Replacing fets on amp, need recommendations

[xSPLoder 10+ year member]

dont you want to replace ALL fets in the amp with matched fets from same lot number?

i have the blueprints for the amp. 1/2 the board has the same fets, the other 1/2 has different ones.

 

[Spooney 10+ year member]

i have the blueprints for the amp. 1/2 the board has the same fets, the other 1/2 has different ones.

Do you know if you are replacing output fets or power supply fets? There could very well be two different fets used in the output section . If this is the case and you are replacing outputs I would probably replace the other half of the outputs as well. They all saw the same stress that killed the others whether they are reading as shorted or not. You also should do what you can to try and verify that the amplifiers drive circuitry is still intact. If the drive circuity has failed as well the amp may very well go right up in smoke again as soon as you power it on. Make sure you verify as well that any of the gate resistors associated with these fets are reading within tolerance . Sometimes we do get lucky and it is just fets but you might as well do all you can to figure that out before you power it up and destroy all of your newly replaced fets

 

[xSPLoder 10+ year member]

Do you know if you are replacing output fets or power supply fets? There could very well be two different fets used in the output section . If this is the case and you are replacing outputs I would probably replace the other half of the outputs as well. They all saw the same stress that killed the others whether they are reading as shorted or not. You also should do what you can to try and verify that the amplifiers drive circuitry is still intact. If the drive circuity has failed as well the amp may very well go right up in smoke again as soon as you power it on. Make sure you verify as well that any of the gate resistors associated with these fets are reading within tolerance . Sometimes we do get lucky and it is just fets but you might as well do all you can to figure that out before you power it up and destroy all of your newly replaced fets

it would be the input fets. i have the same exact spec fets going in that is being taken out to a T. all the fets on the in 1/2 were already replaced before I got it to finish it off

 

[Spooney 10+ year member]

Input fets = Power Supply fets . Definitely check those gate resistors for all of the power supply fets and the driver transistors as well. If you have a scope it would be a good idea to verify that the gate drive waveform looks good as well. Do you know if the amp is using a TL494/594 IC or an SG3525 to control the power supply?

 

[maylar 10+ year member]

Dow 340 was the compound of choice when I designed power supplies for a living. It was originally developed as an antisieze compound, and works great on the threads of your soldering iron tips. I can't believe how expensive it's become though. The other 2 choices would probably be fine too.

As for Kapton tape, they make insulators sized for the devices so you don't have to make your own. I always preferred mica to Kapton. But a lot of modern transistors have insulated cases so insulators aren't required.

 

[Spooney 10+ year member]

Dow 340 was the compound of choice when I designed power supplies for a living. It was originally developed as an antisieze compound, and works great on the threads of your soldering iron tips. I can't believe how expensive it's become though. The other 2 choices would probably be fine too.
As for Kapton tape, they make insulators sized for the devices so you don't have to make your own. I always preferred mica to Kapton. But a lot of modern transistors have insulated cases so insulators aren't required.

Mica I believe is superior in thermal conductivity to the kapton tape but that tape is so easy to just slap on the mating surface for the fets. If you have an amp with a lot of fets it can quickly become a pain to place all of those mica insulators and make sure they don't slide out from underneath the fets when you clamp them down. Also in some amps where I tried to use the mica I would find that the screws for the fet clamps would interfere with the mica insulator because some of those screws are much closer to the fets than others. The screws themselves would displace the insulator. When you use the tape you won't have this problem and the kapton tape is easily thermally conductive enough to be used in class d amplifers and has never given me an issue in any class ab stuff either. I will use mica or sil pads where I can though. Just gotta use whats best for the job

 

[maylar 10+ year member]

Mica I believe is superior in thermal conductivity to the kapton tape but that tape is so easy to just slap on the mating surface for the fets. If you have an amp with a lot of fets it can quickly become a pain to place all of those mica insulators and make sure they don't slide out from underneath the fets when you clamp them down. Also in some amps where I tried to use the mica I would find that the screws for the fet clamps would interfere with the mica insulator because some of those screws are much closer to the fets than others. The screws themselves would displace the insulator. When you use the tape you won't have this problem and the kapton tape is easily thermally conductive enough to be used in class d amplifers and has never given me an issue in any class ab stuff either. I will use mica or sil pads where I can though. Just gotta use whats best for the job

It's true that the pre-formed mica's can be wide enough to interfere with mounting clamps. Most of what I've done uses just the single hole with a nylon bushing so that doesn't matter. The big deal with Kapton was getting the assemblies to pass UL specs for high voltage isolation. It's so damm thin the slightest burr on the heatsink would arc. As you said, use what's best for the job.