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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Need a 72 Mhz AM receiver
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 |
#2
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Need a 72 Mhz AM receiver
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 If you're very near to the transmitting antenna, I'd think that even a simple un-tuned crystal radio would be sufficient. A length of wire, a 1N34 diode, maybe a small capacitor, a crystal earphone, and a clip lead to attach to ground would oughta do it. If you want to get fancy, add a series-resonant circuit (small air variable cap and a hand-wound inductor) to tune it to 72 MHz. A 100 MHz (or faster) oscilloscope with a wire connected to the BNC jack would probably pick up enough to show you the modulation, if present. |
#3
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Need a 72 Mhz AM receiver
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. ** Sounds impossible. 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. ** I bet it was low and badly affected by rain too. 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? ** Err - ever heard of a radio scanner ? .... Phil |
#4
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Need a 72 Mhz AM receiver
Seems like you should be able to take an old FM tuner and modify it a bit, buit actually I think I might hav rean even better idea.
What about a cable ready TV tuner ? Not just the tuner, rip out the flyback and all that ****, they don't use keyed AGC anymore. I think channel 3 or 4 should tune that. then just figure out how to get a little more selectivity if you need it. Maybe shift it tuning a bit to put it near the sound carrier, but instead of feeding a limiter nad an FM detector, just feed an AM detector. Possibley you could use the bandpass filter for sound on the video. Not sure, the split carrier sets might not work right because of the synchronous detection. Know what ? Fukit, just use the video carrier and filter it right, you are ooking for audio frequencies right ? Unless there is something within 20 Khz of it you won't hear it anyway. Just feed the video detector to a small amp and speaker. You wil haqve to find a set that can tune it in. You will have to set the AFT to get where you want, masybe even modify that a bit. as long as it works off the jungle IC with normal AFT you should be able to do it. I would say use the stuff from an old VCR but it is likely to be too crammed to do any modifications. What do you have laying around that's really old ? (you know I used to pick up cellular on UHF before they removed the channels above 69 :-) (that was a LONG time ago) Anyway, yup, do it with an old TV. Don't use a black and white, they are too rare. I wouldn't mind having one just for the hell of it. In fact there is a guy on Craigslist wanting to buy them who posts from time to time. |
#6
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Need a 72 Mhz AM receiver
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#7
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Need a 72 Mhz AM receiver
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#8
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Need a 72 Mhz AM receiver
On 03/04/2014 05:20 PM, Phil Allison wrote:
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? ** Err - ever heard of a radio scanner ? ... Phil TV set between VHF channels 4 and 5? Yer average Bearcat won't tune TV bands (or slightly big guardbands). You need a wideband comms receiver like an ICOM or an AOR. |
#10
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Need a 72 Mhz AM receiver
wrote in message
... .... 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 It depends on where you want to check the signal. On the transmitting end, with 1W power, a simple diode into a resistive divider and speaker will do. Totally frequency-insensitive, but you can probably check the carrier with a frequency counter separately. On the receiving end, you'll have to look for some rather sensitive wideband receivers. This one will tune from 100 kHz to 1.3 GHz in 1 kHz steps: http://www.yupiteru.co.jp/products/r...500/index.html Yupiteru does not directly sell their products overseas, but several Japanese online shops do export. As Dave has noted, ICOM and AOR (both also from Japan) make several other receivers. These tend to be even more pricey but also have more selectable filter bandwidths and demodulation options. The Yupi above has AM, narrowband FM and wideband FM. Sensitivity ca. -6 dBu at your frequency and modulation type. Regards, Dimitrij |
#11
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Need a 72 Mhz AM receiver
"dave" Phil Allison wrote: 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? ** Err - ever heard of a radio scanner ? Yer average Bearcat won't tune TV bands (or slightly big guardbands). ** Some of the cheaper scanners do seem to miss 72Mhz. You need a wideband comms receiver like an ICOM or an AOR. ** My 20 year old AOR 1000xlt ( hand held ) scanner covers the whole range from 100kHz to 1.3 GHz. Demodulates AM, FM narrow and FM wide at any frequency too. As do many others. They go for under $100 on Ebay. Thousands of uses .... ..... Phil |
#12
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Need a 72 Mhz AM receiver
On 03/05/2014 07:23 PM, Phil Allison wrote:
"dave" Phil Allison wrote: 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? ** Err - ever heard of a radio scanner ? Yer average Bearcat won't tune TV bands (or slightly big guardbands). ** Some of the cheaper scanners do seem to miss 72Mhz. You need a wideband comms receiver like an ICOM or an AOR. ** My 20 year old AOR 1000xlt ( hand held ) scanner covers the whole range from 100kHz to 1.3 GHz. Demodulates AM, FM narrow and FM wide at any frequency too. As do many others. They go for under $100 on Ebay. Thousands of uses .... .... Phil I have the 8200b3. I had to get one for work. I could never justify the price just for the hobby, as the house was already full of "scanners". It also has NAM (narrow AM), helpful for listening to HFBC, helpful for Air Band. I rarely listen to either on the beast. The "b" means "blocked", unfortunately. |
#13
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Need a 72 Mhz AM receiver
Don't you have an oscilloscope? Loosely coupling the scope input to the transmitter, (laying a wire nearby), you should be able to see if the signal is being amplitude modulated.
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#14
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Need a 72 Mhz AM receiver
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#15
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Need a 72 Mhz AM receiver
On Fri, 7 Mar 2014 19:21:59 -0800 (PST), "
wrote: Don't you have an oscilloscope? Loosely coupling the scope input to the transmitter, (laying a wire nearby), you should be able to see if the signal is being amplitude modulated. According to the original posting, he's 9 miles from the transmitter. A 100MHz scope will work, if he can find a 9 mile long scope probe. Also, if the modulation is FM, one can't easily see it with a scope. However, the easy way is to read the FCC ID off the transmitter or a receiver, look it up on the FCC ID data dumpster, and reverse engineer the specifications. 72 MHz is the band assigned for RC (radio control) aircraft only (no model cars allowed): http://radiocontrol.wikia.com/wiki/72_MHz http://www.modelaircraft.org/events/frequencies.aspx I would not be surprised if this system is nothing more than a repackaged RC device. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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