Yamaha RX-479 PSU exploded

[Satdude]

Hi, I have a Yamaha RX-497 receiver, and one day I came home to a burnt smell and the breaker was tripped. Upon opening the receiver, I saw extreme damage to the main power board. C254 was exploded, R261 was burnt, D255 was shorted, Q254 is open circuit, D261 (bridge rectifier) is also open, traces to IC252 (TC4013BP) are vaporized right off the board. Not sure about any other damage, I'll have to desolder the flip flop to test it. Also the transformer T251 is getting hot when plugged in. What could have caused such extreme damage just sitting plugged in? Only repair that was done in the past was a replacement of C254 when the receiver didn't power on anymore. I can of course replace all of the components mentioned, but I don't want to have the PSU blow up again. Can this fault also take the main board with it? If there are any experts in these Yamaha receivers please let me know. I attached some pictures and a schematic of the two boards that are interconnected.

[petehall347]

i might be checking ac and dc voltages at t251 .

[Satdude]

So after replacing the MOSFET Q254 K3850 with a 2761i, C254 with a .22 WIMA Capacitor, IC252 TC4013 with a Tesla MHF4013 CMOS D-FlipFlop, D261 SN1B60 recitfier, D255 with a 10V zener (2x 5.1V) and repairing the burn traces, the amp came back to life. We found out the board touched the metal casing, so I put some electrical tape there to stop it shorting in the fu ture.

[stj]

why the hell does it have a mosfet that shorts the bridge rectifier output?

[redwire]

why the hell does it have a mosfet that shorts the bridge rectifier output?

It's a triac Yamaha has made using the mosfet+bridge rectifier, like an SSR. But this takes much less current overall to turn on, compared to a triac. The section is powered by a capacitive dropper C254.


Any power surge, mains transient, lightning, electric utility wiring cross-connect etc. would kill the circuit as in the pictures. There should be other electronics damaged in the house as well, which can prove it.
You need MOV's or gas tubes for surge protection.

[momaka]

That sounds like a clever and a silly circuit at the same time.

In any case, I would suggest OP to use an X2 -rated cap for that position, given that the capacitance is rather small. While X2-class caps won't necessarily always go open-circuit in the case of a surge / over-voltage, they are usually a little more tolerant of abuse / line voltage spikes. So using one as a capacitative dropper might yield better reliability.

[redwire]

It's only 22nF, a very low power circuit.

In many Yamahas that mosfet or dropper cap frequently fails. 2SK3850 rated only 600V, C254 630V - in something blocking mains. I think they needed an MOV to cover mains surges but then you'd need a fuse too, that would break the piggy bank lol.
The circuit is clever and hard to troubleshoot - they have power-on reset off, zero-cross switching, undervolt detect IC251, overvolt detect IC253 with the 10V VSB.
But a say 1,000V spike will kill it all like OP has shown.

[Satdude]

I think the damage happened after the metal case somehow touched one of the pins of the chip, because there is a darker spot on the case and one of the legs was completely gone. The amp was fine for 2 years but was recently moved and now did this. I added some electrical tape to the case to prevent it shorting in the Future.

[redwire]

I think that is from smoke and soot, blast damage going on the case. I'm not sure if the case is grounded, it can be through HDMI to the TV etc. - oddball paths. But you could be right.
I would consider adding a surge-protector power-strip maybe. Not sure how good power quality at your place.