Help: H/K 730 Thermal Paste Advice

[giacomo1987]

I am fairly new to the vintage receiver collection/repair scene, having just recapped my dad's 330C a few months ago after I got my new 730 about 5 months ago. This week I started a recap project for my 730 and got to the point where I needed to do the power amp board. If you aren't familiar with the 730's structure, the main power amp transistors are on the opposite side of every other part on the board. They plug into sockets through the heat sink and both the heat sink and the transistors are fastened to the sockets with screws. In other words, I need to remove the heat sink and transistors to recap the power amp. My main question is, what do i do about the thermal paste? Is there anything I need to be careful about? I am not about to just remove the parts and re-paste the transistors without knowing exactly what to do, so a little bit of advice would be helpful to get my project back underway. With that in mind, if its as simple as it seems and I can just re-paste the transistors without worry, what is the best kind of paste and what kind of paste should be avoided? I can post pictures if need be.

 

[BinaryMike]

I clean up all of the old paste and use sil-pads instead of mica insulators almost always, but some folks dislike 'em. It's very important to insure that no burrs are present before reassembling with sil-pads. The newer ones are quite durable and highly engineered for thermal conduction. Unit-to-unit variation is almost nonexistent.

 

[giacomo1987]

So what you are saying is, I can clean off the grease and use only the sil-pads? My primary concern is grease on the prongs and inside the sockets possibly preventing electrical conduction with the transistor and the board (I don't know if this is even a problem, but I am assuming the worst).

 

[megawatt]

I use both sil-pads and thermal coat. I find both work equally well. if you are using so much paste that it is getting on the transistor leads, you are using too much. use as little of this stuff as you can. It does not take much.

that is right... you don't need grease with the sil-pads.

 

[Bigears]

You are looking for the minimal application of thermal paste...just enough to 100% coat the surface but no more. The idea is to improve the heat conduction between two metal surfaces that have micro gaps....the paste fills those gaps with a highly temperature conduction material.

 

[dshoaf]

Just beware of some pastes used to mount large processor chips for PCs. This stuff is conductive and will be a problem. Examine any paste you plan on using but I do, also, support the sil-pads. They're so much easier to work with and are more effective.

Cheers,

David

 

[PacificStereo]

I've always been of the "replace like-with-like" school of thought when it comes to devices on heat sinks.

Sil-pads are great, if the application will allow it. However, they all have a thermal resistance that's higher than mica and grease. Sometimes, you just can't get away with it.

 

[jan_stevns]

I've always been of the "replace like-with-like" school of thought when it comes to devices on heat sinks.

Sil-pads are great, if the application will allow it. However, they all have a thermal resistance that's higher than mica and grease. Sometimes, you just can't get away with it.

Then we are two some old B&o receivers did take a thermal runaway if sil-pads was used - not an easy fault to locate

 

[brutal]

I've always been of the "replace like-with-like" school of thought when it comes to devices on heat sinks.

Sil-pads are great, if the application will allow it. However, they all have a thermal resistance that's higher than mica and grease. Sometimes, you just can't get away with it.

I had several burn throughs with sil-pads, even after polishing to ensure no burs, so I won't ever use them again unless in the original application, metal backed collectors or not. Granted, I'm not likely to run into anything else with the old gear I work on (my own gear only).

Here's my method of application that always seems to result in a near perfect film that ends up with an expected amount of (minimal) ooze. I spread the paste, either on both sides of the mica or the backs of small devices mounted directly to their conductive sinks, with an old wood chisel. YMMV with other thermal pastes than what I use - silicone free Thermalcote.

 

[Oerets]

For most devices a rice grain size of paste spread out is all that's needed IMHO .



Barney

 

[newguy1]

Yeah, you don't need very much at all. Found a good pic here... http://techreport.com/articles.x/13671

 

[Bigears]

I've always been of the "replace like-with-like" school of thought when it comes to devices on heat sinks.

Sil-pads are great, if the application will allow it. However, they all have a thermal resistance that's higher than mica and grease. Sometimes, you just can't get away with it.

Excellent advice which I follow with good results. :thmbsp:

 

[giacomo1987]

Thanks for all the advice! I went ahead and used a similar thermal paste and just cleaned and reused the mica pads. All in all, it turned out to be an easy job. I have yet to test the results as I am waiting for a few more parts for the pre-amp.

 

[BinaryMike]

Sil-pads are great, if the application will allow it. However, they all have a thermal resistance that's higher than mica and grease. Sometimes, you just can't get away with it.

This quoted from the Thermal Design page at homo ludens electronicus:

"When using heat conducting grease between the surfaces, a TO-3 case to heat sink connection has a thermal resistance of about 0.4K/W. But when using a mica insulator between the two, there are two grease interfaces plus the mica, so the total thermal resistance of the connection goes up to about 1K/W!"

I think we must conclude from from this page at Aavid/Thermalloy that silicone rubber insulators are actually far superior with regard to thermal conductivity. The worst case for In-Sil-8 TO-3 pads is half the figure given above.