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The intergrated circuit is most likely gone out. Not much you can do, cost in repair is more than the amp. Check out the peavey vypyr, it's a great modeling amp for about $100.
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I have no idea of what make and model that you have. To find out if the problem id the speaker or your amplifier, remove the distorted speaker and connect a known good one. If there is still distortion-it's not the speaker that is distorting-it's that channel of the amplifier.
Without a schematic you can still trace the circuit to locate the fault.
Put a music source or sound source into the bad channel and a good channel, and with an oscilloscope or with a pair of high impedance headphones with test leads on the ends- you can set up a low z headphone with an impedance match trasformer, make sure you have test prod ends on one of the leads. The other can go to a chassis ground. Start at the input and work your way forward comparing the signal between the good channel and the faulty channel. When you lose the signal or you encounter severe distortion, you are in the vicinity of the defective part.
Common failures are a Deffective 4558 or TL072/TL071/TL074 IC chip. You may have a faulty electrolytic capacitor, or even a faulty bypass caoacitor in general. Bad solder connections, even a broken fim resistor.
With the IC chips, the usual power pins for an 8 pin plastic or ceamic DIP are pins 4 and 8. usual outputs for the 8 pin are 1 and 7 the remaining pins are the inverting and noninverting inputs. 2 and 3 and 5 and 6.
Also check switches and especially those with interupts on the input and output jacks- they can get dirty.
It could be a couple issues in that section. Start by cleaning the contacts and switches to make sure it is not dirt. Also check to make sure the plug you are using is right for the jack- just to make sure everything is connecting as it should and nothing is inadvertantly getting shorted- such a 2 pole plug plugged into a 3 pole jack.
If those are all okay, the issue is in that boost channel section. Check to see if you are getting both power rails at the IC's, measure on a 4558 chip pin 4 to chassis should be -15 volts and pin 8 should be +15 volts to chassis. if you do not have this voltage, check the wiring, check the PC board for bad solder joints and check capacitors. Also check the physically larger resistors.
If you have the voltages, put an audio source on the input and set up a headphone with test leads, or better- an impedance matching transformer with test leads on the high impedance winding- put one to ground, with the other lead, move from IC to IC to locate the area to look at. You can get a data sheet for the 4558 here: http://www.ti.com/lit/ds/symlink/rc4558.pdf It is a low noise version of the 1458 family of op-amps. Check each IC at the inputs and the output, once you locate the stage where inputs are clear and output is garbled, that IC may be the fault, or something in the vicinity- usually it will be the IC. You can get any configuration of the 4558 you need through Mouser or Digikey, and Digikey does ship worldwide http://www.digikey.com
Once you get the new IC in, you should be groovin in no time.
You are probably overdriving the inputs. You haven't told us the source of the inputs and we can't guess where they may come from. You need to read the manual and make sure your source outputs are within range of what this unit requires.
DO NOT USE a guitar pedal in effects loop as it has the WRONG signal levels.
Also, when you boost the gain preceeding a distortion setting in an amp, the amp will just limit out and it will appear that the effects in the amp aren't working... but what is happening is you wind up with too much input signal and the "flat topping" distortion occurs right at the inout of the amp so no further can occur when you select a distortion in the amp.
This may be he characteristic of mixing of the delayed when high distortion is present...
I would go to music store and compare... I tend to think this is the nature of the beast.
If you consider what the waveforms look like with distortion and then add them delayed... they limit out and when added there is a significant difference when adding or mixing waves that are delayed when squared compared to nice sine waves.
initially you want to think bad solder joint, but it think its a bad mosfet or rectifier or diode. it would be hard for an untrained person to identify your problem let alone an experienced tech. it may be time to move on to another amp. in my shop it would be at least 65 bucks for the repair. so where did the "heat"/energy go from that point and what parts were affected. there may be a domino affect. burn one part, not usually. yostamplifier.com
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