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Just buttoned up a Swah build and it doesn’t move. Adjusting the bias and resonance controls allows me to hear a range of wah tones, but the sensitivity and decay controls do little to nothing. Any ideas as to what to look for first?
Derrick
You will need to post clear photos. You need to verify every component again and reheat every solder joint to re-flow the solder. We have a printable resistor card on the Guides Page if that helps quicken the verification.
Based on personal experience, check orientation of D3 – diodes sold as 1N34s aren’t always marked the same way.
Chuck
I removed the diode and socketed it. Replaced it with a different diode and switched it’s orientation back in forth. No difference either way, in fact, it doesn’t matter if it is in at all…..there is no change. I get output when pedal is engaged, I can get “wah” sounds by turning the bias pot, and I can hear a difference when the Resonance control is turned. But the envelope never triggers and there is no effect when the Sensitivity or Decay controls are turned. Again, this behavior does not change even if I remove the 1n34 completly.
I’ll try and get some pics up here in a min.
Derrick
It’s very difficult to see your values and resistor colour bands Derrick
Even if you open the image in a new tab you can’t clearly see the colours or zoom in for a better look
I’m following this thread with interest as Derrick and I seem to have the exact same issue. Very little wah, way less than the name-brand versions. Some change with bias, a little brighter with resonance adjustment, and no appreciable change if/when the N34 diode is removed or flipped. If we can’t figure it out here I might start another thread, but didn’t want to derail Derrick’s post.
I will say that looking at your board the box capacitors seem correct, the electrolytic caps are oriented correctly, and the diodes look right. As Billy said it’s tough to see the resistor values but they seem to be consistent throughout (all the 10ks look the same, etc). And wiring looks solid.
Did you check the Q voltages? I’d be interested to compare yours and mine. Barry listed expected values on the build doc. For quick reference these are:
Q1 7.27 – 3.71 – 3.23 DSG (2N5457)
Q2 5.05 – 3.75 – 2.90 CBE (BC550)
Q3 5.05 – 2.90 – 2.86 CBE (BC550)
I ordered everything for this build from Mouser (I’m sure they hated me on this one). All the parts were individually bagged and tagged by value. It is hard to see the markings even in person. Not sure why I ordered these, but I won’t get them again.
Derrick
My thought is regarding the Germanium Diode.
Is that a Russian Diode? If so the stripe is likely the Anode and not the cathode.Your current orientation in the photo shows it being placed in the board as if the stripe is Cathode same as the board silkscreen.
The other thing regarding the diode is that it needs to have a forward voltage of .27 to .37 for it to make the SWAH work (see schematic). Unfortunately Germanium diodes from different manufacturers are all over the place regarding forward voltage. That is why I list the ones I sell in the shop with those numbers.
But should signal still pass thru the same with or without the diode?
Derrick
I got a voltage on my DMM for the diode in question of .295
Derrick
Here’s some info that should help you narrow down your problem area
Use the schematic to check you get the correct voltage at your VA (9v), VC (5v) and VB (around 3.5 to 4.5v ish) points you will obviously get a little voltage drop through components and your readings will depend on what your power source is actually putting out as you’ll know not all 9v adaptors put out exactly 9v especially if they’re not regulated
You could potentially drop up to 1v through D4, in the power section so just be aware your voltages may be less than shown
This won’t be a problem with the number of sucessful builds of this circuit I was fortunate enough not to run into problems with mine unfortunately I didn’t note the voltages but remember getting some VB voltages of around 3.6v so if you get similar at your VB points it’s fine
Post your voltages for all transistors and IC’s this may help indicate the cause
Input buffer stage (blue) a high impedance input to get as much guitar signal as possible going into the circuit and a low impedance output to drive the next two stages.
State variable filter (yellow). Without getting too techie, this is the part that goes “Wah”. If you are interested, look at the application notes (Google NE5517 or LM13700 data sheet) – warning !! this can be rather boring for normal sane people.
While we need to keep the signal going into the State Variable Filter (yellow) quite low, a slightly higher level signal might be needed for the envelope follower stages (green, pink and lilac).
The “green” stage boosts the signal, the”pink” stage converts the signal into a DC voltage, the DC voltage is proportional to the signal coming from the “green” stage – higher signal level, higher DC voltage level.
The DC voltage output of the “pink” stage drives the transistors in the “lilac” stage into producing a control current (more DC voltage at the base of Q2 means more current), the control current, which is connected to pins 1 and 16, tells the State Variable Filter (yellow) how much it should “Wah”.
Ok, here are the voltages. Hope I did this right.
IC1 Lm13700 IC2 TL074
1-1.247 16-1.246 1-6.42 14-3.67
2-1.11 15-1.11 2-1.11 13-3.7
3-3.67 14-3.669 3-3.67 12-3.67
4-3.669 13-3.669 4-3.669 11-0
4-4.87 12-4.9 5-3.661 10-3.669
6-0 11-7.34 6-3.67 9-3.67
7-4.87 10-4.9 7-3.68 8-3.74
8-3.657 9-3.687
IC3 78L05 1-7.34 2-0 3-4.99
Q1 2N5457 Gate-3.399 Source-3.75 Drain-7.34
Q2 BC550 Emitter-.92 Base-0 Collector-4.99
Q3 BC550 Emitter-.1244 Base-.98 Collector-4.99
Derrick
Emitter on Q3 is 1.244
Derrick
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