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Hello forum,
I just finished my build of SWAH v4 with some success but cannot make it work 100%. The board is functional, no noise, signal is processed and bypass works. The voltages do make sense across the PCB to some extent but no matter what I do, I cannot get proper Vb, Ve on Q2, Q3 transistors as described in the docs. The values seem to be about half of what’s desired, Q1 is somewhere in the ball park, but not quite there either.
The modulation I get from the effect is somewhat shallow and the control pots do not shape the tone as expected. The Sensitivity knob e.g. seems to produce most variation in CCW (minimum?) position and no effect in full CW (yes I noticed the pot has to be wired differently than the other two :-), trust me I checked, three times 🙂 ) The components are all sourced from M*user and are at least mid tier so a reasonable quality is expected and I measured some of them before inserting (not all), that seemed to have checked out.
What I’ve done so far:
– verified all components are to spec and correctly oriented/polarized (including EL caps, diodes, transistors, pots, chips)
– re-flown each and every solder joint, checked for potential shorts in solderAlthough the board is now pretty burnt from the troubleshooting, I had to jump one pad and I replaced R28 with 47R (the voltage numbers shown here were taken before that, with 100R for R28), there is no significant change in measured parameters (i.e I probably did not make it much worse than it was).
For the moment I have two specific and one general question:
1.) is 1.60 V the expected voltage on IC2 p14?
2.) is the ca 1.45 V drop across R28 expected? I have 9.18 Vin, 8.49 after D4, yet 7.13 at VA.3.) May there be any general suggestions as of what to look for in this case? What kind of fault could produce this sort of misbehavior?
Any pointers to things I may have missed are welcome.
Measured voltage values: Vin = 9.18 VA = 7.13 VB = 3.56 VC = 5.05 IC1 1 1.30 || 16 1.30 2 ~0.85 || 15 ~0.85 (noise?) 3 3.56 || 14 3.56 4 3.56 || 13 3.56 5 4.74 || 12 4.75 6 GND || 11 7.13 7 4.74 || 10 4.75 8 3.55 || 9 3.56 IC2 1 3.56 || 14 ~1.60 (with no signal) 2 3.56 || 13 3.55 3 3.56 || 12 3.56 4 7.13 || 11 GND 5 3.55 || 10 3.56 6 3.56 || 9 3.56 7 3.56 || 8 3.35 IC3 I 7.13 O 5.05 G GND Q1 D 7.13 S 4.31 G 3.24 Q2 C 5.05 B 1.57 E 1.50 Q3 C 5.05 B 1.50 E 1.30
Hi Vlad,
I built this pedal and had D3 around the wrong way as the band on my1N34 was on the anode not cathode
Stu
Vlad, Pin 14 of the TL074 is low. It should be about the same as VB There could be a partial short in that area of the board, or a defective opamp stage in the TL074. This could also be loading down the VA voltage. I would look at those possibilities. If you have an extra TL074, try changing it. If the symptoms persist, you can eliminate that idea.
Thanks for the inputs.
@Stu: I have read that thread here and I double-checked the diode because of that exactly. Lifted from circuit, measured it and it is indeed correctly marked and installed. It conforms exactly to this https://www.microsemi.com/existing-parts/parts/54792#diagrams (that’s the actual part I’m using).
@wilkie1: I will acquire some more spare op amps today and I’ll try to replace the chip.
Hello again,
I managed to get the parts and exchanged the opamp. The situation improved somehow but remained pretty much the same in a way 😉Although the voltage on p14 of IC2 has risen to ca 2.73 V, the VA dropped even lower with the new part and the Q2,Q3 still remain a 0.5 or 1V shy of what was expected. Both parts seem to be Texas Instruments TL074CN and I don’t really believe either of them is faulty. Voltages are consistent across the board, the pins on both ICs shifted accordingly.
Any suggestions welcome.
The voltages now look like as follows (with new opamp and R28 at 100R):
VA = 7.06 VB = 3.52 VC = 5.04 IC1 1 1.31 || 16 1.31 2 noise || 15 noise 3 3.52 || 14 3.52 4 3.52 || 13 3.52 5 4.71 || 12 4.73 6 GND || 11 7.06 7 4.71 || 10 4.73 8 3.51 || 9 3.52 1 3.53 || 14 2.73 2 3.53 || 13 3.50 3 3.52 || 12 3.53 4 7.04 || 11 GND 5 3.52 || 10 3.53 6 3.52 || 9 3.53 7 3.52 || 8 3.27 Q1 D 7.06 S 4.28 G 3.20 Q2 C 5.04 B 2.68 E 2.26 Q3 C 5.04 B 2.26 E 1.72
Well, I am pleased to see that pin14 voltage has increased. That chip may have been slightly out of tolerance.
I would not be too concerned that all of your voltages are not exactly as expected. Component variances can cause them to vary somewhat. The more important question is the performance of the circuit. Is it doing what it is supposed to do? Did you try adjusting the Trimmer? How does it compare to the audio sample on the forum? If it is close to that, then I believe you are OK.
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