70CM KW Board Assembly Instructions

First, locate the 35pf trimmer capacitors supplied in your kit. The photo on the left shows the back side of these capacitors, and the reason for this first note: these trimmers must be mounted so they are not centered between the two gate traces, they must be offset so the round part in the center does not sit above the opposite trace. If centered, it will short the two traces together.

Mount it as shown in the next photo, with most of the capacitor body over the bottom trace. Solder into place.

Pre-set the slotted adjustments as shown, with the slot vertical. This will put the capacitors at mid-range, and very close to resonance with the rest of the input circuits.

Install all the components on the input board, with the RG316 input coax baluns installed last (they get in the way of installing the other things).

For reference, a high-resolution schematic is here.

Note the spacing between turns on the input coils.
 

Install all the components on the output board, with the RG402 output coax baluns installed last (they get in the way of installing the other things).

The 250w termination will be installed later.

Note the polarity on the electrolytic capacitors.

The coaxial capacitors (white coax pieces) must be installed 30mm from the left edge of the pc board.

The 10pf chip capacitors must be installed just to the right of the green trimmer capacitors.

The 5pf (green) mica capacitors are aligned across the drain traces with it's right edge even with the place where the traces narrow.


 

Prepare your copper heat spreader and heat sink using the templates and video provided here:

Heat sink drilling template

Copper spreader drilling template (top)

Copper spreader drilling template (bottom)

Video on mounting LDMOS to copper spreader
Note: You may notice the next three photos show mounting for boards using the Freescale LDMOS part; however, the mounting methods are the same for boards using the NXP LDMOS.

Apply a thin, even coating of heat sink compound between spreader and heat sink.

Mount the spreader assembly to the heat sink loosely with three 8-32 screws (the flat washers are not necessary) on the top side, and four 6-32 screws on the bottom (these four screws pass through the heat sink into the holes tapped into the bottom of the copper spreader).

Position the board and termination spacers as shown. The termination spacer should also have heat sink compound applied to the surface that sits on the heat sink.
Slide the input and output boards under the transistor tabs, and loosely fasten with 4-40 screws and flat washers.

The eight 4-40 screws nearest the transistors should be 3/16 long, and the others should be 3/4 long.

Apply heat sink compound to the foot of the 250w termination and loosely fasten down using 4-40 screws and flat washers. These screws should be 5/8 long.

Once everything is aligned properly, secure all of the screws. The four 6-32 screws and the three #8 screws holding the copper spreader to the heat sink, and the two termination screws should be tight. All the rest of the screws (4-40) should be snug and firm only, do not over-tighten.

Using a bit of liquid flux, solder the LDMOS tabs to the board.

Now solder the termination tab to the coupler port.

There are 3 VDD connections on this board, and I usually jumper the outer connections over to the center one with #16 wire, and feed VDD to the center VDD connection (the widest one).
 

The addition of the following components is recommended to ensure unconditional stability by applying degenerative feedback. This feedback is most effective at very low frequencies where these LDMOS devices have excessive gain. Without the degenerative feedback, and under certain rare conditions, self-oscillation can occur and destroy the LDMOS.
  1. 1.Remove the two 4-40 x 3/16 screws located nearest the gate pads, but leave the flat washers in place
  2. 2.Replace the screws with hex metal spacers; be careful not to over-tighten this spacer, it's screw can shear off.
  3. 3.Screw a nylon spacer into the metal spacer, finger-tight only.
  4. 4.Place a solder lug on top, and fasten in place using the 4-40 x 3/16 screws removed earlier.
  5. 5.Solder a 440 ohm 3w resistor to the drain pad and route the other end to the solder lug.
  6. 6.Solder a .047uf capacitor to the gate pad and route the other end to the solder lug.
  7. 7.Solder the resistor / capacitor connection at the solder lug.
  8. 8.Repeat for each LDMOS device (on both sides).


 

In the next step (testing) an output coax lead will need to be installed. Drill a couple of holes into the board spacer near the output port, and use these to secure a copper or tin bracket. This bracket is soldered to the shield of the coax lead, and after securing it to the spacer, the center conductor of the coax is soldered to the output port.
 

Recommended tune-up procedure

  1. Attach input and output coax jumpers. Your driver should be limited to 10w max, and the output should be on a dummy load for the initial testing.
  2. 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 adjust the input trimmer capacitors for max power out. This should be the same setting as lowest input vswr, and the input trimmers should be close to mid-range. If they aren't, spread or compress the turns on the input inductors until they are.
  5. The amplifier can now be driven to full output.