connecting transistors to heatsink

[jwilhelm]

If you're having issues with the thermal paste causing shorts it's likely the wrong stuff. Computer thermal pastes are usually conductive and shouldn't be used there.

It doesn't look like you have much cooling area there.

 

[john123456]

With single pads against the original aluminium chassis plate I was obviously getting a short.

That could have been caused by screws through the holes in the transistors. Didn't fancy taking the chance though.

Maybe in the future I'll tidy this up further and try to replicate the two bolt and paste of the original design. Will see how it goes.

 

[giaspyr]

The way you fastened them with just one bolt in the middle is not the best, the pressure is not the same on all four, the outer ones don't have the best contact with the heat sink.

 

[john123456]

Agreed. That is why I am thinking about adapting it.

However, with the thickness of the material I would say there is still significant clamping force even at the extremes.

 

[Mark Tillotson]

With single pads against the original aluminium chassis plate I was obviously getting a short.

That could have been caused by screws through the holes in the transistors. Didn't fancy taking the chance though.

Did you miss out the isulating bushings for the screws then?

 

[john123456]

They didn't come in the kit so didn't miss them out. The pads were supplied. So you can't really miss something which isn't included.

Have you got a link to something suitable?

 

[jwilhelm]

Those transistors shouldn't need any bushings. The through hole and surrounding area are plastic. If you didn't properly deburr the screw holes you would have issues though.

 

[turk 182]

it's amazing how little conductive things like aluminum dust can contaminate heat sink paste, even a small burr can be driven through a mica or sil pad under torque.

 

[nigelwright7557]

On my first amp build in 1980 I didnt deburr the transistor mounting holes and the burrs broke through the mica pads.
I always deburr holes now and also do a ohms test between transistor pins and heat sink before powering up.

 

[turk 182]

+10 !

 

[john123456]

On my first amp build in 1980 I didnt deburr the transistor mounting holes and the burrs broke through the mica pads.
I always deburr holes now and also do a ohms test between transistor pins and heat sink before powering up.

If I ever do anything like this again I'll certainly be taking those sorts of precautions. Overall with having to buy replacement parts and the cost of the case and all the other bits which have gone into it I could've bought a production amp of some sort... not a NAIM NAP 200... but something.

As mentioned on the cheap Chinese thread, the best of these replicas to my ears is the Quad 405 copy.

 

[john123456]

To conclude, I decided to buy yet another PCB and this time it has the floating heatsink very similar to the original NAIM design. I guess they do it that way for a reason.

I've listed the old boards on eBay if anyone fancies getting some components and PCBs for cheap: Naim NAP 200 clone PCBs - broken | eBay