600w 23cm RF deck kit assembly instructions

Caution: This RF deck looks easy to construct, and it can be, but if you do not have extensive experience working with surface mount parts on high power SSPA's at 23cm, your results can vary widely. Even if you are an accomplished builder of other things, the assembled/tested RF deck may be your best choice instead of the kit; it could save you a lot of time, trouble and expense.


The assembly instructions are next; please read the entire document, and carefully

 

This is what the properly assembled RF deck will look like before mounting to your heat sink; A high resolution photo is here to assist you with parts placement.

The boards in your kit will may be the ones without solder mask (shown in the next photo), but for the purposes of placing components in these kit instructions, I'll be using the green or gold boards as examples because those boards have markings with better visibility.

The RF deck assembled from the kit will look like this one. The solder mask and two of the outer trimming pads on the input and output have been eliminated, yet both green and gold PC board revisions perform the same, there is no operational difference at all. The boards are identical in thickness, material, trace design and copper weight.

OK, let's get started...
 

 

Locate the output board; referring to the schematic and the reference designators on the board, install the 6 high voltage microwave chip capacitors.

The output board is the shorter one.

If your kit was supplied with the seven 24pf high frequency RF capacitors instead of the two 24pf ATC capacitors, use the layout in the next photo for reference:
 

 A close-up of a circuit board

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Because 24pf ATC capacitors are not always available, your kit may be supplied with 24pf high frequency RF capacitors. To get the same current carrying capacity and bypassing performance as the ATC types, double them up in pairs and mount them together laying on their thin sides as shown here.

This will consume four of the seven 24pf capacitors supplied (the remaining 3 are used on the input board later in the assembly procedure).

 
 

 

Note: save the trimmed-off capacitor leads in the next step, they will be used later.

Trim the leads on the electrolytic capacitors to 6mm length and bend them as pictured, about 2mm away from the body of the part. Note the position of the negative lead on the body of the parts, and the direction of the wire bends on each (they are not bent the same as one another).
 

 

Install them like this, positive to the bypassed end of the VDD trace, negative to the ground foil. Leave enough room for the two mounting screws and their flat washers to be placed at the top of the board; note the position of the negative polarity marking on the bodies of the parts, they face inward toward the center of the board.

Now, using a cut-off piece of one of the capacitor leads you saved in the previous step, jumper in one of the trimming pads at the output as pictured here.

How many pads you will need to jumper in to trim the amplifier output properly is going to depend on how it plays when tested; starting with one is a good place, I find them to require as little as none to as many as three. Most of them play best with one or two (at 1296).

The output board is finished.
 


Locate the input board (the longer one) and install the remaining parts. Checking the schematic and the high resolution photo will help you put everything in the correct place, and orient the Zener diode properly.

The input board is finished.
 

 

 

 

If you purchased the copper spreader and then mounted your own LDMOS using the flow-solder method (see the video here),  make certain the area under the LDMOS tabs is free of flux or other material which may hinder good electrical contact.

Slip the input board under the transistor gate tabs. Use it as a lever to bend the tabs upward a bit as you see it done here. Do the same for the tabs on the drain side.

You probably already noted there is no room for mounting screws near the transistor tabs; with high power devices at microwave frequencies, it is very important to get good reliable contact between the PCB and the copper spreader, so we'll be arranging a very tight fit as the boards are finally set into place. You won't be soldering the PC board to the spreader using this procedure, as there are serviceability drawbacks to that method.

The copper spreaders I have available are machined with a .055 trench depth, and that positions the transistor tabs about .005 above the board surface when all is in place. There is some variation (+- .002 or so) in the machining of the trench, so we'll need to fill that gap with a small spacer placed between the pcb and the copper spreader (next step). The spreaders for this RF deck are shipped with spacer material the correct thickness for the spreader provided. If your kit was supplied with the PCB clamp system these spacers will not be supplied with your kit, nor do they need to be installed.

If you are making your own copper heat spreader from the drawing available here, you can eliminate the need for these spacers by changing the trench depth from .055 to .060 as long as you can hold the tolerance to +-.002 (.058 to .062).
 

 

Slip the spacer in place partly under the LDMOS tabs, but not far enough forward to climb up on the LDMOS footing.
 

 

You may need to insert the PCB at a slight downward angle to get it into place, it's going to be a tight fit. If you can't get it set into place properly, with the edge of the pcb in alignment with the edge of the trench, the spacer may have slipped onto the transistor footing.

Check to see if the mounting holes are centered where you see the two 3/16 screws and flat washers installed. If everything looks OK, install those 3/16 x 4-40 mounting screws with their flat washers.

Install the output board with it's shim in the same manner.
 

 

Observing proper ESD safety methods, bend the transistor tabs back into place (level). Using just your clean fingers, push the tabs flat against the PCB.
 

 

Place a drop of liquid flux at the center edge of all 4 transistor tabs, so it flows a bit under the tab when it is applied. Not too much, just a drop will do. Solder the transistor tabs to the PCB as pictured here, making certain of a complete and even flow as you see it pictured here. Clean up the flux residue with iso alcohol and a cotton swab.

Once the copper spreader is mounted to your heat sink, tightening the 8 outer mounting screws will apply a slightly (and beneficial) higher pressure yet to the board-to-spreader contact area.

You're all done and ready to test; here is the recommended test procedure.


 

Note: the installation of a PCB clamp is optional, but recommended.

Installing the PCB clamp on the 600W 23cm RF deck

Some digital EME operators report repetitive full power heating and cooling (1 minute on, 1 minute off) of the rf deck in these modes eventually causes the thin PC board material to flex and lift away from the heat spreader just enough to affect the match. This is noticed when the gain, output power and DC current drop significantly from their usual levels, often by as much 30%. Clamping the boards securely to the heat spreader solves this problem, and restores normal function to those units affected.

Extra mounting screws could not be used for clamping due to the width and positioning of the PCB traces, so a Teflon spacer clamping system was developed. The sheet metal parts of the clamp system are made from flexible aluminum sheet, and will self-adjust to provide the correct pressure during heating/cooling cycles. This PCB clamp can be installed to an existing amplifier in about 5 minutes with only a screwdriver.

All kits, rf decks and 600w 23cm amplifier systems shipped after September 2021 contain the clamping parts. For those purchased prior to that date, a drawing with instructions for making your own clamping parts is shown below.

Here's how to install the clamp:

  1. If installing into a pre-built amplifier, remove the 8 screws holding the cabinet cover in place and remove the cover
     
  2. remove only the 4 screws holding the spreader to the heat sink closest to the LDMOS and put them aside (see the arrows in the photo)
     
    • if installing a new rf deck from a kit (or a pre-assembled one) onto a heat sink for the first time, apply a thin coat of heat sink compound to the bottom surface of the copper spreader; then loosely secure the copper spreader to your heat sink with only the 4 outer screws, the 4 inner ones will be installed with the PCB clamp.


       
  3. Stand the two Teflon spacers on end and make certain they are the same height as one another. These Teflon spacers are cut from 9mm stock, and the cuts are not always the same width; if you have them standing on the wrong side it will not match the desired 9mm height, or match one another. Place one spacer on the input PCB and the other on the output PCB at the locations shown (see photo). Be careful not to cover the gate resistor near the transistor, or the transistor leads with the spacers. If there is excess solder under the spacers, remove it. Place the spacer retainer over the two spacers.


     
  4. Position the clamp assembly over the spacer retainer and secure one side with two of the screws removed in step 2. The flat washers will not be used with any of these screws. The unsecured end on the other side should be 2mm above the board surface when the board is flat against the spreader. If it is not, bend the clamp slightly in the middle until it is (you may need to remove the clamp to do this properly). Once the gap is correct, fastening the other side of the clamp will produce the correct pressure on the PCB.
     
  5. Install the other two screws and tighten all 8 of the clamp mounting screws securely (tight, but not gorilla-tight).


     
  6. If the installation was to a pre-built amplifier in a cabinet, replace the cabinet cover and secure it with the screws removed in step 1.

Construction information for making your own PCB clamp is shown below