Quick check of speakers: disconnect from board output and momentarily connect a 1.5 volt battery. You should hear a "pop" when connecting and disconnecting, if the speaker is good. Speakers unlikely, however. Two at once indicate blowing the speaker coil due to power overload, which also is unlikely from a TV audio amp. Each speaker is connected to an independent stereo output from a dual channel amp, yet each side has the same problem. Blown caps indicate a power problem. Multiple board replacement without problem resolution indicate a problem external to the board. If it's the power supply, it's an output providing voltage just to the audio circuits. You probably need to troubleshoot -- replacing all components multiple times can get expensive.

In my 35 years of repairing electronics, my success with "cleaners" has been much lower than 99%. I'd rate it closer to 5%, but even when there's success, the problem always returns. You can't find a genuine cleaner, either, since trichlorotrifluoroethane has been outlawed for consumers. Cleaning a pot is like sprinkling salt on a bird's tail: if you've gone to the trouble to get that close, just reach out and catch the bird. Moral, try the cleaner (whatever that is), but while you're in there, replace the pot.

Class A receivers such as this are designed to operate in a very inefficient portion of the amplifier performance curve, and then naturally generate lots of heat. Your unit is heating to the point that the amplifier is operating in another region of its performance curve, causing the distortion. I suggest the right place to look is in the final amplification stages, which have large transistors with large heat sinks. Go to Radio Shack and get their cooling spray for troubleshooting, then, when the distortion presents itself, spray one of the transistors (not all) and listen for the distortion to go away. Do this until you find the offending circuit. Once you've found the circuit, the advice changes. Can you solder, without damaging surrounding components? As much heat as one of these power transistors needs to remove it, there's a danger of delaminating circuit board runs. Be careful.

I've done what David suggests many times. Compressed air will not remove soda syrup. Direct access is the only way.

James, it could be capacitors, transistors, resistors, or pretty much anything. It does not appear to be a heating problem, or anything related to time. It is most likely a loose connection, like a cold solder joint (good at first, but fails over time). The good thing is that it appears to be in the power supply section. If it is an old TV, but just old enough to have a multi-function circuit board, this may not be good, but only a technician should be looking. What is the value of the TV? The repair may be more expensive than finding a good used TV at a repair shop (or Goodwill). If it is a very old TV, it may have modular electronics, such that just the power supply could be changed, which you could do yourself. Again, what value do you place on the TV? And, the modular component may be difficult to find. The place where the dangerous voltages exist are in the video circuits, and particularly in the flyback transformer circuits. Look for a large lead going to the CRT (picture tube), note where it goes, and ...

I have an older amplifier (which is what your volume knob controls) that has the same problem. It's as Markus says. There are dead spots on the potentiometer from corrosion. I have repaired them, myself, but it will be easier to replace. You decide. You will have to open the receiver, access the pot, desolder and remove it, then open the pot. This is the hard part, since it was never intended to be repaired. Be careful how you open it, or it will never look the same. There is a wiper arm that is moved across a series of ridges, which is just resistive wire wound around a core. The way to remove the corrosion is to burnish both the wiper and the resistor face with emery cloth. Reassmble and reinstall. Now, since you've destroyed the pot, search the internet (eBay, etc) using the part number off the case of the pot (I hope you saved it).

Umm...I started to pass this by, but had to put my 2 cents in. The condition of the "good" battery, and its cables and posts, is important, when qualifying these previous answers. The "terminal" or post voltage is what the battery delivers to the cables. If the "good" battery is weak, or the terminals are resistive (corroded), the alternator output (voltage) will help hold up the voltage being delivered to the jumped vehicle. Whenever the battery is called upon to deliver current, its terminal voltage decreases, even when another car is not being jumped. If the drain is too high, the voltage decreases dramatically. Having the alternator deliver power will help hold up the "good" battery voltage, and compensate for the assumed lower voltage of the jumped vehicle. My vote is to have the "good" vehicle delivering alternator current to aid the battery. I doubt revving the engine helps much, though.

Danny, 90% sure that it's the power supply. Nothing should ever be thrown away because of a power supply, since they're the easiest subassembly to fix in just about anything. If it is the power supply, however, you should have a technician look at it. A standard warning on electronics is "No user serviceable parts inside," and for good reason. Have you ever been tasered? Getting across 120VAC is worse.

What other symptoms? Power indicator on? If not, check things that affect power, like circuit breakers (things that reset, obviously not fuses). Does it consistently take the same amount of time to shut down? Can you affect the amount of time to shut down by removing heat (removing case, or routing an air source into the receiver)? If so, you can selectively spray a cold source (used to be trichlorotrifluoroethene - outlawed, so go to Radio Shack for current troubleshooting aid) on internal areas to see if the receiver will reactivate. If successful, start isolating to smaller and smaller areas, until offending component is found. I'd have to look at it to provide help beyond this. Unless you have experience, your chances of success are small. A good technician is worth the money. I'm a EE, and been troubleshooting for nearly 40 years.