To do this kind of repair, you usually need circuit diagrams to guide your work. These are sometimes found in the operating manual, but are more likely to be found in a product service manual.

Documents for many products are available online for free, even for fairly old items like this receiver.

In this case, I lucked out because the manufacturer web site had the operating manual as well as the service manual.

If the manufacturer doesn't provide a service manual, you can sometimes find one on the web. Some sites want you to pay or install their software to download the manual; I avoid these because they might be scams. Others want your email address so they can SPAM you, but you can always block them once you get what you want.