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,
by as much 30%. Clamping the boards securely to the heat spreader solves
this problem, and restores normal function to those units affected.
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
Here's how to install the clamp:
- If installing into a pre-built amplifier, remove the 8 screws
holding the cabinet cover in place and remove the cover
- 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
- 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.
- Position the clamp assembly over the spacer retainer and secure one
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.
- Install the other two screws and tighten all 8 of the clamp mounting screws securely
(tight, but not gorilla-tight).
- 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
Construction information for making your own PCB clamp
is shown below