Phillips screw drivers # 1 and  #2

Soldering Iron, preferably a 25 or 40 watt model

Solder,  60/40 resin core only!   I prefer Kester "44"  dia. .032

1 sheet of 220 grit   Wetordry   (Wet or Dry) 3M brand paper or equiv.

1 sheet of 600 grit   Wetordry   (Wet or Dry) 3M brand paper or equiv.

Thermal Heat Sink Compound, Radio Shack Brand or Arctic Silver

Small plastic stick or Popsicle stick

1 Fine tooth double-cut 4 or 6 inch warding file, commonly used by locksmiths ( my personal preference)

1 Single Edge razor blade , commonly call a utility knife blade

1  Electric Drill

1  1/4 or 3/8ths steel drill bit

Saran Wrap , any plastic wrap will do

1 pair of good eyes......1 pair of steady hands.....and a reasonable amount of patience

Now , the first thing you will want to do, after you remove the cover, is to observe!!! Take a good look at the PCB BEFORE you remove it from the heat sink.  Look at the parts layout and the soldered connections.  Locate ALL of the PCB mounting screws and  the screws that hold both front and rear panels to the heat sink.  When you remove the screws, keep them separated, they should go back into the same places where you removed them from.

Don't bend any parts to 'straighten them up' and don't bend any of the open coils while working on your unit.

Gently remove the PCB from the heat sink, by lifting the PCB straight up. Again, now is the time to observe !!  Look at the heat sink for the 'pattern' left by thermal compound of the final output transistors. If your final output transistors were mounted correctly, you should see a thin even layer of white thermal heat sink compound on both the heat sink and the bottom of the final transistor mounting tabs. If you see a lot of compound in the middle of the two mounting tabs on any one final, that means the final mounting 'base/flange' is bowed or warped. The idea here is to lightly file and sand the surface back to a flat condition.  The flange is reasonably thick, and can take a fair amount to 'lapping ' to bring it back flat. I started off by lightly filing the flange to identify both the high and low spots. The important thing to remember is, work slow, keep the file pressed flat and lightly against the face of the flange and use gentle strokes.  As you work the surface, pause frequently to make sure you are not rounding the edges.  When properly done, the surface will appear uniform.  It is not essential that you get every last speck of imperfection out of the surface, use good judgement. Next use the 600 grit emery paper to finish the surface.  You do not want the surface to be mirror smooth. Do not polish the surface with a grit finer than 600 grit. Tests have shown that 2 mirror surfaces when mated with thermal compound will actually transfer LESS heat between them because of the microscopic nature of the mating surfaces. Take note of the brass stand off between the two final transistors too.  You may have to slightly file the standoff to the same height as the final output transistor flanges. You certainly would not want the standoff to hold the flanges up off of the heat sink. You can place a straight edge across all three to check if it's too high.

Before we put the PCB aside, now is a good time to make any solder repairs on any cold solder joints you may have noticed and to remove any offending solder splashes and potential short circuits at the parts outlined.

We can now concentrate on the heat sink itself.  As I stated earlier,  over torqued mounting screws can cause the heat sink mounting holes to become distorted. Even if it's not readily apparent, aluminum will 'creep' over time and lift the final output transistors from the heat sink if too much force is applied.  As the aluminum creeps, the mounting screws will exert less force to hold the transistor to the heat sink, breaking the thermal bond. The result is, the transistor will not be able to dissipate the heat it produces and will fail.  The use of cupped washers to maintain constant mounting force and to distribute that force along with proper fastening  torque  are essential.  Heat and driving a semiconductor beyond it's design limits are certain to shorten it's life.

The next step is to remove any old thermal heat sink compound from the surfaces of the heat sink. Look closely at the mounting holes. You may see slight peaks around the mounting holes. This is a sure sign that too much force was used to mount the final. Using the drill and bit, slightly countersink the tops of the mounting hole. The idea is to remove the protruding mound , and bring the surface around the hole slightly below the surface of the heat sink.  Countersinking the holes is done very gently, do not use a lot of force.  You can use the straight blade of the single edge razor blade to check for flatness at the top of the hole. Don't go nuts with the drill as you are going to sand the top of the holes next.  Use the 200 grit emery paper to sand the tops on the mounting holes.  You will notice while sanding, that the area around the hole will appear to get shinny as you remove some of the black anodized layer and a ring will appear to get bigger and bigger around the mounting hole. You are removing the 'mound' that was formed by the over tightening of the screws and this is normal.  Work your way across the flange bed evenly, stopping to check the area with the edge of the razor. If you don't see large amounts of light between the heat sink surface and the edge of the razor , then your almost done. Next , use the 600 grit emery paper to put a final finish to the area, working lengthwise from one hole to the other. Be sure to clean the surfaces to remove any  filings or emery dust from both the heat sink AND the transistor flanges when finished.

We will now test our work.  This is simply done by applying a thin layer of thermal heat sink compound to the final output transistors mounting flanges and reinstalling the PCB back onto the heat sink.  We are not going to screw the PCB down at this time, this is only a test run. Lower the PCB back into position, straight down over the holes in the heat sink.  After the transistor mounting feet are touching the heat sink, you should lightly push down on the tops of the finals to gently 'seat the flange'.  Next,  you should remove the PCB from the heat sink and look at the 'pattern' that is left on the heat sink.  You should see an even layer of thermal compound on the heat sink, without any gaps or voids. If you see a lot of compound squirting around the outside of the flange outline, you have used too much compound.  This is not one those times where "If two aspirins is good, then ten aspirins is better". Too much compound can be as bad as not enough. I personally use a small piece of Saran Wrap around my finger. This allows me to more precisely control the amount of compound that gets applied AND there's no clean up, plus I don't contaminate the compound.  If your "test pattern" looks good, it's time to reassemble your unit.  Smooth out the compound on the flange and the heat sink with the  Saran Wrap trick and install the PCB.  Replace the screws in the same holes that they were removed from. Be careful not to apply too much pressure to the final output transistor mounting screws, approximately 5 foot lbs. should be enough. Snug them down, but again , don't get nuts, the mating surfaces are flat. We are trying to expel any air and extra compound from between the two surfaces and it shouldn't take a large amount of pressure to accomplish that.  Replace the screws in the front panel and the rear antenna connectors, reinstall the cover and your done.

I heard a guy say that his amp 'runs really cool'.  This to me is a red flag. A high wattage amp should run warm. When the amp is properly driven within it's design limits, I expect the heat sink to feel warm. The heat that is generated by the output transistors has to go somewhere, it's better for the heat sink to get warm,  that says to me that it's doing it's job. It's main job is to keep the power devices cooler by conduction of the heat into the heat sink.  Improperly mounted power IC's will surely over heat and not be long for this world.  When a power device is correctly mounted and properly driven within it's design specs, it should last a long time. 

      Original Flange Mounting

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Initial Filing to identify the high and low spots

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    This is what your looking for

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This Heat sink is from the 200P, But you get the idea...

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Just a little too much compound to illustrate a point........

(Click here for Larger view 69K)