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 it:
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
Amplifier transistor (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 chassis ground.
Don Cantrell, ND6T
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