Samson resolv 65a

Check it with your ohm meter. Is there continuity between the heat sink and -VDD? If yes (must be near 0 actually), then they are ok with just thermal grease. But my guess would be that they're isolated and you need to have the proper insulator and hardware to ensure that the tab does not contact the heatsink. If the heat sink is connected to fins sticking out the back, then it's definitely isolated and you need proper insulator pads.

The grease is not "lubrication". As said, it is for heat transfer.

A "flat" surface is never perfectly flat. When two flat surfaces are bolted together, they only touch on the tips of their roughness. Even if you polish with fine emery on glass, only about 1% of the metal actually touches. The other 99% is air, a lousy thermal conductor.

We fill this air-space with Thermal Grease. Classically a metal-oxide in a light grease. Use a THIN spread. While this used to be only for electronic techs, PC CPUs also use Thermal Grease, and instructions are easy to find.

Electrical insulation is different. (Grease can not be trusted.) Use whatever was there before. Mica wafers are classic. Rubber pads are also used (but usually not with grease). There are plastic films. If the original build used an insulator, then you must also!(*) Usually the original insulator may be re-used. Some go brittle (or get lost), find a replacement.

(*) Older TO-220 metal-tab power devices frequently needed added insulators. In the last decade some of these devices are available in "Insulated TO-220" packages, the metal tab embedded in epoxy, no added insulator normally needed.

squarewave said:

... and you need proper insulator pads.

Click to expand...

Let us not forget that little nylon stepped washer that goes under the screw head.

Gene

i had this mc2 amp  where the driver and powertransistors weren´t insulated from the heatsink .
that was  factorymade .
every time you´d come in touch with the heatsink ....ouch
i don´t know the reason why it was done this way , maybe better cooling because no insulation was necesarry ?

I'd definitely think cooling would be better with just grease if no insulation was needed....... Although Mackie did use some really thin,pliable gasket type stuff on their SRM V1 amps I noticed....It would squish down and form real well but I can't remember if it was used isolating as well or not....I don't think it was but would have to check......

I have now finished repairing this speaker and it seems to work fine now!

I replaced Q22, Q21, Z2 & the burnt R13.

Thanks for all help everybody!

Nice! Enjoy.