On Nov 29, 2009, at 10:18 PM, tahrens301 wrote:
Since I'm looking at using it as a solar pwr'd repeater, what
is the best way to turn the power down (without making a bunch
of spurs along with it)?
Remove sections of the PA, or find the factory-built lower-power PA's for
whatever band you're using it on. Low power VHF's are sometimes seen, but
often they're in the Station configuration, not a mobile. They're almost drop
in replacements for each other, however... so it's easy to move them around
between heatsink housings.
The model# tag is no longer attached, but the final PCB has
the number: PL19D416964G1REVF.
It is VHF, and has 4 power transistors across the board.
There were two major models of VHF PA. Without looking that board up, there's
one with all four transistors in a vertical arrangement with equal-distance
gaps between them, and the transistors themselves are very small (about 1/2).
That PA was the original design, and was known for being HIGHLY spurious.
The later models used bigger transistors (about the size of a quarter) arranged
horizontally with two pairs of drivers obvious in their layout, and a
Wilkinson divider/combiner type setup with large 1/2 watt resistors. Those are
less prone to spur issues.
However, ALL of them used a driver board that fed into a middle power
amplification board.
But... and here's the big but... the engineers at GE knew folks might need
low-power versions and they knew the limitations of the high-power ones. Their
low power PA's are usually nothing more than the driver board, a missing PA
board, and a connection all the way across to the Low-Pass Filter board. The
final output is MUCH less, and the driver by itself won't spur or have any
issues at all.
In the VHF's, there's a feedback circuit to deal with between the PA board and
the driver board. In UHF's, the driver runs flat-out and doesn't care. You
can study the circuit block diagrams and the schematics and board layout in the
LBI's and pull the power section of the board out, deal with the feedback loop
on the VHF PA, if VHF... and jumper over to the LPF with high quality coax run
as directly and short as possible, and you have Voila!... almost a factory
low-power PA.
I've heard that running the amp at less than rated power isn't
a good thing to do.
And you've heard correctly unless you have a spectrum analyzer and put it
through the wringer. On some frequencies, and with some PA's you CAN get away
with it.
But that's not widely disseminated via public forums like RB, because
inevitably someone comes along without proper test gear and puts one on the
air, making a headache for themselves, the other tenants at the site, and
indirectly, ham radio in general by making us all look like fools who can't
properly measure things that any commercial company tech would be equipped to
do. ;-)
--
Nate Duehr, WY0X
n...@natetech.com
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