[Elecraft] Installing the Q7, Q8 Mounting Parts

[W3FPR - Don Wilhelm]

Steve,

If the proper materials are used, the height of the transistor tabs (with
thermal pads attached) will be only slightly higher than the 2-D fastener
(by about the thickness of the thermal pad).  So when the heat sink (rear
panel) is attached, the transistor tabs will be in full contact with the
heat sink.  The newer mounting hardware uses a fiber washer, a phenolic
spacer and the shoulder washer (older hardware used a metal washer, a soft
plastic spacer and the shoulder washer).  The relative height of the 2-D
fastener and the transistor tabs insures that the transistor tabs have
positive contact with the heat sink.

A hint on removing the heat sink without losing the transistor mounting
hardware - cut 2 pieces of electrical tape about 1 1/2 inches long and place
it over the screw heads (on the top of the RF Board).  The tape will hold
the screws in place so the heat sink can be removed and the PA transistor
mounting hardware will stay in place.

73,
Don W3FPR

> -----Original Message-----
>
> I ordered 2 shoulder washers, 2 spacer washers, and a bunch of the soft
> plastic thermal insulators from Elecraft so I can finalize the job of
> doing the upgrade mods on K2 #402.
>
> I was just wondering about the theory of this layout.  The tab of the
> transistor is pressed against the thermal pad by the spacer which is
> tightened by the mounting screw.  I guess you need to only tighten it so
> much as to not puncture the soft insulator.
>
> I am confused because there is a 2-D fastener at this location situated
> in such a way as to appear to be a spacer.  If the 2-D fastener actually
> governs the height of the circuit board above the case heat sink, then
> tightening the screws for the transistor heat sinks would not work
> because the 2-D fastener would prevent the movement.  Tightening the
> screw would only warp the board.  If the 2-D fastener did not restrict
> the tightening of the heat sink screws then why are they there?  You
> would think the 2-D fastener would be the exact height to prevent the
> plastic insulator from being over-compressed, but it doesn't seem to be
> that way.
>
> My old spacers are so warped, probably from heat, that they look like
> truncated cones, wider at the base.  My guess is that in this state they
> were not putting any pressure on the transistor tab to force it into the
> plastic insulator.  In that case its a wonder I didn't burn the
> transistors out.
>
> What about using a mica insulator?  That would never puncture.  Is mica
> worse at being a thermal conductor than the soft plastic ones used in
> the K2?
>
> 72,
> Steve, W2MY
>
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