In article <
14fca79b-407a-43dc...@googlegroups.com>,
Leonardo Capossio <
capossio...@gmail.com> wrote:
>Y removed them from the heatsink, cleaned everything with alcohol (even tried not to touch it with my
>fingers) and put fresh thermal paste (with isolation) between the metal plate that holds the transitors,
>between the plate and the heatsink, and between the heatsink and the case, but still the metal plate is hot
>as hell (can't even touch it) and the heatsink is very hot, but not so much.
You might want to check the metal plate and heatsink for flatness.
If the plate was warped at all, it won't be making good contact with
the heatsink - there would be a few small points or lines of good
contact, and a significant air-gap everywhere else. Filling the gap
with thermal paste won't help very much. Thermal paste helps thermal
conductivity only when it's a very thin layer - just thick enough to
bridge the occasional thin gap between two surfaces which are
otherwise in good direct physical contact. A thick layer may be worse
than not having it at all.
CPU overclockers used to (maybe still) "lap" the top of the CPU and
the contact surface of the heatsink, in order to get the both as flat
as possible. When this is done, the layer of thermal paste between
the two is *very* thin... put a tiny dab on one surface, and then
spread it out thin-and-even with a single-edged razor blade.
Over-torquing the screws which hold down TO-220 or similar
plastic-package transistors to the heatsink can similarly cause
problems. Apply too much torque, make the transistor case and tab
flex a bit, and you end up pulling most of the case away from the
heatsink and thermal transfer gets worse. If your transistor tabs are
bent or warped, lapping them might help.
[Lapping of this type can be done with a sheet of wet-and-dry carbide
paper, mounted on a flat plate of some sort - a high-quality sheet of
plate glass will serve. The usual finishing technique of "start with
a coarser grit, then work your way up through the grits until you get
to 1000" would be appropriate.]
The fact that the heatsink itself is very hot does suggest that
something else may be going amiss... maybe the transistor quiescent
bias is too high, or maybe there's a parasitic oscillation taking
place. Using a scope (or an old-style grid-dip meter in absorbtion
mode) to look for RF where it doesn't belong might be a good idea.