Glad I found your post. I want to preface this. I don't know what the proper term is for some equipment but it does work I have been using mine for about 3 years and I got it used, so it could have some years on it. I have my ipod hooked up to it and found that the sound comeing through the speakers gets very fuzzy after abotu an hour. I also notice that the fan isn't kicking on. I have to shut it down and let it cool down before the music comes out clear again. Is it the fan, could it be a fuse for the fan, or a filter, or some other reason that it does this. I hope to not have to replace it and any help is appreciated. thanks
Hard to tell without having a first hand look at it by a competent technician. But, it could be a multitude of problems. Have you tried another source? Eh, it's worth a shot.
This unit uses a temperature sensor(s) in conjunction with the fan circuit. By default, the fan runs at low speed until a certain temperature is reached (I'm not sure on the cutoff, I don't have the service manual on this machine). Then the fan kicks into high speed operation after the sensor detects a high enough heatsink temperature. Now, the fan could be totally dead or disconnected (unlikely, unless the previous owners had a tendency to feed it liquid or lots of dirt), or more likely, the sensor is having issues (fell off the heat sink, but I think these are screwed onto the heatsink) or just gave up.
Making sure the fan filter is still serviceable and in one piece (most of them get nasty and junk, and hence removed and thrown away without replacement) is also helpful. If the grill is clogged, the fan won't move air no matter what you do.
If the fan is not running lo/hi and it gets hot (which it will after an hour) there is a possibility that heat soak is getting the better of the transistors. The thermal junction for the 2SA1303 transistors are 302 degrees Fahrenheit (150 Celsius). But, chances are it's not getting that hot... but sometimes the heatsink paste dries up and just makes a gap between the heatsink... again unlikely here. I'm pretty sure you would know if 16 of those transistors were close to pumping out 300 F
A failing/failed capacitor could do this as well as these caps are probably at least 20 years old, and I'm surprised my amp is nearly immaculate. Heavier used amps though... I feel sorry for them
As to which cap, if any of them are bulging out the top, opened, or exploded, well, there's your answer. That's just a quick way to test them; otherwise you'll have to go probe each circuit to check (or desolder each part and test it, even more maddening). The visual test is probably best here, considering I haven't stated a more obvious DIY route. (below)
The detent switch for the gain control is dirty. Now, chances are that if it was the gain control pot itself, it would do it all the time, hot or cold. But, it's usually easier to clean and lube pots and see if it works. At least you know they are clean.
But, the part where it works great cold, and when it heats up makes it fuzzy, is bad.
There is a problem there directly related to heat, or runtime operation, or both. If you are still trying to use this amp (which I don't recommend), turn the gain to just 1/4 power. I am assuming you have the gain set to full or very close to full. By reducing gain to 1/4, you are now decreasing the power going to the transistors, in turn lowering the TDP of the package. This lowered thermal dissipation should be enough to rule out other issues.
If it still runs just as hot, or still gets fuzzy after the same time, stop using it and take it to a shop.
If it runs longer at 1/4 power before getting fuzzy or runs cooler, that's better, but still not great. You'll have to still take it to a shop, but at least you have some idea what's going on before you drop it off for service.
Either repair is going to require a competent technician.
