Melted LM3886t on a Behringer KX1200

[frogman84]

Hey guys, i am a complete noob, and have just signed up to the forum since it seemed like there are some knowledgeable people here.

I have a KX1200 that i am trying to fix for a friend - it turned on but had a constant tone, even when turned down at volume.

I took the amp out, and after removing the heatsink i found that one of the LM3886t's was all melted. I removed this from the board, and after reading that the two LM's work in parallel, i turned the amp on to see if it would work. It did, and i was able to play through it fine for about ten minutes. Worked just as an amp should.

I gave it back to my friend to see if it holds up, but have told him to keeep it at lower volume levels for the moment.

I have ordered another LM and will attempt to fit it in the spare spot and see how it goes.

My questions were though: Is it ok to be running the amp at all with only one LM circuit in operation?

And also: when i looked at the LM's they still had a piece of plastic between them and the heatsink. I thought this might have been a factory fault and removed the plastic so that the metal of the LM could touch the metal of the heatsink (with some thermal paste applied also). Am i right in thinking that could be a factory fault or is the plastic strip supposed to be between heatsink and back of LM?

Note: Enzo has just informed me that this plastic strip should be left intact, so i shall make sure to replace it. What is the job if these piece of plastic? Does it allow the LM to heat up a little before it starts passing heat to the heatsink? And if there were problems with the LM chips overheating, could it help to have them mated straight to the heatsink?

Thanks guys.
Stuart, Australia.

[oc disorder]

Hi Frogman 84

"My questions were though: Is it ok to be running the amp at all with only one LM circuit in operation?"

I think Enzo has already answered this... not reccomended practice.
Would you drive a 4 cylinder car with only 2 working ?
Perhaps downhill to the local mechanic..if you get my drift.
You don't really want to run the risk of another burn out which could cause more destruction.

"Am i right in thinking that could be a factory fault or is the plastic strip supposed to be between heatsink and back of LM?"

I think you will find it is a mica washer to electrically insulate the back of the IC from the heatsink and to allow (with heat sink compound) thermal conduction to the heat sink.

Years ago I can remember mica being used in slow combustion stoves/wood heaters as windows to the fire.
It can take high temperatures but can easily flake and disintegrate.

These days silicon impregnated rubber (sili pads) are used as they are much less messy therefore eaiser to use in production..I'm still not convinced they are as thermally conductive as mica + heatsink compound but are sufficient in a lot of cases.

You must make sure all the surfaces are clean and perfectly flat and there are no bits of grit or solder particuarly metal shavings etc which could penetrate the mica or not allow good thermal transfer between
two surfaces.

So yes the washer is supposed to be there.

I have found with the boards its eaiser to cut out the component then just remove the debris which won't damage the board as much as trying to remove it in one go.

[Bill Guthke]

most heatsinks are attached to chassie ground.the insulator is to isolate the chip from ground as the tab maybe attached internally to part of the inside of the ic.the strip is either mica or the new silicon insulator that helps to conduct the heat transfer from the ic to the heat sink

[jrfrond]

First of all, about the LM3886: They are designed to be run in parallel as needed. Each one is good for 60W, and they are designed to be "stackable". The power supply voltage and current are upped accordingly.

As to the piece of plastic you found, it is called a Sil-Pad, and replaces the traditional thermal grease and mica insulator. You need to keep the insulator in there, either the Sil-Pad or mica with grease.