Note: the newest RF deck supplied in the kits shown here differs from the
original prototype. Here is the current schematic:
the eight 470pf MC18 mica capacitors, they will be installed on the output
Install them in groups of four, mounted on their thin side as shown. Mounting them in this way helps to even out the RF currents in each one.
Then mount the rest of the capacitors; the 1000pf, .01 and .1uf chip bypass caps are aligned near the transistor pads spanning the the VDD and ground traces, and the 100uf electrolytics are around the corner to the right of them. Note the polarity markings on the electrolytics.
Next, mount the two
TC12 coax transformers, beginning with the longer trimmed ends. There are 4
points to solder...the tips go to the MC18 blocking capacitor traces, and the shields
to the transistor traces.
Ignore the VDD drain chokes in these next two photos, they have been replaced with a bifilar transformer (shown later).
Loop the first coax around and rest it on top of the other exposed coax
shield connections as shown. Note the center conductor of the looped-around
coax rests on top of that coax's own shield, and the shield on top of the
opposite coax's shield. Solder into place.
we had so much fun with that first piece of coax, let's go ahead and loop the
other one around and solder it into place as well.
RG142 balun is next
Install the bifilar VDD DC power feed transformer as shown. The short wires go to the drain pads, and the longer ones to the bypassed vdd 50v pads.
completed output board should now look like this.
Now install all the components onto the input board except for the 4 to 1 ferrite input transformer and 4-turn gate inductor.
A high resolution photo is here:
Pre-adjust the IDQ trimmer resistor almost all the way clockwise, about a quarter of a turn. Be careful, this trimmer is fragile.
the input transformer, soldering it to the board at the five points
Finally, install the 4-turn gate inductor
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 LDMOS tabs as shown.
Turn on the 50v main supply voltage, but not the bias; there should be no current drawn
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.
Shut off the power supply, and remove current limiting.
Please note: if you are using a reflective low-pass filter following the output of this amplifier, be aware that third harmonic levels are high, and some danger to the LDMOS exists if this energy is not reflected back in the correct phase. For this reason, a length of RG142 (or other suitable coax type) is recommended between the output of the RF deck and the input to the filter. Some experimentation with the length may be necessary, but a general rule of thumb is to use a length that will produce at least 60% efficiency. On most installations, and using the filter I offer on the parts page, this would be about 30cm (1 ft). If the length of this stand-off coax is incorrect, you will know...the efficiency will be low (less than 60%).