2 Meter KW Amplifier Kit Assembly Instructions (rev 4c PC board)

I re-designed the original board to minimize unused copper areas, as these spots must contain many vias to ground them to the opposite side to keep them 'quiet'.

At the same time, I noticed that on all previous boards, the main matching capacitors ran warm; while failure of these components is rare, I realized that short stubs of 10 ohm coax (pretty much indestructible stuff) would serve well as the matching capacitors.

The first capacitor is the longer coaxial stub shown here; it's 110mm long, and it's capacitance is 35pf. The second shorter stub is the other matching capacitor, 50mm long and 15pf. These values provided the best match with the revised board, which will work with either the Freescale MRFE6VP1K25H or the NXP BLF578XR  or BLF188XR LDMOS devices.

Here's the most current schematic

Assembly instructions are listed below:

The first components to mount are the capacitors.

The 470pf MC18 capacitors are installed in pairs at the four locations shown. Mounting them on their thin edge helps to balance the currents in the pairs.

Then mount the two 1206 size 1000pf capacitors, the two 2.2uf and two 220uf electrolytics.

TC-12 coax transformers are next. The position of the connection point at the drain traces will affect the match; the loop at the left should be lined up with the transition line where the trace transitions from wide to narrow, on the side of the transition toward the top of the photo. The loop on the right should be placed right up against it on the other side of the transition as shown.

Install the two 10-turn VDD drain inductors

The 35pf coaxial capacitor is next, formed in a way that keeps the open end away from other components.
The RG142 balun is next.

The last component on the output board is the 15pf coaxial capacitor. Position this part so the open end is away from other components.

This is the completed output board.

Except for the input transformer, mount all input board components. The bias has been set up for 12v feed. See the assembly instructions for the 500w 70cm amplifier for other bias feed configurations.

Mount the input transformer. It must be soldered to the board at the 5 points shown.

Slide the boards under the transistor tabs (transistor should have been previously flow-soldered to a copper spreader).

Here is a video showing how to flow-solder your LDMOS to the spreader.

Here is the drilling template for the spreader.

Secure the boards to the spreader with 4-40 x 3/16 screws and flat washers. Do not over-tighten.

Use some liquid flux, and solder the boards to the tabs as shown.

Recommended tune-up procedure

  1. Fasten the spreader to a suitable heat sink using the guide on the right. Use a very thin coating of heat sink compound between spreader and heat sink (too much, and it will actually impair heat transfer).
  2. Attach input and output coax jumpers. Your driver should be limited to 2w max, and the output should be on a dummy load for the initial testing.
  3. Attach ground, bias (12v?) and VDD (50v) wires to the RF deck, but do not apply power yet.


  1. Turn on the 50v main supply voltage, but not the bias; there should be no current drawn

  2. Turn on the bias and note the idling current drawn from the 50v supply. Adjust IDQ for 2 amps. Note: the current drawn by the bias supply (usually12v) is not what you are measuring here...you must measure the idling current (IDQ) the LDMOS draws from the 50v supply.

  3. Shut off the power supply, and remove current limiting.

  4. Drive the amp with about 1/2w, and check the output; it should be in excess of 250w.
  5. If all OK in step 6, the amplifier can now be driven to full output. Typical performance is 2w in, 1kw out at 50V VDD and 27 to 29 amps. Many of these RF decks will produce 1250w with 2.5w drive (50v VDD, 32 to 35 amps).