Technical > Electronics > random hiss from speaker - capacitor?

random hiss from speaker - capacitor?

1. What caused the cap to scorch? Might be a symptom of a larger issue.

2. Is there a cell phone anywhere the speaker [or the amp or the wiring] when it randomly hisses?

Thinking it was the proximity of the two resistors - they were both touching, so I gently gave some 'breathing room' there.

There is a 3G tablet (and sometimes cell phone) nearby most times, but the amp is in the sub, which is on the floor - about 2-3' of distance b/w devices and the amp. Unlikely to interfere, but not a bad thought, and certainly a possibility. We could try having those moved slightly.

reply to amungus
Do both channels do it?

Can you move it to another location and still get it?

Is the cap really scorched? I haven't seen that much in low voltage equip.

Could be a cap or transistor.

What ever happened to signal tracers?
--
--
--
"Facts do not cease to exist because they are ignored." - Aldous Huxley

"I’m not familiar precisely with exactly what I said, but I stand by what I said, whatever it was.." - Mitt Romney

reply to amungus
Ceramic caps usually don't go 'intermittent'... Just for grins, I'd take a closer look at those power resistor's leads for 'cold' solder connections on the board, and 're-wet' them with fresh solder if needed...

-Tursiops_G.
--
If You're Unsure, "RTFM"... If You're SURE, "RTFM" Anyway.

That's what I thought too. That those little ceramic guys were pretty solid. Worth asking though, since that's the only visibly bad area of the board.

Might check for cold solder, wouldn't surprise me at all...

As for channels - yes, since the sub contains the amp circuitry, it seems to send a little hiss to L/R channels.

reply to amungus
I'm not likely to suspect the Cap either. The resistors did the scorching. Those ceramics typically don't care. For a noise like that you'll likely have to get an idea of the signal path and try to follow it back. Typically this comes from a bad amp module, if that's what it has.

reply to amungus
no one noticed the cap at the bottom of the red line with it's wrapper coming off? it looks like it overheated!.

Yep, I missed that one. Being an electrolytic, it could be a source or the trouble. Either way if it did get hot enough to melt the plastic, it likely got hot enough to do further damage. Unlike the ceramic electrolytics are heat sensitive.

reply to amungus
Also lets not forget those resistors, if they are getting that hot, it's very likely that the solider joints have become crystallized and fractured, which is a very likely source of noise.

reply to amungus
How random is the random hiss, does it turn on constantly for 15 mins or more, or is it more like psssssssssssst stop psssssssssssssst etc with random length bursts and pauses over a few minutes?

If it's the second scenario, I suspect it's RF breakthrough from taxis, police or utilities radios where the carrier is breaking through, but modulation is not strong enough to hear any "signal". i.e. distant signals a mile or so off.... however, if a cab stopped right outside his house to check in, it might bleed through audio, (and blast him out of bed at 3AM if he left it on and turned up )

reply to amungus
Oh no, a nick in the heatshrink, containment is breached, she's going to take out the whole of Metropolis!!!

i.e. looks like nothing to me, unless your eyes are fooled by the diagonal cap behind it.

reply to AndrewG2
I see dust.

reply to AndrewG2

reply to amungus
Well said... it would of course be desirable to see round the side of it. But I've seen plenty that catch a mechanical nick like that on top, even "brand new" and some where the heatshrink had an imperfection like that (i.e. didn't shrink quite right). So on available information, even though I've also seen plenty of bulged, leaking and damaged caps also, I'd put down my 2c on that side of the equation.