@moira The buzzing is probably coil whine in the transformer, from some subharmonic of the switching frequency that happens to be in the audible range.

It's possible there's a bad capacitor somewhere, like in the feedback for the controller, or if the power transistors have a snubber circuit.

@toroidalcore it seemed too low pitch for transformer whine but I didn't think about lower harmonics so maybe that's it

capacitors are easy to check, at least. ^_^

(this is not the same model as the one with the splody cap and the values are all different so ... sure hope I have some lol)

thanks ^_^

@moira You can get weird effects if something goes a little bit unstable, or unstable during certain load conditions. It's kind of hard to say without poking around with a meter or scope.

Good luck!

@toroidalcore @moira if it sounds more like buzzing, it could be the core laminations becoming, er, de-laminated. try squeezing the core (in the direction that squeezes the laminations together) and see if that helps at all.

admittedly this is more common with older, larger transformers.

@wohali @toroidalcore oh it's buzzy for sure

i'm kind of afraid of operating it with the case off given the previous [POW] incident tho'

@moira

Do you have access to an oscilloscope by any chance? I'd be curious what the output looks like when it's doing that.

@wohali

@toroidalcore @wohali well guess what's not making the noise and not cutting out anymore

so it must be heat dependant and there must've been some heat even tho' i didn't feel any

@toroidalcore @wohali WHELP we're back

big voltage drop and the buzz, gimmie a sec for recording

@toroidalcore @wohali and it's leaking 60hz (at very low AC voltage) at all times, I didn't check that before

also I thought the buzz was higher pitched than I'm hearing now but I could be just remembering that wrong

@moira

Is that AC-coupled? That looks like the AC component on top of the DC output. What DC output voltage are you expecting from it?
@wohali

@moira @toroidalcore that's not terrible AC overlay, but it could be improved with better output filtering.

i assume you're not getting the voltage you expect here? or is it just a buzzing?

as always with these things, suspect the electrolytic capacitors. look for any leakage or funny smell.

here's a very good intro video to how SMPS work if you need one: https://www.youtube.com/watch?v=3FyXqNcqvRM

@wohali @toroidalcore oh now when it buzzes it's dropping voltage like a mofo

and quick, too

this is all new, none of this was happening until this afternoon so something yearns to fuck off into the sea

@moira @toroidalcore there's not a lot going on in this design.

always suspect the electrolytic caps first. especially if this has been running hot somewhere, they'll be the first component likely to fail - and are often the first component cheaped out on by inexpensive contract manufacturers (China and elsewhere).

if you have a cap tester i'd pull each of the electrolytic caps and run them through it. be sure to discharge them first! you can probably skip C1 for now unless it smells or shows obvious signs of failure.

@wohali @toroidalcore the two little caps near single IC (very similar, separated by resistors) are showing weirdly high ESR in-circuit so that's fishy

the "nichicon" C1 is showing less than half its rated capacity in circuit which is also kinda sus

(I have a dedicated ESR meter, PEAK Atlas ESR70 Plus, sometimes you can get hints from in-circuit components with it)

@moira @toroidalcore ah, ok, i don't expect everyone to have an ESR meter, esp. not an in-circuit one.

the caps near the meter there are may not be as critical, esp. if they are just decoupling for Vcc on pin 7. (the 8-pin package won't have pins 11 and 12 broken out, they'll be connected internally.)

C1 is going to be the post-rectifier filter on the input side and, while it'd be interesting to look at, is probably something you're not setup to probe with your current gear?

you could check that the IC is at least getting the power it expects at pin 7 safely, if that's all over the place the controller isn't going to be doing the right thing at all.

@wohali @toroidalcore the esr is one of the relatively few fancy tools I have

both it and my true RMS multimeter say this 150uF cap is more like 70uF so that seems kinda bad