(This is a wiki post, please add and correct!)
Look and see if others have had the same problem. Use the Discourse search facility to find topics where the module you’re building is discussed.
Inspect your work. Verify all components are correct values. Look for solder connections that are rough, irregular, skimpy, dull, or missed completely (we all do that!) and reflow them. Look for solder bridges between pads and remove them.
Check your power connection. If it’s a Eurorack style power cable, make sure the red stripe connects to the -12V pins at both ends (module and power supply or bus board).
Get out your multimeter. You have one, right? If not, get one. You absolutely need one, and it needn’t cost much.
Disconnect power and check for shorts:
Something not addressed in that video is: If there are 10R resistors or diodes in series on the power rails, shorts elsewhere in the module will not be found by checking at the power header. Check again at the other sides of the resistors or diodes.
Also make sure there are no shorts in the cable between ±12V/ground.
Make sure all diodes, transistors, voltage regulators, voltage references, and polarized capacitors are oriented correctly. Double check if you’ve substituted one transistor type for another: the two types may have different pinouts.
Ensure that the sleeve of all jack sockets is connected to ground. If you’re using a non-metal panel and are running into issues with unwanted noise it can sometimes be helpful to solder a ground wire to the metal body of all potentiometers and other panel mounted hardware.
For the ICs, if you have an IC that got too hot to touch (or started smoking, smelling, etc.) you probably have one or more of the following problems:
- IC is installed backwards. You can tell just by looking. If you don’t know how to tell which end of an IC is which, see here. Also make sure the chip is seated properly and all pins are actually in the socket, not bent under. You did use a socket, right? With power turned off, you can check continuity between each IC pin and the socket pad.
- There are wrong voltages on the power pins. With power turned off, check continuity between the power rails and the ICs: Make sure any +12V pins connect to +12V power, -12V pins to -12V, ground pins to ground, +5V pins if any to 5V regulator. Check the datasheet or ask here if you don’t know which IC pin is supposed to connect to each. Again, if there are series resistors or diodes on the power rails, you will need to check continuity from the other sides of those. As an additional check you can remove the IC, apply power, and measure voltages at the power pin locations, but probably wait to do that until you check:
- Non power pins are shorted to power rails. With power turned off, check continuity between other pins and +12 V, -12 V, and ground. In some cases continuity is okay and in fact part of the circuit design, but in particular op amp output pins (pin 6 on a TL071, pins 1 and 7 on a TL072, pins 1, 7, 8, 14 on a TL074) should not be shorted to ±12 V or ground. For other pins consult the schematic or stripboard layout to see if the connection is intentional. If there is no continuity there still might be a problem if there’s a low resistance to a power rail, so go back over it with the resistance measurement setting and be suspicious of anything below about 1k or so. And, again, if there are diodes or resistors on the power rails, check back to them, not all the way back to the power inlet.
- Chip is damaged, defective, or mislabeled. Always possible, and you can try swapping in a new chip to see if it works, but the chip might be running hot because it’s damaged or it might be vice versa — in the latter case, swapping in a new chip will just result in frying another chip, so don’t try it until you’ve exhausted other possibililties.
Connect power and check to make sure no components get hot, or start to smoke or smell. If so, disconnect power and don’t connect again until you’re confident you’ve found and fixed the problem.
With power on, check the IC power pins again, this time to see if the proper voltage is there. Measure with the red probe on the IC pin and the black probe on a point that connects to the power supply ground.
At this point you should be confident you have power where you need it, and you have the right components in the right places oriented correctly. If things still aren’t working it probably has to be a missing, wrong, or, bad connection, a solder bridge, a damaged or defective part, a damaged or defective circuit board, or a design error. Tracking those down will depend on the particular circuit and the problem you’re having.
One strategy is to start at the front end and trace signals to see where they start going wrong. An oscilloscope is great for this, but if you don’t have one, in the case of a module that produces or processes control voltages, you might be able to do that with a multimeter. For modules that produce or process audio signals you can use an audio probe as described here:
It also helps to refer to a schematic (more helpful than a stripboard layout) if possible. Even if you don’t know much about reading schematics you can probably figure out what is connected to what. For example, look at this schematic:
This circuit has one chip, a TL074 quad op amp chip labeled U3 (there isn’t a U1 or a U2, don’t ask, there were reasons…), which has four op amps (U3A, U3B, U3C, and U3D) corresponding to the four yellow triangles. Each op amp has two inputs (+ and -, on the left side of each triangle) and one output (on the triangle vertex at the right). Audio signals come in on the left, and you can see they all connect to pin 2 of the op amp. Then the output, pin 1, connects to another input, pin 9, via a resistor. And so on. You can probably expect to hear audio signals at the output of op amps that the signal connects to. If the signal connects to a + input you should be able to hear it there too. If it connects to a - input you will usually not hear it there, but you would hear it upstream of the resistor it gets there through. In this case you should hear something at pin 1, pin 9, and pin 17 (all outputs; pin 7 is an output but the audio signal doesn’t go into that op amp). You would not hear anything at pins 2, 9, or 13 (- inputs) but you would on the upstream ends of resistors R1–3, R10, and R15 (input resistors to the - inputs). None of the + inputs have audio signals going into them. Pins 10, 5, and 12 have DC voltages from the potentiometer RV4 so you could measure there with a multimeter and see what happens as you change the pot. Once you figure out where the audio or control signals go wrong, you may be able to figure out where the problem is, or you can describe your findings online and ask for help.
If you need help, again, search here for topics about the module you’re building and post a message in one of them. That’s generally regarded here as better than opening a new topic, because it makes it easier for people like you needing information to find it in the future. Of course if there is no such topic, start a new one. Be as specific as you can about:
- exactly what you’re building (with a link to a web page about it, if it’s something unfamiliar)
- what you’re putting into the inputs and where you’re sending the outputs
- what’s not working and what happens instead
- what is working
- what you’ve done to try to diagnose and fix it
It may be helpful, especially if you’re building on stripboard, to post photos of both sides of your build.
Note: If you post a schematic or stripboard layout, add an “UNVERIFIED” label to the image unless you’re certain someone else using the same schematic or stripboard got it working with no modifications or substitutions. Otherwise people encountering the image on a web search without context may not realize it might not be good.
One last thing. No one likes reading a story and discovering it doesn’t have an ending. Let us know whether you end up solving the problem or not. If it turns out to be a dumb mistake, that’s all right, we all make them and we have a dedicated place for them. It also is gracious to acknowledge the help you’ve received; whether it was useful or not it was well intentioned, and saying what was useful may be of help to other people with a similar problem.