I have a Sony STR-KM7 AMP and the subwoofers have stopped working?

cpu stands for central processing unit no idea where you find one but perhaps a dealer can order one from SONY

Its normal as long as it does not show any malfunction.For cooling there must be fan rotating,in your model its only under very high tempratures for fan to be triggered to rotate.

First question is why?

It's not clear if this has EVER worked or what is new in the scenario. The STR manual has setup instructions for supporting a sub. If you want to prove the sub itself works you could jack any source like, say, a CD player, into it to prove its amp will respond BUT IT WILL BE LOUD if it works.

Generally speaking, an amp attempts to protect itself from heat, shorts, overloads and operator exuberance by refusing to turn on or stay on.

Overloads can be from excessive periods of high output or marginally low impedance loading by the speakers; and shorts would be wiring issues or a speaker blowing up.

You should be able to feel if it's hot. WHY is it overheating? Make sure it has sufficient ventilation on all sides and that vent holes are not blocked by dust balls. Ensure the fan (if equipped) is running as designed (some only operate on demand). Clean dust and debris from it.

If the amp comes back on after cooling, you're lucky. They only have so many self-protection cycles in their lives so continuously resetting or cycling their power without addressing the cause can do more harm than good.

If it protects immediately on a cool power up you should disconnect the speaker connections and try it 'naked'. If it comes up then diagnose which lead(s) are shorted. If it does not come up the problem is internal and should be left to an experienced and competent hands-on tech.

Probably not if it takes RCA input, but the receiver needs to be informed of the sub's presence and hw you want to share low frequencies among the rest of the speakers and itself. Then you need to set the relative levels using test tones, which also proves the sub works.

http://www.retrevo.com/support/Sony-STR-DH700-Receivers-manual/id/23341dj691/t/2/

Page 102

Set the subwoofer to YES (Page 43)

It is based on your sub-woofer type..whether powered or passive sub woofer..
Assume that you have powered one..RCA sub woofer jack is the best to feed it ..you should use RCA sub woofer output on your receiver as well..good luck

Is the audio for the center channel driven by its own amp? No other channels on it or affected?

Assuming you're not cranking it up to "11" to rattle the walls...

I think what you're hearing is the protection relay dropping out - removing power from the output stages of the amp when the amp thinks something is amiss. If it turns on OK after being OFF for a while and repeats the loop you probably have a heat issue.

A solid click OFF that is not resettable would possibly be a short in the speaker path - shorted speaker wire or speaker.

Check the amp to see if it was hot to the touch or getting warmer up to the point it clicked off.

Is it ventilated properly with no heat-producing components below it? Clean out its vents and make sure it can breath. If it has a fan make sure it's working and has good airflow.

Power Amp IC gone in your set, replace it.

sometimes these subwoofers are for builtin amplified subwoofers..so if this is the case it will not work