'Blown Amp'

[Asbo Sovfinger]

Tried fitting some bits and pieces to my Mk3 yesterday and a small problem appeared when trying to run the sub from a Kenwood KAC-714 mono amp.  Wired it all up in accordance with the instructions, fuses checked and replaced etc  but when trying to actually run it.....nothing.  Played around for a while, still nothing til the cry of 'we've got flames....well, smoke' issued forth my glamorous assistant.  The boot no smells slightly of TCP and although the amp is still getting power, in as much as the light is on, there is nothing coming from the sub.

Much later after some dash removal fun fitting the cd changer cable to an 'impossible-to-remove-no-matter-how-ingenious-the-home-made-removal-tools-got' head unit I took the back off the amp and it's obvious that 2 resistors have fried.....smelly.

Any ideas?  I'm hoping that it's just one of those things that's been waiting to happen in the guy's car who had it before me and it just gave out when we wired it up rather that anything serious with the wiring in my motor.  The only thing we could think of is that the spekaer in the sub enclosure got stuck and therefore created a massive resistance path (my glamorous assistant, Rich's, word not mine), but we both think that the fuse should've stopped any major problems before smoking occurred.

At the end of the day the gear was all free from a mate, so it's not really cost me anything except time on a Sunday afternoon, but it was all working in his car so something has happened somewhere.

[Ben Lessani]

Its usually not the resistors inside that burn out, its the MOSFETs that cook and the smell is the heatsink paste baking.

You can get the part no. for any blown components and replace them like for like from Maplin/RS. You'll need to find the root of the problem though, otherwise you're likely just to blow the same components again, and the going rate for low end amp's MOSFETs is about £4 a pop.

Do the obvious really, check for burn marks or shorts between traces. Lookout for any capacitors that are swollen/leaking - they are usually the most common cause of PCB failures, and can lead to a MOSFET popping.

Check the resistance over the terminals on your speakers, make sure the amp is stable down to that value (2/4/8 ohms).

Or maybe you just wired it wrong

[Asbo Sovfinger]

I think I understood about 10% of that. 

Here's a pic of the damage:



The paste around the heatsink appears to be ok, still white and the smoke definitely came from around the phono sockets which are in the general area of the nast brown smudgy burn mark highlighted.

Anyone make anything of this?

[Ben Lessani]

I'd take the heatsink off that transistor just to the right of the blown resistors and check it out.

Do me a favour and take a photo of the board side on - so you can see all the components from the side - that way you can see any deformation of the caps.

[Asbo Sovfinger]

You had to wait until I screwed it back together again didn't you?!     

2 minutes

[Asbo Sovfinger]

Best I could do



Bear in mind this amp was given to me for nothing so I'm not too worried about it, it was more to brainstorm some ideas about whether it was ckufed before I got it (or was just about to go) or if it's something we did.  Although I'm a bit ignorant with cars and electrics in general my mate who was with me is pretty savvy.