Marantz receiver hi level hiss

[Hesky]

For some reason 2 years after I brought my Marantz SR4200 av receiver it has started to produce a really loud hiss, it’s by no means a quiet preamp compared to some, but lately it’s got silly bad. I use external power amps and they are not the source of the noise. Even when I have a digital source feeding into the onboard dac it still has an abundance of noise at the preouts of the receiver. The amplitude of this noise is also unaffected by volume attenuation and tone control adjustment. Even stranger, if I allow it to cool down completely and then turn it on again it is nearly silent but then the hiss starts to reappear back to the same annoying level. I don’t fancy buying a new av receiver as a good one can cost 1000's and home cinema isn’t my main audio passion. I'd sooner spend that sort of cash on diy class a amp projects.
Before I take it apart I thought it a good idea to ask if anyone recognizes this problem and could possibly help me identify the course, I have my suspicions but it would be cool to hear what people think.

Cheers
Hesky

[jitter]

but then the hiss starts to reappear back to the same annoying level

Does the hiss become progressively louder, or does it suddenly appear at full volume?
Is it white noise or pink noise?

[Hesky]

I'm pretty sure it is white noise as its completely random sounding. I used a frequency generator which produces pink and white noise and the noise from the amp sounds almost identical to the generated white noise at the same level. The sound is quite constant, it may get a bit louder when running a little hot but its there from the moment you turn it on.
Took it apart to have a look at the general layout and see if there was anything obvious.


Preout Stage


Power Stage


Input Volume Stage


Decoder Stage