Rev 1.03
14
It is difficult to remove and replace transistors
later! So before soldering, check once again that
the Q7 transistor (IRF510) is in the correct place
on the side of the wide opening of the heatsink.
Check that everything matches the PCB
silkscreen and this diagram (right).
Once the heatsink assembly and transistors are
fitting perfectly into the PCB, you can solder first
the heatsink mounting pins, then the transistor component leads themselves. A high wattage
soldering iron may be needed for soldering the heatsink pins, as the heatsink will try to conduct
away heat which may lower the temperature of the joint below the melting point of solder. You
could leave the heatsink pins unsoldered if it seems troublesome. After soldering the transistor
leads, cut off the excess lead length.
Note that the pads for the heatsink mounting pins are not connected to ground (or anything else).
The PCB top side has no copper or tracks of any sort under the heatsink; this is to ensure that
even if the soldermask accidentally got scratched, there would be no contact between the heatsink
and ground or any other electrical connection.
3.6
A recommended modification
This modification will be included as
standard on any future PCB revisions. It
involves the addition of a 10-ohm resistor
at the gate of Q7, the IRF510 final PA
transistor. The PA does work without this
resistor, and indeed it will produce slightly
higher power output. However at certain
supply voltages and frequencies, instability
can occur. When this happens the amplifier
superimposes a low frequency oscillation
envelope on the wanted RF output. Fitting
this 10-ohm resistor resolves this potential
problem, at the cost of some power
reduction. The measurements made later
in this document were made with this 10-
ohm resistor fitted.
The modification involves cutting one track
on the BOTTOM side of the PCB, and
soldering a 10-ohm resistor on the bottom side of the PCB. The resistor location is shown in the
above circuit diagram fragment which shows the IRF510 PA. The 10-ohm resistor is supplied in
the kit and has colour code brown-black-black-silver-brown.
