So, Where do I start ?

[aneng]

Hi!

I want to be able to repair (mostly) audio gear (music stuff, amps, effects units, mixers etc).  I have a low-level qualification in electronics and have managed to repair stuff before with help from others and schematics etc.  However, my knowledge falls short of being able to look at a schematic and work out for myself what kind of voltage / signal I should be getting at various points (assuming there’s no test point data) - mainly because I only have a vague idea of what I’m looking at.  If I don’t have a schematic then I’m stuffed beyond testing for blown fuses, bad joints and obvious things like burnt out components.

How do I go about getting the knowledge I need to be able to fault-find circuits, specifically with repairs in mind ?  Is it different, in a practical sense, from the standard electronics studying of learning all the theory etc (I don’t mind doing that if that’s what it takes), but if I don’t really need to know Horowitz & Hill inside out just to pin down a knackered diode in an effects pedal, then all the better.

I’ve had a look at the ‘learning resources’ bit but that’s rather overwhelming and knowing which to pick is difficult.  I’ve read Michael Geier’s “Home to diagnose and fix everything electronic” and I’m two-thirds through Forest Mims’ “Getting Started in Electronics”.

I’ve got a bit of gear (Hakko iron, 20MHz Scope, DMM, component analyser etc), but just lack the knowledge to property fault-find - especially if I don’t have a service manual spoon-feeding me.

Thanks in advance folks 

[rstofer]

Without a schematic, troubleshooting is going to be very difficult.

Perhaps you could start with the datasheets for the various components.  At least you would know which pins are inputs and which pins are outputs.

You could sketch out a block diagram just by considering what the gadget does.  A power amplifier will have some power transistors, usually on a heatsink.  There will be some circuit driving the transistors.  You will need to figure out what amplifier configuration you are looking at: Common Emitter, Common Base or Common Collector.

https://www.electronics-tutorials.ws/transistor/tran_1.html

For amplifiers, you can always put a sine wave in one input and chase it around the circuitry with a scope.  I might not do this if I were working on vacuum tube circuits (unless I had a 100:1 probe) but for transistor circuitry, sure.

Obviously, you are going to need the datasheet for every component in the device.  Simple components, like resistors, capacitors and diodes can be figured out without a datasheet but transistors don't have a standard pinout.  Proprietary ICs are going to be a problem; you won't get a datasheet.

I don't mess around with radio or audio, I have no specific suggestions other than just practice.  Take a device that works, chase a signal around and build up a block diagram.  Put the diagrams in a binder for future reference.

[ericasdf]

Knowing the theory is important. If you know how a certain type of part works, then you can generally figure out if what is going in vs coming out makes sense. I am guessing you are mostly interested in the analog audio stuff, as digital basically goes into a ADC and comes out of a DAC. Everything in the middle is digital, and that is a totally different ball game. For analog, starting with knowing how diodes, transistors, and amplifiers (op-amp theory) work will get you a long way. Then look into high/low pass analog filters. An audio mixer is just an op-amp with multiple inputs paths. A class or two might help, but getting an engineering degree isn't necessary.

But some of just knowing what to look for comes from experience. Like the previous post stated, play with stuff (preferably working stuff). Get a cheap frequency generator so you can pump a set frequency into the system and just play around with it. Vary the frequency and see how the response changes. Probe all the various inputs/outputs of the circuits and see what they look like. Pull up the datasheets of the parts and then figure out how they work and then test that block and see if its doing what you would expect. Watch out for vacuum tube stuff, they are high voltage and can toast your scope if you are not careful.

And lastly, don't get discouraged. Getting where you want to be will take a lot of work.

[Shock]

However, my knowledge falls short of being able to look at a schematic and work out for myself what kind of voltage / signal I should be getting at various points

You can assume in an amp it's going to be supply voltages and an audio signal. With some digital circuits dependent on the display and controls or type of processing the amp has. Familiarity along with knowing common voltages used in similar circuits helps, or look at the components being used. Troubleshooting an amp you would input a test signal which allows you to see when that signal is missing or distorted in some way.

If I don’t have a schematic then I’m stuffed beyond testing for blown fuses, bad joints and obvious things like burnt out components.

Learn how to test every component and what works best in an out of circuit. Don't be afraid to measure.

How do I go about getting the knowledge I need to be able to fault-find circuits, specifically with repairs in mind ?  Is it different, in a practical sense, from the standard electronics studying of learning all the theory etc

The Art of Electronics may improve your theory but it won't directly help your practical repair skills initially. You need to do hands on troubleshooting and attempt repairs. There are no great books on repair mostly because of the absence of all the tips and tricks you will pick up over the years and methods good repairers follow subconsciously.

There are very few decent youtube channels on repair (some crap ones get far too much attention) nothing on audio really springs to mind. Perhaps try some of these:
https://www.youtube.com/user/kev31779/videos
https://www.youtube.com/user/Maxxarcade/videos
https://www.youtube.com/user/essoterric/videos

I’ve had a look at the ‘learning resources’ bit but that’s rather overwhelming and knowing which to pick is difficult.  I’ve read Michael Geier’s “Home to diagnose and fix everything electronic” and I’m two-thirds through Forest Mims’ “Getting Started in Electronics”.

Don't quit learning basic electronics. You can be taught repair like a monkey and just repeat those skills over and over but knowledge along with the practical will increase your end game potential.

I’ve got a bit of gear (Hakko iron, 20MHz Scope, DMM, component analyser etc), but just lack the knowledge to property fault-find - especially if I don’t have a service manual spoon-feeding me.

Add a simple hot air station at some point and an audio signal generator, you should also have flux, wick, and a hand solder sucker.

Obviously the service manual is the first thing you want to grab if you can but don't expect to run before you can walk.

[Harb]

My suggestion is to work on smaller projects.......maybe small kits etc for a while and build up your knowledge of the "Blocks" used in the equipment you want to master.
After a while you will see these "blocks" in the larger circuits you are working on, and be able to recognise and take a good guess at what it is you are looking at and what to expect to see at the end of your test leads.
Having no schematics is always hard, even for old hands, so don't feel bad.....you will eventually get used to Seeing things as smaller blocks and get used to working without schematics when you have to........believe me its a lot easier these days with the net.....once it was near impossible to find them at all .