> Several years ago I Modified a GE Mastr II mobile to be a 220 > (35 watts) repeater and placed an Advanced Receiver Research > peramp with a resistor, capacitor and diode added for lighten > protection.
You should indicate the parts are for DC power line protection. Not for protecting the GasFet from glitch events that might cause the device to fail. > http://homepage.mac.com/pseabolt/mods/220.htm > In a few years I modified another GE Master II for an 800 MHz > repeater case and used the same pre-amp. (60 watts) You used an 800 MHz ARR Preamplifeir in the same type of hardware configuration? > A few months ago I did another modification and added and > external 220 amp. the amp is 28 volts and does over 200 watts > but I am running about 100 watts. I see you are using a surplus Uniden ARX-2125 amplifier... but I don't see any mention of how you biased the amplifier after the modification? Do you know what class of service the amplifier is biased to operate? > I placed a new AR2 preamp on the repeater and after a week > the sensitivity went from .05 mv to open the squelch to > .7 mv. The GasFET in the preamp was gone. I replaced the preamp > with the orginal preamp than had been working for about 10 and > a couple of weeks later, it gone too. I would expect a failure to happen unless you're running some serious protection/filtering in front of the preamplifier. You might want to use uV for your micro-volt label... otherwise folks will assume you mean milli-volt or mV. GasFets will handle only so much unwanted energy before they go on vacation. > Can you guys think of anything that would cause two amps to go > in this short of time? Is this just coincidence or could the > extra power have something to do with the failures. You don't say what type of duplexer, preselector and filtering you have in front of the receiver preamplifier, but it's probably not near enough protection. You also need to address the rf amplifiers bias point and rf output filtering concerns. Rising to the 100 watt plus output level might be only a 3 to 6 dB increase for a lot of folks... but shear quantity increase of possible problematic and/or gremlin RF related to the repeater operation is much more. The no free lunch rule applies and you must pay even more attention to the details versus operation at lower power levels. Pull the modified Uniden Power Amplifier out of operation until you have a handle on what is going on. Tell us what type of filtering you have in front of the receiver and preamplifier. s.
