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KLV-351P
Identifying the Problems:
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removing the PCB from the heat sink, take a good look at the
heat sink where the final output transistors were mounted AND
the bottoms of the final transistor mounting tabs. Note
that the 'pattern' of the heat sink compound will tell you if
you have a problem. If you notice a lot of compound on the
center of the mounting tab and very little on the tab ends,
then you have the distortion I mentioned. On the 351P, be
sure to look at the small power transistor on the corner of the
PCB as well. On the unit I inspected, I noticed that the
transistor was only 'half seated' to the heat sink. The
pattern of heat sink compound on only 1/2 of the transistor
itself was the tell-tale sign that something was wrong. Upon
closer inspection, I noticed that the hole in the heat sink was
not drilled deep enough for the mounting screw. When the
screw was tightened at the factory, the screw bottomed out in
the hole before it pulled the transistor to the heat
sink. A simple cupped washer under the screw head
fixed this problem.
Here you see that there is a lot of thermal compound in the center of the mounting tab. These finals were not fully seating...
This transistor was not making full contact either..... The "Big view" picture clearly shows both the solder splashes and a cold solder joint.
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351P Heat Sink after PCB removal (click here for larger view 58K ) (click here for BIG view 306K)
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351P Finals -- Mounting Tabs (click here for larger view 109K) (click here for BIG view 576K)
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351P Small Power Transistor (click here for larger view 93K) (click here for BIG view 417K)
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