Troubleshooting
"Never think up reasons not to check something."
-Enzo (music-electronics-forum.com)
The Basic and General Method:
Rule #0: Be safe. Learn about electricity and safety regarding electricity before opening up anything. Be especially cautious with anything high voltage (i.e. vacuum tube circuit), anything blowing fuses/breakers, and anything suspicious in the line power (mains) wiring side (frayed power cables, poor previous repairs with electrical tape, etc). Build a dim bulb limiter for any kind of blown fuse repair to avoid needlessly blowing more fuses. Always a good idea to use the limiter whenever you're first powering on something that has had its power supply or power amp wiring changed (like changing a power amp module). This can save you in the case of a wiring mistake.
Step #1: Test! If things are safe (see above), test the unit before opening it up. Note everything experienced while testing and get as complete a history of the unit and the reported problems before opening it up. Many repairs stop here with "no problem found."
Step #2: dc conditions. Do not apply any input signal and check dc conditions starting with the power supply, then all active devices, then everything else. Printing out the schematic and writing down the voltages is a good idea.
Step #3: ac conditions. Apply a test signal and trace. Write down the input voltage and note how it changes at each stage. Write these notes on the schematic.
Step #4: Think! The ac and dc conditions probably don't match what electronic theory would predict at some step along the way. Try to think it out before replacing anything. Write down candidates for replacement and why.
Step #5: When the best candidate seems apparent, try in circuit testing (this has limitations). Lift one leg of a suspect component for easy out-of-circuit testing. Replace suspect components. After replacement, recheck as far back as Step #2. Return to your list of candidates if the replacement makes no change.
Connections!
Don't forget to test for basic connections in the circuit. Circuit boards crack, solder pads lift, cheap circuit board traces can just disappear sometimes! Print out the schematic and highlight each connection as you verify it. Remember that a broken ground connection will make your voltage measurements wrong, so when you find strange voltages in steps #2 and #3, don't forget to check for broken connections everywhere, including ground. If you disassembled something, check that you didn't break a ground connection normally achieved via a screw into the metal chassis. If you did, replace the connection with an alligator clip or temporarily solder a wire jumper.
Step #6: If stumped, take a break and come back to it. Seek help on forums like music-electronics-forum.com, diystompboxes.com, freestompboxes.com, group-diy.com, etc. Post the relevant dc and ac conditions and schematic to aid others in aiding you. Re-read the quote from Enzo at the top of this page.