This is never going to be a complete guide on how to debug your build, but rather a collection of suggestions and observations on the subject. I’m assuming we’re working with non-lethal voltages typical for most stompboxes, i.e. in the region up to about +18V. Special care needs to be taken when working on high voltages and this won’t be covered here.
I welcome any corrections and suggestions on how to make this resource better. Let me know.
Keys to success:
- Work systematically – It’s a good idea to keep a print-out of the layout in front of you and mark off components etc. as you check them. The corresponding schematic can also help if you need to do something like tracing the audio path from input to output.
- Be patient – Sometimes it helps to put your work away and look at it with fresh eyes the next day.
- Sit back and consider the problem – This is often easier when you gain more experience, but you can almost always use logic and deduction to narrow the scope of the problem. Does everything work except the tone pot? Start by checking the pot, the pot wiring and the components immediately connected to where the pot wires connect to the board.
- Trace cuts – Flip the circuit board so you’re looking at the copper trace side. Make sure you’ve got the correct number of traces and that they’re at the correct spot (all layouts should have a trace cut count). Remember that the trace cuts on the layout are mirrored when you look at the trace side of the board. Note that there’s usually several trace cuts underneath ICs, and a layout usually also has a list of hard-to-spot trace cuts that are hidden underneath other components. Also make sure none of your trace cuts are incomplete, still providing a conductive path. Performing these checks right when you start, before you solder any components, is good practice and will eliminate a few problems down the road.
- Jumpers – All layouts have a jumper count; make sure you haven’t missed one (some might be partially or completely covered by an IC socket). It’s also easy to misplace a jumper or have it too short/long by one hole.
- Resistors/capacitors – Like jumpers make sure you haven’t forgotten or misplaced any. Also check that you’ve used the correct value. It’s easy to pick a resistor that’s 10x off because of the similarity in color coding. There’s plenty of good phone apps that can help you with the resistor and capacitor coding if you haven’t learned them by heart yet.
- Electrolytic capacitors – With electrolytics you also have to observe the polarity (unless you’re dealing with bi-polar/non-polar ones, but that’s quite rare). Most radial electrolytics will have an indicator showing you the negative side. Electrolytics on the layouts are indicated by a shaded band for the negative side, but I’ve also added a “+” indicator to make it easier (note that on some of the older layouts you’ll find a “+” on the shaded side; always go with the “+” indication).
- Tantalum electrolytics – Same as with the regular electrolytics except the component often has markings indicating the anode/positive leg instead. These are also quite fragile and susceptible to over-heating, so you have to be more careful not to damage them.
- Diodes – With these you have to observe the polarity. Most diodes come with a band indicating the cathode/negative side, which you should match against the diode on the layout. As with most semi-conductors they’re also a lot less forgiving of over-heating.
- Transistors – Make sure you’ve got the pinout right; this may differ between manufacturers. The illustrated transistor orientation on the layout may or may not be correct, but all layouts have a note specifying what the pinout should be. Find the datasheet of your specific transistor (google works) and match the pinout according to the note on the layout. (A good reason why you should use sockets for your transistors).
- ICs – Observe the orientation of the IC and that you’ve used the correct version (e.g. TL072 and not TL071). Make sure all the IC legs are properly inserted into the IC socket and not bent to the side.
- Solder bridges – Kind of self-explanatory, you’re looking for solder bridges between adjacent tracks or across trace cuts. It might be a good idea to touch up any border-line cases, or perhaps redo the joint altogether (solder sucker and new solder, less of it this time). While it’s not an elegant solution carefully running a knife along each of the lengths between the tracks could work; even a tiny solder bridge barely visible with a magnifying glass is enough to cause an electrical connection and throw your circuit off. And finally, the continuity tester on your DMM is great at finding adjacent tracks that are bridged, just keep in mind that sometimes they -should- (e.g. jumpers, diodes and very small resistors).
- Cold solder joints – This is another common one. You’ll hear people saying that a healthy joint looks shiny and a dull one is a potential problem. This is probably true for old school lead-solder, but I use the lead-free stuff and all my joints end up looking dull no matter what, so keep that in mind. If you look closely you can most likely spot any joints that look suspicious, but your DMM continuity tester will also be of help here. Re-flowing the joint in question is often enough to solve the problem.
This can be a great help in getting closer to the problem, but there might be some additional thinking required too. Remember that you have to power the circuit in order to get anything meaningful out of this.
- Make sure you’ve got a healthy battery or power source before starting.
- When applying power to the circuit, if the only thing happening is a component or the battery becoming hot you probably have a short somewhere and you should immediately turn the power off. Use other means of finding the problem, a visual inspection perhaps.
- All voltage readings are performed relative to ground so clip your black DMM probe here to free up one of your hands.
- You usually only read the voltages off of active components, i.e. transistors and ICs.
- Ideally you’ll have a full chart of expected voltages at every pin on the circuit, but even lacking this there’s a minimum of things you could verify.
- All ICs require a +V and a ground connection; look up the datasheet to locate the correct pins. You’d expect the IC +V pin to be at- or close to the +V your feeding the circuit, and the IC ground pin to read at most a few mV.
- Some ICs run at +5V and not the typical +9V, in which case you’re likely to also have a voltage regulator in your circuit (78L05 or similar; looks like a transistor). Look up the voltage regulator datasheet of your specific manufacturer (these reportedly often differ in pinouts) and make sure you’ve got the pinout correct according to the layout. Now you’d expect to read +V on one of the pins, ground on one, and then +5V on the last one.
- A lot of opamps and dual opamps have their input and output pins sitting at a bias voltage of approx. 1/2 the +V, but this isn’t always the case.
- Keep in mind that all voltage readings are going to vary a bit from circuit to circuit depending on many factors. This is fine.
- If you -do- find one or more pins that are way off here’s when a little logical thinking will help a lot. A detailed breakdown of how to follow up is outside the scope of this document, but a closer inspection of components and tracks directly connected to the pins in question is probably a good idea. A schematic or the ability to trace the connections on the layout will likely be of use here.
Using an audio probe:
Here’s a nifty tool that can help you listen to your signal as it progresses through the circuit; R. G. Keen’s improved audio probe. You build this little thing, connect it to a small amp (ideally with a volume control), use the clip to connect to your circuit ground and touch the probe at various points on your circuit to hear what your signal sounds like at that point.
Now, just randomly “listening” to any point on your circuit isn’t likely to get you anywhere. What you need to do is keep a copy of both the schematic and the layout in front of you and slowly start to follow the audio path. You will need your circuit powered up and some kind of signal injected into the circuit input; you could keep strumming your guitar with one hand (or get your friend to do it for you), or maybe build a signal generator/sine wave oscillator of some kind to create a signal for you.
With everything set up start by verifying that you can probe and hear your signal at the “input” of your circuit. If you’re not getting an input signal debugging the rest of the circuit is moot; look at your setup, switch wiring etc. Once you’re getting a signal at “input” you start probing the signal along the audio path of the circuit. Here’s where the schematic will help you identify exactly what the audio path on the layout is. Let’s say you follow the signal through an input capacitor and into the first opamp or transistor gain stage, and there’s no signal at the gain stage output; you’ve just potentially discovered where the problem is (now’s the time to look for mistakes near that opamp stage or maybe verify the IC is getting power).
This can be a very powerful debugging tool for some circuits, but it can also be a great way to adjust the bias on those JFET preamp emulator circuits with all those trim pots (start probing the signal on the first JFET output (drain pin typically) and adjust the trimmer until you can hear a nice, beefy output signal, then continue to the next JFET gain stage, and so on).