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Leif Neland

sendte dette med sin computer:
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.

I just wonder: Is it not possible to put in a spare transmitter while
you work on this? I'd be rather unhappy if the building burned dowm
while waiting to get the alarm repaired.

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.

Methinks somebody should look into their procedures...

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.

When you do the "divide and conquer (sp)", you could work the other
way. Can you hear/see the tones going into the modulator?

Leif

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