Need help with blown amplified M-Audio speakers

[ogandou]

Hello, I've been redirected here from the ax84 forums, hopefully someone can help me.

I have this pair of amplified M-audio speakers, the AV-40. About 6 months ago or more, I was listening to some music on my PC while my wife and I were doing the bed. Pulled out the heavy, thick blanket from the bed... I take it, and I shake it before folding it (by that point I'm standing about 4 feet from my computer and the amplified speakers - and just so you know we have carpet in the bedroom...), and as I shake the blanket, that freak, humungus static discharge occurs. Immediately my speakers start going bzzzzz, and I can't get any more sound from them other than this loud hum. Crap...

I'll gloss over the impossibility to get support from M-audio and the difficulty of finding someone to fix those. So I'm thinking, probably fooling myself, that maybe, just maybe I can fix them.

I've opened them, checked the voltage at the output at the transformer as well as at the bridge rectifier, everything seems to be fine there. But the stereo amp chip (TDA7265) runs HOT as hell. Gives off so much heat that it burns my hand when I touch the back of the speaker, literally - despite the somewhat bulky heatsink it's attached to. And it wasn't that hot before the "accident", I would have noticed it. It's THAT hot.

Since it's sort of cheap, I'm going to try swapping the TDA7265 with a new one to see if that could be the problem. I was just wondering if someone had an actual idea of what the problem could be before I do anything, or if I should check a few more thing before putting in the new chip - I don't want to fry it because something else is broken that I don't know of. I'm not all that familiar with troubleshooting that kind of equipment so I could simply be headed for failure! We'll see...

Anyway, I thank everybody in advance for any help you could give, it'd be most appreciated.

[!]

It would help if we knew if the amp circuit just uses TDA7265 or if there is other active circuitry before it, like transistors or an op-amp. A nice top-down picture perhaps.

Without any audio signal being fed into the speakers, and of course being careful not to electrocute yourself, use a multimeter to measure what the voltage is at the amp chip's audio input pins for left and right channel. Without there being an audio signal supplied this should be a VERY low voltage ideally approaching 0V. If it's much above 0V you probably have something shorted or leaking in the circuit prior to the amp chip and so would trace the circuit back to the next active component and so on. If it's not much above 0V it's probably the amp chip (TDA7265) itself.

[simon7000]

The TDA7265 uses input capacitors so if it gets hot it is bad,

[!]

^ I'm not quite following what you mean simon, while the datasheet shows input coupling caps, it's not necessarily the case that you HAVE to use them right before the chip (nor even at all if there is no DC present). It's very likely the amp circuit does have the coupling caps somewhere, but they could be right after the input, before an opamp or something discrete which then goes to the TDA7265.

[simon7000]

With 600 mv of output offset by itself, there will be input capacitors!

[ogandou]

OK, hopefully this picture will help but I'm not sure... I'm going to try to measure the voltage you mentioned. I've already replaced those 2 big caps on the top left, they seemed to have leaked.

[ogandou]

Voltage was 5 mv... I'm pulling this thing off. Is it safe to power the whole thing with the chip removed?

[ogandou]

Well, I put in the new TDA7265, and it works like a charm! Woo!