@longoverdue
Hi Edward,
My experience with old electronics is that capacitors deteriorate with age. Look for bulges on the top and leaking brown goo. I fixed a NAD amplifier and a Technics turntable by doing this.
Here is a link for the service manual for your receiver which should help:
https://www.hifiengine.com/manual_library/onkyo/tx-sv525.shtml
HiFi Engine is a very good site for old electronics. You may already know this but it may help others reading this.