Do I need to us thermal paste?

[bcjm]

I am replacing two output transistor in my Sony amplifier. There is a big piece of blue protection plastic looking shield between the transistor and the heatsink. It is still in good condition after I removed the bad transistors.

Do I still need to use thermal paste when installing the new one or can I just put the transistor on top of the blue shield? What exactly does the therma paste do anyway?

 

[tensleep]

You need to use the thermal paste - it helps the flow of thermal energy from the transistor to the heat sink by creating a better bond and eliminating any air between the two. Air is an insulator while the paste is a thermal conductor.

 

[JURB]

I trust you have checked the drivers and the bias regulator, right ?

Anyway, if these are rubber insulators things, they are different than with mica. The rubber ones are not supposed to be reused, though you can. Ideally you keep them stuck to the heatsink, use the goop between the insulator and the new transistor. SPARINGLY. And you use a dawb of it in the middle so when you tighten it it spreads out. This prevents air pockets.

If the insulator is mica, it must be totally cleaned and inspected for delineation. (this is why I do not recommend the practice of systematically redoing all the transistor mounting in amps, if you split that mica you make it WORSE) This is when the layers separate internally. Look through it at something bright or whatever and look for what looks kinda like a bubble. If it has one it is split internally and is not good. Use at your own risk, what you'll have is alot higher temperature of the transistor to heatsink difference. Thermal resistance actually.

Anyway, with the rubber insulators the book says to not reuse them. If you do reuse them thrermal paste (we called it birdshit) will help because they only seat once. Kinda like the head gasket in your car.

If the rubber insulator just like falls off, you can probably reuse it without paste because the surfaces were apparently smooth enough that it didn't get stuck. It gets stuck because it oozes into the practically invisible nooks and crannies in the surfaces. If it isn't stuck it has still been compressed, but at least you are starting with flat.

In any case, the thermal paste used sparingly and properly is almost always better no matter what, even a totally plastic transistor. With some of those you can actually lap them in and get it even better, but you apparently do not have totally plastic transistors. Very rarely do you find insulators on totally plastic transistors but I have run across them. If you have that situation, eliminate the insulator and use the paste alone. The manufacturer only put the insultor there on a plastic transistor because it was easier than applying the paste.

You didn't mention the model, so I can't get any more specific. But that is how the heat thing works.

 

[bcjm]

Thank you for the information.
How do I check the driver and bias regulator? What kind of check do I need to perform?

My amplifier blows fuse every time I turn it on. One pair of transistor are shorted. That is all I have checked so far.