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