Of course, you could always check the capacitors to make sure there are not that are bulging, even slightly. But from what I have seen in the past, this isn't a problem with the power supply or components related to that - I've only ever been able to correct it by replacing the whole main board. Which today wouldn't be worth the cost. I don't know of any way to test the ICs on the board. And I also don't have a schematic for this one. Yamaha used to make basically two classes of products, those that were "non-commissionable" and those that were "commissionable". The latter were all their really good stuff, which is where your receiver falls, and because of the money in those (at least I think), the schematics weren't easy to come by outside of the company.
Hey... I'm a huge fan of Harmon Karden. I wouldn't dissuade you from going with all Harmon! These days, price differences in high end systems aren't as high as they used to be (or, when you're in your twenties, maybe they just seemed bigger).
I think you would be absolutely amazed at the amount of dimension a larger sound system would bring to a 10x13 space. I have about the same size room with a projector on one wall. It only sits a few people, but when I upgraded to 7.1, I was blown away. I didn't even change the speakers - just added some and changed the receiver.
For replacements, I still do love Yamaha. They still have these two classes of products, but it's not always clear which is which. It used to be that you could get the higher end stuff at tweeter and the lower end at big box stores, but that's no longer the case. They're all mixed in together. But the price gives it away. If you ever wonder why there are two Yamaha receivers with similar specs and vastly different prices - that's why. The more expensive one in this case is a superior product. And I will always love Harmon. In fact, that's my favorite, but it's also expensive. If you're using HDMI, you'll definitely want that on the receiver. It's pretty common today, but wasn't several years ago.