Home Cinema active Subwoofer malfunctioning

[lemofus]

Hello all !

Before I start, I am pretty much a newbie in electronics, I know a few basics, the simple things (Power/Voltage/Intensity/Resistance), and can just about identify resistors, transistors, capacitors and LED bridges (and that probably covers it !). Here goes the problem:

Symptoms
- When I switch the subwoofer on, from "cold" (transformer and capacitors unloaded), a big power jolt is sent through the 10" speaker. (see Video 1)
- Now, the interesting thing is, once the source is constant (film playing in DTS for example), the sub behaves normally. If, however, I pause (amp returns to Stereo/PCM default mode) and resume the film, another huge jolt is noticed.
- Similarly, when I switch from one channel to another, on the amp, an identical jolt is noticed each time. (see Video 2)

Video 1 :

Video 2 :

Picture 1 :

Picture 2 :

Picture 3 :

A few notes
- As far as I can tell, the audio source is not the issue : I've tried using a walkman as sound output, into the sub, and a few other things. The fact the jolt happens when the sub is not connected to any audio source (from a cold switch on) also tends to confirm that.
- The electricity supply in the house is fully earthed and matches expectations
- Importantly : this never used to happen, I know this comes from a mistake I made. I shorted something while the sub was on, having dropped a bit of foil inside the sub (silly, I know). I suspect what I shorted was the output wires (black and red) to the speaker, afterhand having noticed the wire connections were bare (from the connectors to the speaker core)

I have been doing a fair bit of reading, and have started by testing the two biggest transitors, the NPN and PNP transistors most obvious on the pics/videos. Both were fine (I followed a tutorial) apparently. Could it simply be a capacitor discharging each time, due to something else malfunctioning? Or, could it be a capacitor that is simply not doing its job anymore, and the jolt coming directly from the transformer?

I'd love to here your thoughts on this. And beyond repairing this subwoofer, i would love to use this as an opportunity to acquire some basic electronics skills and learn from this, the amp and board being pretty simple overall (it would seem!). I have a multimeter and soldering iron ready for action!

Any advice would be much appreciated!

Tony

 

[SeasonedTech]

Having read your post, and wondering why about the "tinfoil" thing, it seems to me that some sort of "damage" was done to this amp.
What you're asking as far as repairs, particularly without a service manual/schematic handy is virtually impossible.
Troubleshooting it due to customer errors (the tinfoil) increases the problem.
In addition, trying to get a schematic on these chinese-made electronics is in itself a laugh.
With I could be more help, but even as a professional bench tech, this is out of my ability.

 

[shrtrnd]

SeasonedTech pretty-much told you the way it is.
If you're going to try to do this yourself, I'd start with the capacitors.
With the power disconnected, short each of the caps between terminals, to make sure
they're discharged. Put an ohmmeter across each cap to look for a short or open (normally electrolytic caps will display inceasing or decreasing value on the ohmmeter display,
to show they're charging off the ohmmeter voltage of the test)
It may not do you much good, there may be a part in there that's specifically designed
to neutralize your audible bump, but when you're out of ideas, that's one to try.
Good luck, and keep your tin foil away from your electronics.