Where can I get Manual for Sony STR-DE635 - Sony Audio Players & Recorders

Maybe the Preamp driver IC, looks like a car audio ic on the main board.
Often with age the heat generated by the ic will destroy the solder.Makes it look like lava rock in bar-b-que grill. Which
causes an open circuit in the preamp driver to the audio power
stage.
Simply resolder them, most often restores audio. Should have
a bottom removalble cover.
If not, a fuse normally blows if ic or channel is shorted. Check if
display will say Power off, Protect, etc. That means there is a problem with a channel.
Most Sony receivers use darlington transistors, they short easily !!
I am an old school component level service tech, I think I do have a schematic for this model and many others.

first turn the actual volume up on that channel on the reciver then make sure your speaker wires in the rear are using the same lenght each

Use this link. It is a PDF file you can download or print out if you need to.
http://esupport.sony.com/US/perl/model-documents.pl?mdl=STRDE635&LOC=3

Nope, power supply it would not even power up.  The capacitors are fried

Maybe, it worked for me. my rear speakers workt ramdomly, often not. so i re solder the relay for the rear speakers, now it works fine everytime, a hole movie and some random on and offs, it can't do any harme to try=)

It may need soldering of the display, could also be a bad resistor or transistor in the power supply.

If you still need it just search for it on google you will find it;) or you can get it from me.

sorry my expertise is in appliances, youl need to repost you question to another tech

Turn your volume down,press the tape monitor button.then turn up?

I suspect intermittent 31V supply, filament supply or bad connections on fluorescent display.
Check Q801 31V reg , R802, zener on base of Q801 located on DC board. Inspect all boards for bad solder joints, reflow and tighten pcb grounding screws.