Mosfet Issue

[sr1200]

I have an amp that has blown a mosfet. It looks as though this has been replaced before and from what im finding on line about this particular speaker/amp, its a common issue. The current mosfet is a 20N60 600v breakdown voltage 20 amp drain current.
If I were to put a mosfet that had a bit more current drain on it, would that help at all or is there some other spec that I should look for to perhaps give this a little more longevity?

TIA

 

[JohnRoberts]

That depends on why it failed... was it a punch through (over voltage) or melt down (over heat)?

Does the manufacturer have any advice?

JR

 

[sr1200]

From what ive seen and the amount of thermal paste that was caked on... im gonna go melt down. From the stuff I've found online, this was a very common problem with this amp. Replacing the part fixes it for a while, and depending on how hard its used, it may just burn out again. The fuse (7A) was blown as well (which was to be expected).

 

[JohnRoberts]

Back in the day I used to grind the tops off of TO-3 transistors to review the failure modes. Plastic devices are harder to open up without damaging the silicon.

I am not sure what the thermal paste tells you.

JR

 

[sr1200]

People tend to try an over compensate for a design that barely fits within thermal spec, thinking that somehow it will "fix" the issue. I see it in other non audio related devices all the time (people that build their own computers often do this). Caking on thermal paste can work against heat dissipation though, especially when you have a heat sink the size of a cinder block like this thing does.

 

[JohnRoberts]

People tend to try an over compensate for a design that barely fits within thermal spec, thinking that somehow it will "fix" the issue. I see it in other non audio related devices all the time (people that build their own computers often do this). Caking on thermal paste can work against heat dissipation though, especially when you have a heat sink the size of a cinder block like this thing does.

Ineffective band-aids to fix marginal designs are rarely done by the original designer but often by others in response to field failures.

Thermal resistance junction to ambient has multiple variables. Heat sink grease is generally helpful to fill tiny gaps and air holes to improve heat transfer. Its hard to use too much, as tightening the power devices will squeeze out excess thermal grease.

JR

 

[Newmarket]

What is the manufacturer / model ?

 

[sr1200]

Ineffective band-aids to fix marginal designs are rarely done by the original designer but often by others in response to field failures.

Thermal resistance junction to ambient has multiple variables. Heat sink grease is generally helpful to fill tiny gaps and air holes to improve heat transfer. Its hard to use too much, as tightening the power devices will squeeze out excess thermal grease.

JR

Theres nothing "pressing" this out unfortunately. In some cases, yes, by applying pressure to the heatsink, it will even out the mess, but this... this is just a spoon full of thermal paste on the back of a bunch of TO packages and a GIANT heatsink that the parts are only connected to via this paste. There are screws that connect the board to the heatsink, but based on what im seeing (the impression of the mosfet and other components in the goop) theres a good millimeter or more between the package and heatsink.

EDIT: Though, you may be correct in this, as i can tell im probably not the first or second one inside this thing.

 

[sr1200]

What is the manufacturer / model ?

Its a QSC K12.2

 

[JohnRoberts]

Theres nothing "pressing" this out unfortunately. In some cases, yes, by applying pressure to the heatsink, it will even out the mess, but this... this is just a spoon full of thermal paste on the back of a bunch of TO packages and a GIANT heatsink that the parts are only connected to via this paste. There are screws that connect the board to the heatsink, but based on what im seeing (the impression of the mosfet and other components in the goop) theres a good millimeter or more between the package and heatsink.

EDIT: Though, you may be correct in this, as i can tell im probably not the first or second one inside this thing.

That sounds very wrong and if true could lead to multiple problems. I have seen TO3 power devices secured with only one screw, and some modern power amps use spring clips to press the devices down onto the heat sink.

It is not paste to glue devices, it is a thermally conductive heat sink grease or compound with relatively low thermal resistance to improve heat transfer.

In fact to3 power transistor collector leads are connected to the circuit by the metal screw attaching the metal case to the heatsink. Most designs use a thin mica insulator between the energized metal can and the heat sink.

Please recheck how your power devices are attached?

JR

 

[radardoug]

You need to screw those devices down And as John said, check connections. All that paste will hinder, not help

 

[sr1200]

No mica here to speak of on any of the parts, just a ton of semi dried paste. (which ill need to scrape off and replace)

Theres 3 main boards (4 if you count the tiny power input where the switching is done). A preamp board, an LCD / UI board and the "power" board. Everything is connected by ribbon cables to each other. The main out that goes to the speakers is a 6 or 8 pin molex lookin thing. Its VERY tight and everything overlaps.

There is a strange plastic and metal bit that was supposed to be used to press the TO's onto the heatsink (it kinda looks like a razor blade in a plastic housing) But there was so much of this paste on here that the parts didnt actually make contact with the heatsink. Which is why im still going to guess its a thermal issue for now. Once i get the replacement parts in im going to see if i can put some decent higher grade thermal compound on there instead of this clay and have the mosfets actually make contact with the heatsink. Ill grab a pic next time im in there. Whomever replaced the part before me did a pretty heinous job of soldering it and left some nasty old solder on the solder side that they didnt clean up. Looks like they used plumbing solder theres so much lol.

 

[sr1200]

You need to screw those devices down And as John said, check connections. All that paste will hinder, not help

That was my concern when i opened up... it looked like someone used a whole container of the stuff. I removed about a teaspoon of it from one of the pairs and theres still a ton on there. I plan on cleaning all that up and attempting to do it "right".

 

[JohnRoberts]

I found this image... on EBAY https://www.ebay.com/itm/1161017153...oc:2&msclkid=2a4d8e1415cd102e9d28e83b988730cf

It looks like one screw in between each pair of devices holding a spring clamp, using thermal compound and thermal pads (insulators) between the devices and the heatsink.

Using too much thermal compound does not help, but should not hurt.

JR

 

[sr1200]

Yep thats it, now picure it with like 50x more thermal paste smeared all over that heatsink.