Problem with SWAH (Super Auto Wah)

[awgearhart]

I just finished breadboarding the SWAH from GuitarPCB, but I think I have an issue. I've been trying to adjust the internal trim pot to get a good sound, but it seems like regardless of what I do, the SWAH won't decay... it kind of stays the same filtered sound after striking the note (similar to leaving a wah-wah in a set position).

Is this a problem with the biasing? Or could I have incorrectly wired by pots?

Current configuration:
Sensitivity - B100k
Resonance - B50k
Decay - C1M

[awgearhart]

I guess a better way of putting it is that the pedal won't "sweep..."

[duck_arse]

we can't [ yet ] see how you have wired your pots. and we would need a circuit dia to compare your built parts against, if you know what I mean.

edit : and yes, breadboards count as builts.

[awgearhart]

I'm reading a post about sweeping... I'm using BC550B transistors where someone says BC550C works better... don't know if this is confirmed.

Also saw something about replacing C8 with a lower cap, and using germanium diodes instead of silicon...

Any help would be appreciated

[Mark Hammer]

And you know that the BC550 pins are in the right place?

[awgearhart]

Yes, pins are in the right place. I've tried the BC337 and 2N5088 to substitute the 550's, but still no sweeping sound.

[awgearhart]

It doesn't sound like the decay or sensitivity knobs are making any effect on the sound.

[awgearhart]

Is this a buffered pedal? I noticed the board requires wires to S4, S5, and S6, and I don't have them wired to the 3PDT yet. Could that help?

[awgearhart]

It looks like my voltage measurements for Q2 and Q3 Base and Emitters are quite low (~0.6 V)...

I've been looking all over the internet for help, but can't find anything that's useful.

[awgearhart]

Also: My Q1 (2N5457) reads 0.001 V at the Gate... this is obviously an issue but I can't figure out where it could be based on the schematic.

[antonis]

Just for reference..



Check for R3 proper connection to IC2B pin 7..
To verify for VB circuit presence, chek any of ICA/C/D non-inverting inputs..

[awgearhart]

Here are my voltage measurements all knobs CW... maybe this will help:

Q1: 6.56, 3.245, 0.001 (DSG)
Q2: 4.92, 0.518, 0.405 (CBE)
Q3: 4.96, 0.404, 0.075 (CBE)

IC1p1: 0.056
p2: 0.000
p3: ~2.6
p4: ~2.5
p5: 0.022
p6: 0.000
p7: 0.022
p8: ~4.4
p9: 3.42
p10: 0.28
p11: 6.22
p12: 0.022
p13: ~2.1
p14: ~2.1
p15: 0.000
p16: 0.066

IC2p1: ~3.59
p2: ~3.49
p3: ~3.4
p4: 6.45
p5: 0.079
p6: 3.544
p7: 3.362
p8: 0.932
p9: ~3.3
p10: ~3.4
p11: 0.000
p12: 3.544
p13: 0.087
p14: ~0.8

IC3 input: 6.49
IC3 output: 4.97
IC3 ground: 0.000

[antonis]

Something very wrong happens with IC2..

pin 4 should measure 9V
pins 5, 6 & 7 should measure the same as pin 3/10/12..

P.S.
Check R29/R30 values..

[awgearhart]

For some reason, they each are measuring about 25.7 kOhms... maybe they are somehow running in parallel?

[awgearhart]

P.S.
Check R29/R30 values..

When on the circuit board, R29 and R30 measure to be 25kOhms. When I removed each resistor, I remeasured them off board and they were 47kOhms... it sounds to me there is an unintended bridge somewhere, but it's not visually present. Could there be an issue with the board?

[antonis]

Judging from your measurements on pin 5 seems like R29/R30 junction point is (almost) grounded..
Check PCB for short somewhere inside GREEN rectangles or check for shorted C11..
(ingnore red rectangle for the moment..)

[awgearhart]

I measured the capacitance on C11, it was basically 0 nF. I tested a new cap and measured 100.1 nF, but when I replaced it on the board, it measured 0 nF again.

[antonis]

So, C11 discharges when on PCB..
(actually, DMM current, used for capacitance measurement, leaks out of capacitor plates..)

Check for possible short between C11 pads, with capacitor in place..
(if it isn't OK, remove cap and re-check..)

[awgearhart]

I can't see a short, but clearly there is something wrong. I'm wondering if I just remove R29, R30, and C11 and just wire them directly to each node.

[antonis]

I can't see a short,

DMMs help us to see better..

P.S.
Remove R19 & R20 and take measurement between IC2 pin 5 and GND..
(you should read "open"..)