Before you do anything make sure you limit the current the amplifier is drawing from the mains. This to prevent any further destruction. Inserting a light bulb in series with the mains line and the amp is a common practice for this.
Is it the correct one?
I hope the following list of procedures helps:
- There are common failure modes you should investigate first. These include:
1. Failing of solder joints in parts under stress (jacks, large capacitors and resistors etc.)
2. Corrosion and wear of components (connectors, potentiometers, jacks, switches)
3. Components dying of old age: capacitors, fuses...
4. Components dying from heat: capacitors, output transistors/chips
5. Input problems: Input stage is the most vulnerable for static discharges and other signal "peaks" that may destroy the active device(s) of the input stage. If the circuit has effect loop jacks, jacks for PA in etc. These are also input stages. If you use them frequently, check them.
- There are also common identifiers for particular failures such as:
1. 50/100 Hz hum: likely a ground loop somewhere, could be caused by a cold solder joint
2. 100 Hz buzz: error after rectifier, likely a failed filter capacitor somewhere. The louder the hum, the more upstream the capacitor.
3. "Wailing" transformer: A short circuit
4. Smoke: The part in question is drawing too much current (short or open circuit?)
5. Hot parts: usually a sign of a short circuit or oscillation
6. No sound but no clear indication of failed parts: Usually a sign of an intermittant signal path. Check for failed/oxidized jacks, switches, potentiometers, solder joints. Proceed to check for dead transistors or opamps or failed capacitors. These should likely be on the signal path. Also check out that gain stages have a proper rail voltage.
7. Problems that appear/disappear after a while: likely heat related issue, check for cold solder joints
8. Problems that appear/disappear gradually: likely a problem with aging capacitors
9. Occasional "pops" and "crackles" & increased hum: old filter capacitors. The increased amount of hum is often hard to detect without a reference but the other symptoms are an excellent indicator of this problem. Do not confuse with similar noise coming out from dirty, worn potentiometers.
- Specific amplifier models have their common failure modes. Google if you can find information about them or discussion about similar problems.
- Look for burned parts or failed solder joints (this should be obvious). Never assume that by replacing them you fixed the problem: Often you are just repairing the damage caused by the real problem.
- Fuses sometimes trip due to old age but often they do it for a reason. You can replace the fuse (if it blows or has blown) but never power up the amp without a current limiter after that.
- If you don't have a schematic then sketch out the circuit parts you are investigating and download all neccessary datasheets. If you can't work without a schematic then you likely should not be working at all. If the schematic is rare, you will likely sketch one out faster than what it would take to hunt one down. This is also about the only way to assure you have the right
- Figure out what voltages the circuit should have in different nodes, check if they are correct. For example, it's likely best to start by testing the rail voltages. After that the DC offset of output, bias current/voltage (if you know it), opamp supply voltages and opamp input/output voltages, base-emitter voltages etc.
- In case you get no sound out of the amp: If nothing is burning (or if the current limiter light bulb shows no indication of high current draw) it is rather safe to hook up a signal generator to the input and follow the signal path by probing. This will tell you where the signal breaks and thus will help you to locate the problem. You need an oscilloscope or an AC coupled, high input impedance amp "probe" that you can "stick" into the signal path and listen whether the signal is present (or distorted).
- When you find an indication of a fault you must know the circuit well enough to cook up a theory of what might be causing it
- Now that you have a theory of what might be wrong, the search area has likely been narrowed down a lot: you can proceed by testing for failed parts. You know how to do this, right?
- Fix the failed stuff you find. Also, figure out why it failed and check if you need to fix some more stuff. If this turned out to be a dead end you must concentrate on alternative theories about the fault mechanisms, do more testing etc.
- Once you think you're done, power up using a current limiter and see if there are more signs of problems.
Edit: I added up few things that I missed. Also, I feel a need to add a warning: Eventhough the rail voltage(s) of transistor amps are generally quite low most of them deal with mains circuitry that can kill or severily injure you. Usually you have little need to tamper with the mains circuit but at least you must know how to avoid touching the live parts. There are also other safety measures that you should be aware of, they are quite thoroghly discussed in some of the sections of that larger troubleshooting guide.
One important measure is to drain the filter caps - or at least to assure their charge has "bleeded" away once the power is cut off. This is a minor issue in old, low power amp that the Squier is but becomes rather important the higher the capacitor charge is both in terms of voltage and stored energy. In tube amplifiers touching some of the capacitors without draining them at first can kill you, in solid state amps the charge is hardly lethal but the high short circuit current may kill the amp itself. There are two mechanisms for this: Either the capacitor dies due to excessive current or some of the transistors (discrete or integrated) die since you basically create an ESD, which is poison to most semiconductors. This is an issue even in low power amps - and the more they have stuff like digital circuits etc. the more vulnerable they are to static discharges. Also, the stored energy may shock you nastily and if you wear jewellery (you shouldn't) it can heat up and burn you if the metal becomes suspect to short circuit currents.