On this one they don't sell just the transformer inside on the board, But that is the part most likely to have failed anyway as they will over time. One of the biggest killers of theseisi unplugging them after they have been powered for a long time. Te reason is that this part is live all the time when plugged in even when its off.
So the part heats up and thats fine, but then it cools if unplugged and usually cracks the tiny wire windings inside.
The best way to fix this is to replace the supply board.
You can get that from Sears parts direct or this site I prefer if they have one, I also have other sources if we end up needing to go there.
Please check : https://www.partstore.com/OpenSearch/Model/Onkyo+Corporation/Onkyo.aspx
Or call them as I would recommend as theydo not list all parts and products on site;-)
In any case, I will stick with you until this is complete.
Please rate kindly thus far, it won't cost you anything to rate my service to you kindly,
Then you can reach me by simply replying to this message anytime.
Thank you and have a great day Dave!