Motorola in car problem

[conse]

Hello all, first let me start off by saying that I work for Oklahoma State University Parking Enforcement, and we use a Motorola VHF system. We kind of have a problem, we have a Motorola in car radio in one of our Crown Victoria's and we're having some transmission issues. The radio broadcasts fine, and receives transmissions as well. However, on the listening end, (if you're broadcasting from the car, using the car radio) you can hear a "humming" noise it's a little lower in volume than the voice speaking. It seems to fluctuate as well, meaning when the car accelerates, or decelerates you can hear this as the "hum". Higher when it's accelerating, and a drop off and lower hum when you start to decelerate, steady at an idle.
When transmitting from the car, no problem is evident, and the radio makes no unwanted noises or appears to be in any dysfunction. However you can only imagine how annoying this is if you're on the listening in with a hand held, or at our dispatch center, when the Vic is in stop and go traffic! My question is, what would cause this? I've speculated an exposed wire in the setup, or maybe a loose one. I've looked but I've not been able to find anyone who's able to help. Our "radio specialist" hasn't found anything either, but debate is in the air if he's an actual specialist, or just a bus driver from our other department. Any help on this topic would be appreciated. If you need more information please don't hesitate to email me and let me know what you need!

Conse
Stillwater, OK

 

[firefive76]

What model of radio is it? I'm going to guess a M1225...

 

[conse]

What model of radio is it? I'm going to guess a M1225...

You're exactly right, these are small photos but this is the radio:

M1225™ Series Mobile Radio - Motorola USA

Let me guess, this is a common issue with this series?

 

[firefive76]

Figured thats what it was. These radios are notorious for transmitting buzzing sounds. We have several of these radios that have been sent it to try to get them fixed, and are usually returned with "nothing wrong" probably why this series has been discontinued. The only thing that seems to fix the problems is get a new radio. A filter might help, or new alternator put in the car, other that that, time to pitch the radio.

 

[conse]

Thanks for you help! We've been pushing them to give us a repeater since we have a lot of dead spots due to buildings and such. As it looks now, our new radio system will be in place towards the middle of the year. So hopefully that might entail ditching these old base radios, and some of our old GP300s! (don't laugh, we get treated like dirt with our budget, all our radio equipment is hand me downs from the OSU police department!)

 

[firefive76]

No problem. From what I have seen, it seems that the CM300's don't have any problem with the electrical noise going over.

 

[conse]

We actually have a CM200 in our van, and it's never given us any problem. Always crisp, and clear.

 

[druhe]

Check the alternator

The alternators on the Crown Vic's are notorious for causing interference as described in the original post. I don't know how many times we've been called for radio repair and it turned out to be the alternator.

 

[N_Jay]

Swap the radio and see if the problem follows the radio or the car.

Get a new "radio specialist".

 

[DPapay]

Alternator whine

Sounds like good old fashioned alternator whine. Try moving the ground wire to a completely different point. Sometimes you cannot get rid of it. We make our own filters and they seem to work most of the time. You can use equivalent parts, values can vary also. These are what we use.

Toroid - 5711-RC

Capacitor - 140-TG472M1E-1840-RC

 

[firefive76]

I have seen the M1225 do these in Crown Vics, Impalas, Tahoes, Chevy Pickups, etc. They are well known for picking up ANY electrical noise and sending it over in radio transmissions, wether it be alternator, strobe lights, and even LED lights. We've taken them out of cars and replaced them with CM300's, and haven't had a problem with the CM300's.