What could make a high pitched whine a PSU?

[Spork Schivago]

Hi! I have a hisense 55h6sg TV here and I'm going to start a thread in the TV repair sections tomorrow, properly.

But out of curiosity, what would make a high pitched whine in the power supply? I thought it was coil-whine, but seeing how it keeps blowing the same 1 watt resistor next to the primary capacitors, I'm starting to think it has something to do with one of the transformers.

Would a transformer ever make such a noise like that? Kinda cool to hear the resistor explode. Put a fire proof little guy, 1 watt in there and it sounded like a .22 cal going off! Scared the shit out of my wife, but I was kinda expecting it.

TV showed a bootup logo before it exploded though.

[R_J]

Re: What could make a high pitched whine a PSU?

A short in the secondary can load the primary causing the whine, the power supply is trying to work but is driving into too much of a load, could be a shorted diode in the scondary.

[Spork Schivago]

Re: What could make a high pitched whine a PSU?

I was going to check the diodes and was leaning towards them, thinking the resistor was handling too much of a load, caused by a shorted out diode.

I don't understand the whine though. Is the resistor making it? Is the diode making it? Where does it actually come from?

[R_J]

Re: What could make a high pitched whine a PSU?

The circuit should be running at lets say 60-80khz. With a heavy load or short the circuit will be running at a lot lower frequency and the wave form is likely very destorted, maybe around 15khz (audible) the noise you here is coming from the transformer windings.
In some circuits the drive ic can actually cause it to go into a burst mode during a short, in that case you would here a chirping sound from the circuit as it trys to start but fails, trys again, and repeats.

[Spork Schivago]

Re: What could make a high pitched whine a PSU?

The circuit should be running at lets say 60-80khz. With a heavy load or short the circuit will be running at a lot lower frequency and the wave form is likely very destorted, maybe around 15khz (audible) the noise you here is coming from the transformer windings.
In some circuits the drive ic can actually cause it to go into a burst mode during a short, in that case you would here a chirping sound from the circuit as it trys to start but fails, trys again, and repeats.

Thank you for that explanation! It really did sound like it was coming from the transformer, but to me, I was thinking perhaps the transformer might be bad. But after seeing the startup pic on the TV, I figured I must be wrong with my evaluation. Once my wife comes home from work, I'll start checking the diodes and see what I find.

Thanks for the info.

[Spork Schivago]

Re: What could make a high pitched whine a PSU?

R806 and R811 look damaged, however, the through-hole diodes all seem to test fine. I used my DMM and my DCA meter to test them all, they all seem to check out. VD902 is a surface mount component. Through-hole diodes have the VD### silkscreen schema, so I'd imagine that's a surface mount diode, but might be hard to test.

Tracing the burnt out R806 resistor, where I cannot read the markings, that goes to a bridge rectifier. But I don't see how that could be bad, seeing how a startup image successfully showed on the TV. But I guess I should pull it and test it anyway, just to make sure.

Running out of ideas here now. Besides replacing the burned out resistors and maybe trying to somehow test that surface mount (possibly?) diode, I dunno what else to do.

Cheap board, cheap TV.