On Nov 25, 2009, at 9:53 PM, ke5hbb wrote: > To the group I am new at building ,with the help of a friend we put together > a uhf mobil. at 45 watts out I had a problem with the pa . A small copper > loop beside the power adjust pot. burned thru the board and took out a cap.I > was able to get it back online but am worried about overheating . mobil units > don`t have large heat sinks. What is the normal power setting? These small > copper loops act as a jumper from one trace to another. What are the correct > term for these loops? > > Dave 73
There were different sized PA's available in the mobiles. What's your combination number stamped on the front of the rig? That'll tell what it originally shipped from the factory as. Most were 100W, though. The failure at the jumpers between the boards is common if you're not driving the PA into a perfect 50 ohm load. The late model station (not mobile) PA's added a Z-matcher to the Low-Pass Filter section of the PA for this reason. Also, as someone else mentioned, those jumpers can act as inductors to some extent (although I disagree that they were intended to) and one of them burning up can be an indication that there's problems "upstream" on the high-power board. The fact that it burnt up makes me think you have at least the 100W NON-continuous-duty PA. Most of the others don't have enough "oomph" to fry that connection between the high-power board and the LPF. Most people install fans on the mobile version of the PA's heatsink when using them in repeater service. The GE MASTR II PA can become spurious at low power levels, starting somewhere USUALLY around half power and down. Your PA may have burnt up because of that also (it will draw INCREDIBLE amounts of current as it tries to put out 100W on/near your main frequency and spurs all up and down the band. Get a spectrum analyzer on it. Turn it up from 45W (usually we run our 100W PA's at 75% power or greater) and since it's a mobile, get fans on it so it'll stay cool. Also, I highly recommend a Z-matcher or better yet, and isolator on the output of MASTR II PA's. Give them a 50 ohm load and they'll usually run forever. And if you can, there's TONS of stations on the used market these days as Public Safety heads for narrowband and/or digital stuff. Take your exciter, receiver, and all the "guts" from that mobile that are already tuned up for your frequency and stuff 'em in a station/repeater chassis with a non-fried UHF PA, and you'll have years and years of service from it... -- Nate Duehr, WY0X [email protected] facebook.com/denverpilot twitter.com/denverpilot
