shevalier wrote on Today, 06:39:

It seems that noise from the reference has started to affect the dead-time control. Not because the capacitance is greater, but […]
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It seems that noise from the reference has started to affect the dead-time control.
Not because the capacitance is greater, but because that’s how it was designed from the outset; it’s just that the old capacitor has simply dried out.
I’d clean up the power of the TL494 itself and see what happens, because, manufacturers cut corners on every aspect of the TL494’s powering itself.

Ideally, a Cvref bypass capacitor of ~0.1 µF should be fitted (this is missing in 99% of power supplies).
Either a film capacitor or an X5/7 rated for 25+ volts will do.
And the Rvcc+Cvcc circuit for the TL494 itself should be located in the immediate vicinity of its pins.
In 80% of cases, this is missing; in 10%, only Rvcc is present
R – 4.7–22 Ω (SMD, 0805/1206)
C – 4.7–1 µF X5/7 rated for 50+ volts.
Radd in series with C_dt is often present. It should be approximately up to 5% of R_dt. It will slightly reduce the high-frequency noise passing through C_dt. SMD 0805/0603.

Cvcc and Cvref – either SMD, if there is a ground trace nearby, or for through-hole mounting. ‘A roof over the house’, capacitor simply on top of the chip. This was a common method for decoupling memory chips during the ZX Spectrum era 😀.

It turned out that there actually is a Rvcc+Cvcc circuit for the controller, it's that 4.7µF cap next to the 0.68µF cap in the earlier pictures. And there's also a 22Ω resistor as Rvcc. So I just added a 680nF cap between Vcc and ground to filter out high-frequency stuff that the electrolytic cap can't handle, and a 100nF cap between Vref and gnd. It looks like adding a resistor between Vref and the cap would have been a bit messy, so I skipped it for now. Then I powered it on again, and the whine was still there, but the odd thing happening on 12V rail was reduced.

I also decided to probe 12V line and Vcc at the same time, and it appears that there's something happening at the same time.

So far everything was good, but then things got weird. I heard some crackling and saw smoke rising from the PSU, after which I removed the power cord from wall. It looks like a big film cap next to the high-voltage capacitors was the source of the smoke. Also, it looks like one of the primary switching BJTs is now shorted between base and emitter.

I wonder what the heck just happened? The recap or the modifications done shouldn't have caused the PSU to blow up...

EDIT: Actually Vcc is ramping like that at 100Hz frequency, not 1kHz. So that comes from rectified mains frequency. But the question still remains that why was that visible from output? It doesn't explain why it blew up, either.