[SOLVED] Need help diagnosing a "no sound" issue from subwoofer

[Turrican3]

@Alex Wolf

Thanks mate, I really appreciate this.

I'm actually more than willing to repair the Infinity amp... to tell you the truth, I already fiddled a bit with it but it's waaay too complex for my current knowledge (which is very very VERY basic) and to make things worse I'm always wary when there's 230V AC around.

But perhaps with a fair amount of help / guidance, who knows, I might actually end up succeeding at this repair.

[Alex Wolf]

Wait I think I saved one shot so perhaps it may help everybody see what I was dealing with.

What's shown in the attached photo is the back of the subwoofer with the amplifier in a working position, you can see the inner part of the plastic box with conductive paint.

Perfect! Now I can be clever (😁) and say that it doesn't look like a paint, but like a metallization. You can see this suspiciously shiny "paint" in many places, especially very often in laptops and the like.

Now, the amp was initially mounted in a way upper position. And it was VERY tight. So my conclusion is that the solder side of the electronics that can be seen here were making contact in at least one point, making a short or something like that.

As a matter of fact, as soon as I removed the amp from the chassis (or simply shifted the installation in a lower position, as per photo) it started working again.

Okay, let's do a little analysis.

Here are photos of the insides of this module (with some of my notes):



As I can see, there is sufficient clearance between the switch-mode power supply board (SMPS) and the metallized case. The same cannot be said about the audio power amplifier board (PA) installed at right angle.

I guess the most likely reason lies somewhere here:

[Alex Wolf]

I'm actually more than willing to repair the Infinity amp... to tell you the truth, I already fiddled a bit with it but it's waaay too complex for my current knowledge (which is very very VERY basic) and to make things worse I'm always wary when there's 230V AC around.

But perhaps with a fair amount of help / guidance, who knows, I might actually end up succeeding at this repair.

It is absolutely right to be wary of mains voltage: it is a high energy source with relatively low impedance (speaking of a household outlet). And more importantly: it is a high-voltage AC source for which the impedance of the human body is very low, as a result it can be very dangerous, and even fatal injuries can be caused. So working with mains powered boards, you really must be as careful as possible, ideally use an isolation transformer (it will not protect against electric shock, but the circuit will no longer be referenced to the mains voltage, and the impedance of the power supply will be much higher). As a workaround a pair of similar small mains (50/60 Hz) transformers can be used for this puprose, for example: [230VAC->24VAC]->[24VAC->230VAC]. It goes without saying that measuring instruments must comply with CAT II 600V at least. And avoid grounding your body when you do this.

In all other respects, no one knows the answers to all the questions. But those who don't ask questions at all will never know anything. 😉

[Turrican3]

I guess the most likely reason lies somewhere here:

Yup that's my not-so-educated guess too!

In all other respects, no one knows the answers to all the questions. But those who don't ask questions at all will never know anything. 😉

Absolutely right!