Help reading a schematic

[sndansby]

Hi,

I am looking for a tutorial on how to read a schematic. I am trouble shooting a pedal that I am having trouble with but I do not know where to begin with the signal path. Well I know where to begin but after that I am confused.

[sndansby]

I am trying to trouble shoot this pedal: https://forum.pedalpcb.com/attachments/pedalpcb-leprechaun-jpg.3801/

[jimilee]

So, start with the in at the top left. Following the path, the signal travels through r2 and c2. r1 and c1 go to ground. it's helpful to look up the pinouts of the ICs to further see how the signal travels through them. Working on the leprechaun what troubles are you having?

[midwayfair]

You're unlike to find a tutorial on "reading a schematic" because there are too many ways a circuit can be put together to even begin making such a tutorial. You can glean something from reading circuit analyses on places like GeoFx and ElectroSmash, but I honestly don't know if the information is generalizable if you see something that wasn't covered in them.

This is about as close as I can get to a tutorial for reading a schematic for an audio circuit:

1. Get a chart with schematic symbols.
2. Print the schematic, or bring it up in a program like Paint that will let you mark it up.
3. Pick a color and note it as "audio path." Start at the input for the audio signal, which will be marked on the schematic. Your goal is, like a maze, to follow the audio to the output.
4. When you come to a symbol you don't recognize, stop, identify the part type, and go read the Wikipedia page on that part type.
5. When you come to a fork in the road, so to speak, your goal is to identify what kind of circuit is formed by the cluster of components.
    Everything useful is a voltage divider. You can have a resistor voltage divider, which will send some portion of the signal in one direction and some portion in another. Take a typical volume control. Look at where the signal starts, and look at where it can go. What would a volume pot look like if it were just resistors? You can have a resistor-capacitor voltage divider, which will impede the signal based on frequency. You can calculate what frequencies are cut if you want, or just identify in broad strokes what is happening (e.g. are high frequencies being cut, or low frequencies, or both?), but you need to recognize them because they're common. Transistors are also a voltage divider -- the -istor part is from resistor -- but they are able to amplify the voltage or current of a signal depending on their usage. You can look up basic transistor circuits to see something with only a couple parts to learn how they work. Op amps are made up of transistors and you can even look at their circuit by looking up the datasheet for the part.
6. Do the same process with the supply voltage -- you can identify the power filtering, and when the voltage is limited or perhaps divided (as is often the case when supplying the half voltage for a dual op amp).

Learning circuits that way is a process, and it will take a while to analyze complicated ones. Start with simple circuits (like the ones labeled "noob" in the Madbean store) and learn to recognize blocks of circuits, this will reduce the number of parts you need to examine when you open a new circuit. Many parts are also analogous to others -- different types of transistors might have different symbols and they might have different pinouts in the real world, but their schematics are drawn the same way; heck, they're often drawn the same way when they're tubes.

The other thing you should do is experiment. Make an audio probe, grab the schematic for the pedal, and just stick the probe somewhere in the circuit while playing a guitar chord. This might be useful if you're not sure whether something is audio or not, but it's definitely useful for understanding what happens to the guitar signal at any given point in the circuit. It's also an extremely effective troubleshooting tool, because if you look at the schematic and have some inkling that "hey my guitar signal shouldn't go missing here" then you've probably found a problem. And breadboarding circuits is a great way to understand them from the ground up.

[Zerro]

That link is only for ones who knows some password :@( It's always usefull to attach the schematics; not everybody has time to search at web to find what you are talking about. But I suppose, you are building Leprechaun? For newbie is such a building really big chalenge.

[jimilee]

What John said, Electro smash is a fantastic resource.


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[harryklippton]

http://beavisaudio.com/techpages/hiw/HIW.png

I found this graphic from Beavis audio really helpful in starting to understand how circuits work

[sndansby]

Thanks for the advice I am working through it today and will let you know what I find out.