White Goo on Transistors

[jeffshort2000]

Hi,
I am replacing the output transistors on my Onkyo receiver ,and it appears that the 1st stage transistors have the protection circuit applied with some sort of white goo that resembles heat sink compound.

When I removed the 2 tranisistors from the board it looks more like silicon.

Does anyone know what I can use for this when I re-assemble it with the new parts? It was hard like glue...

[HK26147]

There are different varieties of heat sink compound:
I always put fresh compound when I replace and make sure any insulators are in good shape.

MCM carries them both Silicone and Silicone free, as well as insulators (mica and silicone).

[jeffshort2000]

Just ordered my parts from MCM friday and I am hoping to source this locally. They hit me with 8.99 shipping when it was shipped from my own state!

[HK26147]

Sorry to hear that;
Those "under minimum orders" usually end up with the S&H being more than than the item...
Whenever possible I wait or build up an order w MCM - they have frequent sales.
Sometimes I have to go to RS.
It been a long time since I've been in Cleveland but it's there locally.

I check with AllElectronics a lot - they have a flat $7 S&H fee,
They may not be the cheapest on everything, but they have an unusual variety of things, and I've saved a lot on hard to find items, and never have a problem with their service

[HK26147]

Try Newark:
6400 Rockside Rd, Independence, OH‎ - (216) 328-9173

[jeffshort2000]

Thanks! the only one that I knew of was the expensive TV parts place in Bedford.

[jeffshort2000]

So my thoughts are that I have to get some of the silicon variety. for this application? I have the standard grease for the power transistor to heat sinks. Correct?

[HK26147]

It's hard to make a living doing small electronics repair.
There were a lot of those places ( I use to visit one in downtown Cleveland ).
But it hard to cost justify the capital & training ( inventory ) to fix items items that are made cheap and disposable.
Most repairs places won't touch any electronics for less than $75 min.

If you find one of these guys ( check other electronic related suppliers as well ). I like to know, you can find some unusual deals sometimes.

I believe most use silicone for power transistors.

BTW: Some related stuff
http://sound.westhost.com/heatsinks.htm#9

[jeffshort2000]

The best place around here is the electronic surplus. It was in downtown cleveland and then moved to the near east side.

They just recently moved again to Mentor so it is a drive to get there. I need a reason to go out there to justify the hour drive.

I have been classicly trained in electronics and can fix most stuff to the component level. I would not want to do it for a living though.

For this application, I have a TO-220 transistor 1/2 of the push-pull first stage, with a small signal one leaned right next to it. There is no heat sink, but a pile of white goo connects the 2 transistors. I do not think that it is heat sink compound as the small transistor was really stuck to it.

[HK26147]

I am unfamiliar with that receiver;
Perhaps someone with familiarity of Onkyo protection circuit scheme can clarify.

But; Do both channel have the same "attachment".
Was it affixed to the metal base of the TO-220?
If both channels are arranged like this:
I'm guessing that the intent was to use the smaller semiconductor as a thermal feedback device on the TO-220?