The heat sink
provided with the BITX40 is adequate for lower power levels but
bigger and better heat sinks provide for longer component lives and
peace of mind. If you are going to install the transceiver in a metal
box then you might consider using that enclosure as a heat sink.
Here is how I did
In the early stages
of installation I positioned the board as close to the rear of the cabinet as was
convenient but it still left about 1/4” (6mm) gap between the Power
(Q15) and the back panel when the original heat sink was removed. I found a small
block of aluminum that fit nicely. I drilled a hole toward one end to match the hole in
the tab of that IRF510.
When the mounting
spacers had been attached to the BITX board (so I could tell where
the transistor was located in
respect to the panel) I marked and drilled the transistor's
attachment hole first. I then placed the mounting
bolt through the transistor, spacer block, and rear panel to
temporarily hold it before marking and
drilling the four holes to the attached mounting spacers of the BITX.
After the board is
mounted use a shouldered nylon or ceramic washer to insulate the
mounting bolt from the transistor
tab which is connected to the collector. Don't let it short circuit
to anything. A mica or silicone pad
between the spacing block and the transistor completes the insulating
job while still conducting heat. If
you use a mica insulator be sure to use heat sink compound on both
sides of the insulator. A
silicone pad does not need it.
If you cannot find a
suitable aluminum block then look for any heat conductive metal.
Copper, brass, even steel.
Is the output
transistor too far from a cabinet panel for this modification?
Consider mounting an “L” bracket to the bottom of the case and
bolt that to the transistor using the same method. This piece
of metal should be fairly wide and thick enough to conduct away the
heat. Not only does it relieve the transistor of the duty of
supporting additional mass, it provides support.
Always check and
recheck your work during and after installation before applying
power. The mounting tab of the IRF510 is directly connected to the
drain of the device and is at full supply voltage as well as RF
output power. Be certain that it is well insulated from the case or
Don Cantrell, ND6T