This PA uses two MRF899 power transistors, driven by two
MRF897s. The specifications show the MRF899 rated for 28VDC max, and the
MRF897 for 26VDC max (2V less "nominal"). See specifications, below. The
PA is very easy to convert to amateur use, requiring addition of only two
RF connectors, 20+A 24VDC connectors, and a PTT port. With only 4.5W drive
available, and a 26V @ 20A power supply, 230W continuous output was
reported. This PA is self-limited to about 250W maximum output by the
internal control board. Adventuresome folks who have removed the control
board and are powering the PA boards directly have tested this unit with
27 and 28VDC power supplies, but only for short periods; it is not known
how long the transistors would last if supplied directly with more than
26V DC. Leaving the control board in place assures voltage regulated to
24VDC will be applied to the PA boards, alleviating any concerns regarding
1. Replace the in and output connectors with connectors
of your choice. Picture
Control board end of mod'd PA Closeup, PA control board
I used a small relay with 5V coil connected to the 5V
regulator U5 (Picture). Ground the other side of the coil to
enable bias. The 0.02ohm resistor is used to measure the total
collector current, and will limit the output to 250W. Bridge this
resistor to disable this limitation. I decided to stay at 250W (HP
measurement) and keep the limit in place to protect the transistors.
If you keep this safety in place and overdrive the amp, then you
will notice severe distortion from this circuit switching the bias on and
off at the current limit.
There are also "150W" version of this PA in existence.
They look almost identical from the outside (seepictures below - click for
zoom), but are different on the inside.
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