Burning smell from Infinity Kappa 202a

[Compman55]

Finally my 202a gave up. It still works but puts off a burning PCB smell. It is not a typical electonics smell, but rather a nasty fiberglass smell. I looked and one of the power pins came unsoldered. It did not arc or burn anything in the amp. I slid the chassis out just enought to resolder all the power connections as they all had cold solder joints. Amp still work but puts off a horrible smell. I did not see anything in the amp discolored, but I did not slide the chassis out completely.

I did not reinsulate the FETs with kapton tape yet because there are no shorts, once I solve this smell, I will consider ripping it apart and doing that.

[event horizon]

Check the power supply capacitors to see if any have "leaked". If you can't see any burnt PCB that'll be about the only other thing unless something is blown that is likely to stink things out.

Take it from me that a PSU cap that has leaked will definately make a smell Look for possible liquid on the PCB around the caps, or the top of the caps where you might find a cracked case which has been ruptured by internal pressure.

[Compman55]

No leaking caps, that was my first thought as well.

[Perry Babin]

If you've broken the (physical) connection between the transistors and the heatsink, you need to pull the amp out and remove ALL of the insulator material from the heatsink and from the backs of the transistors. Then you need to replace the insulator material (kapton tape is probably the easiest). While you have it out, you can investigate more thoroughly.

I'm assuming that the original insulator was a silicone coated kapton and that much of the silicone stuck to the semiconductors when the board was removed.

The problem could be that one FET failed and continues to burn when powered up. Bad electrical connections on the power connectors could also do it but there would be obvious signs of the board overheating.

[Compman55]

Amazingly, and I don;t know how, but the insulators all stuck to the mosfets. I shined a light inside and none were loose, there was also a coating of white paste on them as well. The smell happened before I slid apart, and the amp has no short to the case. Could running on only one power connector fry the step up transformer inside so it is not visable?

[1moreamp]

Originally Posted by Compman55

Amazingly, and I don;t know how, but the insulators all stuck to the mosfets. I shined a light inside and none were loose, there was also a coating of white paste on them as well. The smell happened before I slid apart, and the amp has no short to the case. Could running on only one power connector fry the step up transformer inside so it is not visable?

Your amp has been opened before and the lessor tech used heat sink compound to try and compensate for the freshly made poor thermal contacts using the original sil-pads over again created.

I own two of these myself, and I rebuilt them several years ago. And Perry is dead on 110% telling you the truth about the damaged sil-pad issues. They all pull apart upon dis-assembly, and all must be replaced. I have never seen a situation to the otherwise.

And this amp requires TWO weco plugs due to its very high current draw. Don't be surprised if you find your Weco male pins de-soldered from the main board because of running the amp with just one power plug...They can be a real pleasure to re-solder back on to the PC board once the deed is done.
Spare parts for these are only available from dead amps by the way, so you might think about the fact spare parts do not exist for this era of amp, unless your tech has a great bone yard to pick from....Been there, done this, several times in fact. I even got the T-shirt...

[Compman55]

2 Points I need to verify. I did not "run" the amp with one power connector. I always have both plugged in, but being the pin came loose, it lost contact with the PCB completely. Therefore in my opinion, for how ever long I don't know, it was essentially running on one plug even though both were plugged in. I wonder if this could have overstressed somthing inside.

Secondly, I agree also about the sil pads, I saved your post from back in 2006 and am willing to pull apart and do all that with the kapton tape. However I don;t want to do this until the unit is fixed. As of now there is no short to the case so the sil pad must be OK enough, but not optimal. The warranty stickers were still intact, but maybe the grease I see is just melted sil pads.

[1moreamp]

Originally Posted by Compman55

but being the pin came loose, it lost contact with the PCB completely. Therefore in my opinion, for how ever long I don't know, it was essentially running on one plug even though both were plugged in. I wonder if this could have overstressed somthing inside..

I would say very much so YES it is possible that a bad connection is the heart of your issues I have replaced a many of these Weco pins over the years for bad connection issues and outright failures. But then again I also have replaced many of the older Rockford connectors also. Please LMK what you find, I am rather interested on a personal level to see whats causing the issues you describe, call it personal curiosity...