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 [email protected] http://facebook.com/denverpilot http://twitter.com/denverpilot
