Fender power chorus squeals when powered up for 30 seconds,plays ok for 10 minuted then fades to nothing ,no sound .If the amp is left for a while and powered up again it does the same thing over again.

SOURCE: Fender Hot Rod Deluxe. Turns on, sounds very good

More than likely a bad solder joint. Where is the next question. How confident are you with voltage measurments? The rails in tube amps are high and can kill you so if you are not inclined to do so please don't. The reason I suspect a solder joint is that it is a failure over time, cold solder joints expand and contract with heat. When they heat up they break connection. Not saying 100% that is your problem but it is very likley.

Testimonial: "Hey thanks for responding!...I am not savy at this so I will take it to an expert before those tuibes blow on my face. Thanks again!"

SOURCE: I own a Peavey XR696F

check any and all filter,cross-over,and de-coupling capacitors in every circuit especially on the left channel side,but check all capacitors anyway on both channels,as one channel can affect both left and right channels, also a capacitor can heal itself over time and do the same thing again later,"Cold-Solder" joints can do this also, on a printed-circuit board, HINT:heat and cold can cause problems so,try heating, joints,caps,etc. or buy a can of freeze mist(to cool components) at an electronic parts store and gently heat or cool each suspect component waiting for the outcome,or wait till the condition starts and then try the cooling,because the component has already been thermally warmed up by the circuitry power

SOURCE: I have Fender Hot Rod Deluxe and Blues Deluxe

I want you to do a test. while using the amp plug a pair of headphones into the preamp out... note you will only hear stuff on one side if they are stereo phones and the sound may be fairly weak... while the unit is operating correctely then unplug phones and use it. When it fades out plug in phones compare the level when it was OK... If the preamp output fades lower as well, then we know problem is in the preamp section...

We can do a lot of troubleshooting without opening the amp.

I see nothing in the scheamtics that should be an issue causing this...

Since it didn't fail for the tech, think "what was different?". Are you connecting something else? Foot pedal? Temperature? :Loudness of playing?

Now what else is in common to these two units... AHA! The guitar...

If your guitar has a battery in it, the 20 to 30 minutes fading would correspond to a battery failing and then recovering after you unplug the guitar for a while.

SOURCE: Fender Hot Rod Deluxe. Works great

As voltages drop, electronics gets unstable as it goes down and becomes noisy... this may be the nature of this beast. Opening the standby might be the best way to avoid the problem.. Here is your schematic for service:

http://elektrotanya.com/fender_hot-rod-deluxe_sch.pdf/download.html

Scroll down to "get manual" and click to download. IGNORE other download links.
Opening the standby switch turns off power to the main amp so noise generated in the preamp will not be amplified.