Troubleshooting Yamaha RX-A730

[SwiggityCricket]

Hi all,

I have an interesting failure with a Yamaha RX-A730 I picked up at a thriftstore a while ago. I used it for about a week when it stopped turning on, so I researched all sorts of failures, troubleshooting, how to reset the thing, etc and wasn't able to get very far. Similar receivers from Yamaha seem to have a "green cap" failure but I don't appear to have the same symptoms as them (also, I have no idea what a green cap is other than maybe a bypass cap?) I did try a few things but eventually put it on my shelf for later. Fast forward to last week when I was bored and decided to get it running again. I plugged all the boards back in and it magically worked for a few days until this morning.

Sometimes the red power indicator LED will turn on, sometimes it won't. When it is on it is flashing very rapidly; I don't mean the slow flashing that indicates it is in protection mode, I mean really fast to the point where if I wasn't watching for it I'd think it was solid on as usual. If I press the power button it will then flash like it's in protect mode, but I can't do the "disable protection mode" button combo to get anything to actually turn on. I don't hear any relays, etc.

So I started by measuring the output voltages from the power board. Page 140 of the service manual. Here is where it also gets weird. Sometimes the 5.6V rail is 5.6V, sometimes it's ~3V. The 3.3V rail swings between 2.9 and 3.3 depending on how the 5.6V rail is doing. Which makes sense, because if I'm reading the schematic correctly the 3.3V is derived from the 5.6V. That would also explain why the power LED is flashing like it is.

So I unplugged the supply board from everything else and tried measuring the output voltages there. 3.3V was nothing (expected), 5.6V was anywhere from nothing to 3V to 5.6V. Sounds like a cap or regulator is failing to me, but I'm having trouble reading the schematic. I am an electrical engineer but I've only been in the industry for 2 years and I'll be honest, I haven't done much engineering or real hardcore circuit analysis yet. Especially not with these kinds of power supplies.

The left side of the Sub-transformer is basically the recommended application implementation for the STR-Y6753 power IC. The right side is very similar but with a few changes to fit the transformer Yamaha used. (Datasheet here, pages 25 and 26)

I don't really know where to go from here or what specific components to test. I haven't hooked my oscilloscope up to anything yet but I bet there's a lot of ripple on the 5.6 and 3.3 lines, which would be an issue for the microcontroller(s) down the line. I don't have a dedicated LCR tester (just a multimeter). Any thoughts or advice is greatly appreciated.

One more thing, I am able to hear some screeching from the PS board when it is plugged in, and faint high-pitched clicking that slows in speed as power drains from the caps when it's unplugged. I know coil whine is a thing but this sounds more sinister than that. I don't know much about supply failures but maybe this means something for a cap or two, etc.

Service manual is here.
I've attached just page 140 of the pdf to this post for easy reference if you don't want to download the whole thing.

[SpecialK]

Measure AC ripple on the 5.6V line by using the ACV setting on your multimeter.  Probably you do have a bad electrolytic cap.  I would think 3.3V is derived from that supply but you do need to have a bit of overhead, so if the line sags the 3.3V will drop too.

[SwiggityCricket]

Ripple was kinda jumping all over the place, but it wasn't zero so that's bad.

I replaced all four of the smaller electrolytic caps on the power board and whadduyah know, it works now. Not sure why I didn't just do that as a precautionary troubleshooting step. It was only four caps, took like 10 minutes. Lesson learned, thanks for the help!