Sil-pads question

[northpoint]

Finally got a Kenwood KAC-927 amp fixed up. Had to replace the output transistors in it and clean it up a bit. At idle it seems to draw 1A and the led does change from green to orange- which on this model means its ready to go. However, When putting the aluminum case back on and powering it up it immediately spikes the current and accordingly a voltage drop. I am thinking about the power and output transistors have no sil-pads on them and thus the collectors might be shorting out with the heat sink and case. There was a lot of heat sink compound on the heatsink where they meet the case and where the transistors bolt to the heatsink. Im thinking at one time it had sil-pads but someone just decided to put heat sink compound on them instead and leave it like that.

Thats my theory. I cannot find anything else touching the mainboard except a stand off from the case cover that gets screwed to the motherboard for support. I isolated that to make sure and still had the problem.

So, Is it fair to say that when dealing with amps that basically any transistor should have a sil-pad between it and the heatsink? This would isolate them from conducting thru the heatsink and causing all kinds of havic?

I am asking this because when this arrived on my door step I do not remember any sil-pads during tear down. I cannot think of any other reason that this would short out when the cover is put on.

Educate me please. Attached is a pic of the areas I am referring too.

(Part# 7) cover is Aluminum and so are heatsinks (part # 725).



Seems to me the backs of the transistors should be isolated from the heatsinks with more than just a layer of compound.

Sorry if this is a bit rambling and long winded.

NP

[northpoint]

Re: Sil-pads question

The answer to this is yes. You must isolate the transistors from the heatsink because the metal backing on the transistor is not isolated from the pins on the transistor. This is what is probably causing the amp to suck up 6amps when put in the case. The case is acting like a big heat sink and has the heat sinks from transistors attached to it. Not a good thing. But now I know.

[euromatlox]

Re: Sil-pads question

Mica insulator sheets can be used instead of Sil-pads. Can be easily found for TO-3P and other common transistor cases. Thermal grease should be applied on both sides of the sheet. However it's better way to put the grease into transistor surface, and only the heatsink side of the Mica sheet to be greased. (Transistor contains the grease for the other side.)

Regular white thermal grease is suitable for most cases (like amplifier transistors).

The mechanical connection is usually done by metallic screw. In many cases the transistor's screw hole is in the plastic (non electrically conducting) part, if not, a small plastic 'hole insulating bush' is needed also. (Google: to-220 hole insulating bush - and you will find a picture.)

https://en.wikipedia.org/wiki/Mica

I personally have replaced some old amplifier's Sil-pads with Mica Sheets/thermal grease. I think Mica/grease is more effective, but needs more work than using Sil-pads.

Some transistor cases are all plastic (non conducting). Those do not need any insulator, just some grease.

[R_J]

Re: Sil-pads question

What transistors did you use? those should be all plastic.

[redwire]

Re: Sil-pads question

If you were running the amp with transistors off the heatsink, they heat up real fast and you can damage them.
You need insulators (i.e Silpads/mica) on the power transistors AND insulating bushings for their mounting bolts. Silpads alone don't do it because the screw grounds out the tab; in TO-220 parts.
No insulators are shown in the assembly pic, so your transistors probably are plastic insulated and don't need silpads.

With a multimeter, beep out for continuity from chassis to the power transistor's tab (middle pin). That will flush out any ground faults.

[northpoint]

Re: Sil-pads question

Just want to cover all the posts and thank everyone for replying:

I feel stupid. I ordered in some transistors - C1864 / A4886 to finish off a car amp. I did not notice that the had a bare metal backing. So, When I tested the amp out of the case it basically ran fine in initial power on tests. So, I assembled into the case and noticed a large current drain and voltage drop. I took it out and tested again and it was fine.

So, What I had done is not notice that one of the transistors was different than the others in that its packaging was not isolated from the heatsink. The others are coated to be insulated. Since they are fastened by screws I basically reordered with the proper package from Digikey. The originals I found on ebay and its probably my fault for not noticing the packaging of the transistors.

BTW --- Thank you very much for the info on Mica insulator sheets. Thats the way I will go for the power supply TO-220 mosfets.

I feel pretty stupid on this. But hey, I learned.

Concerning testing without heatsinks, I do an initial power up test to see if it kicks up and how much current the amp draws at just a ready state. Anything else it has to be on the heat sinks.

Thank you all for the education