Help with TO-3 power transistor heatsink

SuperBassSam · #1 05-08-2011, 04:54 AM

Hi all,

I am repairing a Hughes & Kettner Bassforce XXL which was entirely missing its power transistors. Using the schematics I ordered the correct ones and know where they fit into the circuit. However I have the heatsink left inside the amp and I'm a little confused as to how it fits together (it was disassembled in a bag when I got it). I've attached some pictures of how I've put it all together and just wanted to check it's right (obviously minus soldering & thermal paste) - I assume the screws & nuts connect the case of the transistor to the board?

Any help would be extremely useful!

Thanks a lot

Jerrold Tiers · #2 05-08-2011, 08:47 AM

Not familiar with that unit.... but.....

I see the two finned parts are on a metal base..... in which case as long as that part is on the board correctly and does not short any traces, the rest of the mechanical assembly should be pretty self-evident as to how it fits.

DO get the transistors in the right places..... assuming they are different. And remember the insulators.

Yes the screws make the connection to the case in that type assembly.

That assembly is a little goofy, but it apparently works.

SuperBassSam · #3 05-08-2011, 02:01 PM

Hi Jerrold thanks a lot for replying. Yeah the metal base sits on the component side of the board and there are no traces or anything which it comes into contact with, I am fairly sure that's how the heatsink goes as I can't see any other sensible way to do it, I just wanted to make sure I wasn't making any huge mistakes!

I had indeed forgot insulators (or rather not realised I needed them, which now I think about it is a dumb thing to think.) I have done a little research and ordered them so thankyou very much for that. With the mica insulators do they need thermal paste between them and the transistor and also the finned section? Bit confused about where I should put it besides between the finned parts and metal base.

Thanks a lot, really grateful for the help!

BassmanPaul · #4 05-08-2011, 02:16 PM

If you buy the grey type of insulators they don't need heatsink compound. In fact if the heatsinks are not mounted an anything conductive and insulated from each other you bon't even need the insulators.

SuperBassSam · #5 05-08-2011, 02:24 PM

Ah okay, so would the grey type be the mica ones or not? And it also looks as if the finned piece of metal doesnt conduct electricity so that's good. So does that mean that if I don't put the insulators in I would need thermal compound between transistor and finned piece?

BassmanPaul · #6 05-08-2011, 02:32 PM

Quote:

Originally Posted by SuperBassSam

Ah okay, so would the grey type be the mica ones or not? And it also looks as if the finned piece of metal doesnt conduct electricity so that's good. So does that mean that if I don't put the insulators in I would need thermal compound between transistor and finned piece?

The sinks are plated and while they do conduct the plating does not. In this case, as the sinks are independent it doesn't matter if they carry a voltage. You will still need the compound.

My preference is to use the grey insulators anyway. Compound is so messy to deal with, it dries out and I just don't like the stuff.

Jerrold Tiers · #7 05-08-2011, 09:17 PM

I would not count on the anodizing being a good non-conductor, unless that was a design feature....... of which I am not sure.

otehrwise the two sinks will be connected together by the plate.

Now, it IS possible for a design to be OK with the cases shorted together, if they are the output node. There are amp designs like that, there are amp designs where the cases are grounded, and the output is at another point in the circuit, etc, etc.

I flat DO NOT KNOW how that circuit is set up. I don't know what H&K was doing there. The *most common* setup would require them to be isolated, and probably would require insulators.

I would grease the finned parts to the plate and both sides of the insulators, if they are mica, or if they are orange plastic ("kapton"). If they are a soft material, typically gray, greenish, or bluish, they are a silicone pad. Grease does not help and may actually hurt in that case.

if you do need grease, remember that you need very little..... do not slather it on, a thin coating is good, because all it is doing is to fill up the tiny spaces where the surfaces don't quite touch. Any more and it stops helping and becomes part of the problem.

WingKL · #8 05-08-2011, 10:10 PM

You might want to try the Aavid Ultrastick compound as a superior alternative to messy greases that dry out. Do not use those silver impregnated thermal compounds meant for CPUs. I found out the hard way that they aren't as insulating as they claim to be. Silicone pads do work but I personally worry about being able to punch clean correctly sized round holes in them for the transistor leads/ screws.

SuperBassSam · #9 05-09-2011, 05:41 AM

Thanks for all the replies everyone.

Jerrold I will use the insulators as I have ordered them now, I would assume that even if they aren't needed by the circuit they aren't going to do any harm being in there? And thanks a lot for your advice about where to grease, definitely cleared it up! I ordered the mica ones so I will use thermal paste on both sides. I have used it successfully on CPU's before so I feel fairly confident applying it.

WingKL, I checked out that Ultrastick and it looks brilliant, a lot better than the Arctic Silver which I was planning to use. The trouble is I can't find anywhere to order the Aavid compound, tried eBay and everything. Do you know anywhere where I might be able to get hold of some? Also being in the UK makes it more difficult as well. Would something like Arctic Silver Incorporated - Céramique do the job? It isn't silver impregnated and while the Ultrastick seems to be paraffin based I can't really find anything similar anywhere.

Thanks a lot!

WingKL · #10 05-10-2011, 02:59 AM

I think I got the Ultrastick from digikey but I can't seem to find it there anymore. It apparently is available from Allied but I don't know if they ship to UK. I would not use any thermal compound specified for CPU use, just to be safe. CPU's typically run at low voltages and the higher voltages on power amps could arc through the compound. I used the Arctic SIlver silver bearing stuff and it arc'ed and vapourised(!) a trace on my amp PCB and took out a transistor. Luckily I was able to jump the trace with a wire. Don't take that chance.

SuperBassSam · #11 05-10-2011, 09:51 AM

Yeah I looked on Digikey and couldn't see it, and I also don't think Allied will send to UK. I've emailed Aavid themselves asking where I can get it. Ouch at the arcing! Don't think I will be using that then :P Thanks for all the help anyway, if I do get it working I'll post a new thread detailing the repair.

chucko58 · #12 05-10-2011, 11:21 AM

The Arctic Silver Ceramique will be fine in this application.

Jerrold Tiers · #13 05-10-2011, 07:17 PM

Quote:

Originally Posted by WingKL

You might want to try the Aavid Ultrastick compound as a superior alternative to messy greases that dry out.

That stuff is the very best.

It actually comes from "Power devices" which is part of Loctite, which is part of Henkel, and the Henkel website is very nearly the worst I have ever seen, so you probably won't find it there.

Power Devices calls it "Thermstrate TC", that *may* help you find it. The repackaged stuff from AAVID is just as good. Digikey had it, Allied does have it.

Aavid Thermalloy - 100300F00000G - Chemicals - Chemicals & Adhesives - Allied Electronics