Debugging your Circuit

No matter how many pedals I build they don’t always work, at least not on the first try! Anything can go wrong. Debugging is a skill and an art, that is not hard to master.

It’s impossible to name all of the possible things that can go wrong but, I can give you a list common categories of problems and how to test for them.

  • Power
  • Switch Wiring
  • Short
  • Mis-biased
  • Misplaced parts
  • Everything else

Testing procedure

Here is the testing procedure I use. I do these things every time I make a pedal after I solder the last connection and have it all in the box ready to go!

  1. If you used sockets for any parts you can leave those parts out of the circuit for the first few steps!
  2. Connect my amp and guitar without power to the pedal. Guitar > Pedal > Amp. Step on the bypass switch. In one position you should hear the guitar, in the other nothing. This tests the bypass mode. If that’s working the switch wiring could still have problems but is most likely working. If you don’t hear any audio check the wiring around the foot switch and the jacks. Most likely there is a problem here.
  3. Step on the bypass switch. Does the LED turn on in one position. If so then wiring is even more likely to be working! If not check the wiring around the foot switch, and check the LED and the LED current limiting resistor. The LED might be backwards, make sure the Anode of the LED is on the positive side (on my PCBs this is the square pad) the cathode should be on the negative side and the LED may have a notch or flat side denoting the cathode. You won’t hear any audio if you left your socketed chips out.
  4. Before putting chips into the circuit let’s check the voltages and see if they are what we expect. You’ll have to examine the circuit to figure out what values you are looking for. Here are a couple common things to look for:
    • Dual op-amps, like TL072, should have 0v at pin 4 and 9v at pin 8
    • NPN Transistors, like 2N5089 and 2N3904, should have about 4.5v at the collector pin 3.
    • The PCB should have 9v at the positive power connection and 0v at the ground connection.
    • PT2399 circuits need 5v. These circuits use a regulator. The regulator should show 9v at pin 1, 0v at pin 2, and 5v at pin 3. The PT2399 should show 5v at pin 1.
    • Check your circuit schematic and see if you can identify any other voltages. Some schematics will show voltages at certain points for testing.
  5. Socket any chips that you have left out. Connect power to the pedal with the back open and touch the parts with your finger or thumb. Nothing should be hot! If something feels really warm or hot disconnect the power. It’s possible to generate considerable heat from just 9V if there is a short! If you are using sockets pull the chips or transistors out and power up again and check the voltages at each of the pins for these parts. See the notes below.
  6. With the power connected and your pedal connected to an instrument and an amp turn the volume down and play something. Switch between the effect and bypass. If bypass is not working check the wiring (see step 2) if the bypass works and the effect is not working you’ll need to debug further.
    • If there is no audio this could be due to a short. This could also be a lot of other problems.
    • If there is audio but it is quiet or “farty” and distorted could be that a transistor or op-amp is misbiased.

Step 6 is the hardest to debug. If everything else is working and you get audio from the effect but the sound isn’t right you’ll need to think about what you are hearing and ask yourself what in the circuit could be causing these sounds. For example if the circuit is too quiet maybe a gain stage or amplifier is not working correctly. In a modulation effect where the modulation is not working maybe the oscillator is not cycling. Here are a few ideas you can try:

  • Look at the solder side of the board closely. You’re looking for shorts, solder bridges, bad solder joints, or anything that looks suspect. Touch up any solder joints that look sus.
  • Look at all of the parts in the PCB and confirm they are the correct values. You can measure them with your multimeter, read the value on the part.
  • Check the orientation of all the polarized parts make sure they are correctly oriented!
  • Test your circuit with the audio probe! See my post about the audio probe.

Good luck with your pedal projects, I hope this helps you debug them!

Some Examples

I just built a couple pedals and followed these procedures, here is the process in pictures.

This is an over drive. Before putting the op-amp in I plug it in and check the LED. If it lights up when I press the foot switch the wiring is probably working.

I also test it in bypass. If this is working on go on to the next step.

I used a socket for the op-amp, so I check the voltage at pin 8. This should be 9 volts.

Next I check pin 3. This should be about half the supply voltage. It should be 4.5 volts but 4.8 is close enough. If it was way off this isn’t going to work I should debug before continuing.

Next I check pin 6. This should also be about 4.5 volts. This one is off by a bit but not far enough for concern.

I think it’s time to plug the chip in and test it out!





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