Power Supply problem for active studio monitor

[jackbb87]

Hi all,

I currently own 2 x Alesis M1 Actives mk2 and the other day one of them went pop when i turned it on. I opened it up to find a blown 5A fuse. I had a look at the board and could not see anything out of the ordinary. So i replaced the fuse and it blew again! This time though it was a loud bang and the fuse like literally blew the glass.

So now i had another look and found a busted resistor and a burn mark between a couple of transistors and a capacitor. My theory is maybe a chunk of the resistor fell off and caused the short.

My Question is:
Is it still possible to save this board by replacing the transistors and cleaning up the burn marks? or shall i throw away and start again

Forgive me i know a little about electronics but i am no means a master.
http://imgur.com/ITGA0Fg
http://imgur.com/MSBnrHV
http://destiny-files.com/image/default_large/BBA0045.jpg

 

[Heliman]

For the fuse to blow like that you have a dead short. Id be checking those transistors and any rectifiers found on the board. The resistor burned up due to excessive current being drawn threw it because of a short elsewhere in the circuit. You can repair any board, the question is time and money vs cost of new board.

 

[Heliman]

I would check Q1 first.

 

[Heliman]

I will also add before you disturb anything on the board, see if you can read all the color bands on that resistor so you know what value to replace it with. Otherwise you'll need a schematic or look at an identical board to get the value.

 

[jackbb87]

awesome thanks mate. Yeh i could read the colours Red Red Silver gold, i got that at 220mOhm with 5% tolerance. I got that on order. Ok ill check the transitors and rectifiers. So shall i start with Q1 and work my way through the circuit. I found a schematic.

 

[Heliman]

I would start with Q1, being on a heat sink it handles alot of power. A short in it would cause big problems.

 

[jackbb87]

Ok ill try that.

Another question is i can see a lot of this yellow goop on the board, im assuming its some kind of insulator. Although around the c8 capacitor the goop has turned brown. Do you know what this would be from? too much heat?

 

[Heliman]

It appears to be glue. Some manufacturers will use glue to secure components in place. Capacitor leakage will normally be wet or oily.

 

[Heliman]

I didnt see that second component on that heat sink in the other pictures. If Q1 checks OK Id check that second one also.

 

[jackbb87]

OK so ive googled how to test these transistors. My understanding is there are MOSFET so i used this technique.
http://www.4qdtec.com/mostest.html

However im unsure as whether my multimeter has enough power to charge the Gate. As when i touch my finger on the G and D the transistor the reading between D and S does not get higher.

 

[jackbb87]

So on Q1 between the drain and the source without putting any voltage on the gate i have a circuit. It seems it is constantly closed. Would that be my short?

EDIT: Ok so Q2 is a dead short between all 3 (Drain, Source and Gate, it also has some extensive heat damage. the insulator that seperates the bolt from the transistor is completely melted. Q2 is exactly the same as Q1

 

[Heliman]

In your case your looking for dead shorts. It sounds like Q2 is the culprit as all 3 leads are shorted. Since both Q1 and Q2 are the same Id replace both of them while you have them out. Be sure to use heat sink compound when reassembling them..