You can garner some info from your voltages Steve

You’ve got 7.24 going into pin 4 (power) 0 on pin 11 (ground) and all other pins with the exception of pin 14 (output) are around 3.62 half the power supply VB as they should be so that tells us your power section is ok

The voltage divider R29 and 30 is working providing VB, you’re getting 5v on Q2 and 3 collector so we know the regulator part is supplying VC

D3 is working it’s got what’s coming out of pin 14 less a little voltage drop through R24 then as expected with a germanium drops around 0.3v unfortunately not enough signal going into Q2 base through Q3 to provide the current required at pins 1 and 16 of the LM13700

So it’s just a case of a process of elimination which will hopefully lead you to the cause – wrong value, cold solder joint, faulty component etc etc

[Steve]

View post on imgur.com

There are some pictures of the board.

[Steve]

Is there a certain set of components that I should be checking?  or everything other than the power supply ones?  I can reflow all the joints and I will check resistor codes.  Any other direction would be helpful.   And other points in the circuit that I should check the voltages for to narrow down where the problem might be coming from?

[Steve]

Alright…  no one spotted it, but it looks like I have a really dumb value in the R17 spot.  I know exactly what happened…. I had a bunch of 47k resistors I pulled out of my breadboard and hadn’t put away yet back into their spot… I assumed they were all 47k.  There must have been another value that I pulled off the breadboard as well.  Looks like I put a 330k in that spot.  Time to go desolder that and put the right value in!

[Steve]

Changed that back to the 47k that I wanted to put in there.  Seems like the sensitivity control needs to be near the max for my strat.  Wondering if I should put that resistor back down to the 22k that was stock?  This is R17 I am talking about.  Thoughts?

[Barry]

You could socket that section and test it.

Glad you got it sorted out.

[wilkie1]

When you changed R17 to 47K, you reduced the max gain of IC2A from 4.54 (100K/22K) to 2.13 (100K/47K).  That reduced the signal going to the OTA trigger circuit. Try the 22K value or less.

[Steve]

Thanks for the confirmation.  I’ll likely go down to the 22k value.  It triggers well enough with my Strat.  It work very well with my SG that is much hotter.

 

Thanks for the help, folks!

[Billy]

Glad it’s sorted Steve

Just finished work and was about to have a look

If possible could you post your working voltages for IC1 and 2 just so I can add it to my notes, your problem would have been obvious checking resistor bands but not from the voltage to pin 2 the voltage looked good with the feedback from pin 1 so interesting to note that

Cheers

[Steve]

Unfortunately I have changed that resistor to the stock 22k value.  I can give you readings now if you would like.

 

Something else that I noticed while play that I am not sure I have fully realised before.  When you pluck the string downwards, or especially when using your thumb on the down stroke, towards the pickup, the effect triggers much more than when you use your fingers to pluck upwards on the string.  This makes sense, in that the initial signal will be stronger if the string is being released closer to the pickup (down stroke) rather than being released on the upstroke when plucking.

I have a good sound now.  Thank guys.

[Billy]

Yes please Steve it was just to get voltage readings for a confirmed working unit

[Author]

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