I work on fire alarm systems in outlying areas that transmit a signal to the fire department via 72 Mhz AM. The signal period is very brief and the carrier is modulated by a series of tones which first pertain to the box ID and then successive tones corresponding to which zone or zones are in alarm. The transmit power is one watt and cannot be increased.

This particular site is about 9 miles from the receiver. The transmit site is low and the terrain is hilly. Five years ago we installed a three element beam to replace the existing omni directional antenna they had. Communication with that omni was spotty at best. With the beam, although the received signal was low, test transmissions were consistent and reliable.

The fire department has recently reported that they have not received a test transmission for some time now. Oddly no one seems to know how long this has been going on but it is at least for the past month or so.

When I arrived at the site initially I found all the lights on. the B+ fuse for the final blown and the unit totally non responsive. I powered down, AC and battery, replaced the fuse, and then powered back up again. The unit seemed to power up normally.

The antenna looks fine on SWR and I ran the self test function and activated a couple of alarms, however the fire department did not get a signal. And although I can see a 1 watt carrier on my meter I cannot tell if it's actually being modulated. So although I know that I have an RF output my problem is that if the unit is only putting out a dead carrier the receiver will ignore it. These Id and zone modulation tones are all in the 500HZ and lower spectrum so they're all audible. So if I had an AM receiver that could tune the 72MZ band and allow me to hear these tones I would then eliminate the transmitter/modulator as being to blame. At that point I would then think about trying to turn the antenna, which is big and heavy, mounted to the side of the house, and not an easy thing to do, especially in the snow.

I'm concerned about the modulation factor because this box seems to have suffered some type of event to lock it up the way it was when I first found it. I also have no idea what caused the transmitter fuse to blow.

Other than my service monitor/spectrum analyzer which tunes .450Mhz to 1.0 Ghz FM, (only) I have no way to hear this AM signal. Does anyone have any ideas? Thanks, Lenny