MTX 405 Amplifier Smoking! Help

[johnkean]

Hey, I was hoping to get a little help with a smoking amplifier. I have an mtx 405 5 channel amplifier. Just today I was listening in the car at a normal volume and I started to hear a popping noise and when I looked at my passenger seat which is where my amp is installed under, there was a cloud of smoke. Thinking that the car was about to catch fire I pulled over and shut down power to the amp to keep anything from burning to badly. When I got home I opened the case and everything looked pretty nice but I did see a coil of green coated copper wire wound around something and this seems to be what got hot and smoked. It appears from looking at where it attaches to the board that one of the wire leads is broken. Is this an easy fix? it appears the wire is epoxied around something but I am unsure of how to proceed. The amp actually still plays just fine for about 5 minutes then starts to smoke. Any help I could get in repairing it would be great. Mtx said they no longer service this model but I would really like to get it working since it is hard to find a good, small 5 channel amp. Ill post some links to the pics of the part that I think is bad or damaged.

[johnkean]

Ok, I really looked closely and under the wires and I see some caps that have bulged and exploded. That explains the gooey clear stuff on the case.
Here is a Link to the picture of the board.

http://i395.photobucket.com/albums/p...5/SANY0001.jpg

[Perry Babin]

It sounds like you may be referring to the inductor for the class D channel.

Can you post a photo of the inside of the amp, the area where the component is located and a good quality close-up of the component?

[Andrew Eckhardt]

I can't be sure from the picture, but it is possible that the green wire'd inductor is an input filter and the caps that smoked are the rest of the B+ filter. A simple problem like this would explain why the amp seems to run OK. Unfortunately the wire harness runs right over where what looks like one of the capacitors in the bank failed. If you find that only one capacitor vented then you can replace it and everything should be ok. If more than one capacitor bulged and/or exploded you may have something more serious wrong with the power supply and will probably take an experienced tech to fix.

[Perry Babin]

The component with the green windings is the output filter for the class D amp. This inductor and the capacitors that have apparently blown remove the carrier (rail to rail square wave) from the class D amplifier's output.

Many of the MTX amps that used this type of inductor had problems with shorted windings. The windings tended to short where the terminal winding met the rest of the windings. This caused the capacitors to fail in those amps.

If the outputs haven't failed (they are probably OK since it's not drawing excessive current/blowing fuses), all you'll have to do is replace the blown caps (replace all in the group), insulate and resolder the inductor.

When you reinstall the inductor, you should place a drop of adhesive (Goop or similar) at several points at the base of the inductor. Don't completely fill the area under the inductor. It will make it very difficult to remove it if there is another problem in the future.

You MUST remove all of the electrolyte (gooey stuff?) from the amp. It's corrosive and conductive and will cause more damage if it's not removed completely.

[johnkean]

Here is a link to the swelling caps that blew up and the clear stuff that came out of them.

http://i395.photobucket.com/albums/p...5/SANY0006.jpg

http://i395.photobucket.com/albums/p...5/SANY0004.jpg

[Andrew Eckhardt]

Originally Posted by johnkean

Here is a link to the swelling caps that blew up and the clear stuff that came out of them.

http://i395.photobucket.com/albums/p...5/SANY0006.jpg

http://i395.photobucket.com/albums/p...5/SANY0004.jpg

That is an aweful lot of capacitance for a class D output filter, if that is what Perry is saying it is. In any case you'll need a pretty beefy iron to desolder that inductor and clear the short, if that is what happened.

[1moreamp]

I have seen this before and those are 22ufd NON-polar caps and they are dust.. I also have replaced them before and had the amp go right back into safe and proper operation. They use a large number of them to add up to a number somewhere just over 100ufd NON-polar value. I think it was a size related issue as to why so many and so small. These caps can be tough to find also, at least I had fun locating enough of them to repair two of these a few years ago..
So this is a common issue I have seen on several occasions. And this repair could be as simple as just replacing all of those blown caps and restoring the filter section to normal operation...

[Perry Babin]

1moreamp:
I think these may be polarized caps. I can't remember ever seeing non-polarized caps in an MTX amp. If you look between the wires of the first photo, I think you can just see the negative markings on the caps.

Andrew Eckhardt:
Some class D amps use as much 330uf across the outputs. One example is the old MTX 81000D. It had four 330uf in series parallel across the output.

johnkean:
Don't lose the thermal pad that's on the cover. It's needed to help sink the heat away from the inductor and into the top cover of the amp.

[Andrew Eckhardt]

Originally Posted by Perry Babin

Andrew Eckhardt:
Some class D amps use as much 330uf across the outputs. One example is the old MTX 81000D. It had four 330uf in series parallel across the output.

I guess for a sub channel it wouldn't be that surprising.