Installing the Q7, Q8 Mounting Parts

[Steven Pituch]

Hi all,

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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[Don Wilhelm-3]

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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[Steven Pituch]

Hi Don,
I guess my mistake was assuming the same steel washer was used.  I just
looked it up in the new manual and I see they have been replaced by
fiber washers.  The same fiber washers that are used to mount the speaker.

Since I thought that only the phenolic spacer was different, and since
Elecraft didn't have a kit, just ordered the shoulder washers and
phenolic spacers.  I just looked at the fiber washers under the speaker
and they are partially compressed, but I will micrometer their
uncompressed parts and try to get them at Ace Hardware.  I notice that
these washers are not used in the 150 W tuner or 100 W amp kits
otherwise I could swap them out with those, which I haven't yet finished
building yet.

If Ace doesn't have the fiber washers I will place another Elecraft
order as this sweet a radio deserves to have all the parts correct.  I
was playing with the new KDSP2 last nite and wow!

Thanks,
Steve
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